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5 | <title>Radio WWV/H Audio Demodulator/Decoder</title> |
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6 | </head> |
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7 | <body> |
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8 | <h3>Radio WWV/H Audio Demodulator/Decoder</h3> |
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9 | |
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10 | <hr> |
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11 | <h4>Synopsis</h4> |
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12 | |
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13 | Address: 127.127.36.<i>u</i> <br> |
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14 | Reference ID: <tt>WWV</tt> or <tt>WWVH</tt> <br> |
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15 | Driver ID: <tt>WWV_AUDIO</tt> <br> |
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16 | Autotune Port: <tt>/dev/icom</tt>; 1200/9600 baud, 8-bits, no |
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17 | parity <br> |
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18 | Audio Device: <tt>/dev/audio</tt> and <tt>/dev/audioctl</tt> |
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19 | |
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20 | <h4>Description</h4> |
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21 | |
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22 | This driver synchronizes the computer time using data encoded in |
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23 | shortwave radio transmissions from NIST time/frequency stations WWV |
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24 | in Ft. Collins, CO, and WWVH in Kauai, HI. Transmissions are made |
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25 | continuously on 2.5, 5, 10, 15 and 20 MHz. An ordinary shortwave |
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26 | receiver can be tuned manually to one of these frequencies or, in |
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27 | the case of ICOM receivers, the receiver can be tuned automatically |
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28 | by the driver as propagation conditions change throughout the day |
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29 | and night. The performance of this driver when tracking one of the |
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30 | stations is ordinarily better than 1 ms in time with frequency |
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31 | drift less than 0.5 PPM when not tracking either station. |
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32 | |
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33 | <p>The demodulation and decoding algorithms used by this driver are |
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34 | based on a machine language program developed for the TAPR DSP93 |
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35 | DSP unit, which uses the TI 320C25 DSP chip. The analysis, design |
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36 | and performance of the program running on this unit is described |
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37 | in: Mills, D.L. A precision radio clock for WWV transmissions. |
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38 | Electrical Engineering Report 97-8-1, University of Delaware, |
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39 | August 1997, 25 pp. Available from <a href= |
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40 | "http://www.eecis.udel.edu/~mills/reports.htm"> |
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41 | www.eecis.udel.edu/~mills/reports.htm</a>. For use in this driver, |
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42 | the original program was rebuilt in the C language and adapted to |
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43 | the NTP driver interface. The algorithms have been modified |
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44 | somewhat to improve performance under weak signal conditions and to |
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45 | provide an automatic station identification feature.</p> |
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46 | |
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47 | <p>This driver incorporates several features in common with other |
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48 | audio drivers such as described in the <a href="driver7.htm">Radio |
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49 | CHU Audio Demodulator/Decoder</a> and the <a href="driver6.htm"> |
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50 | IRIG Audio Decoder</a> pages. They include automatic gain control |
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51 | (AGC), selectable audio codec port and signal monitoring |
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52 | capabilities. For a discussion of these common features, as well as |
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53 | a guide to hookup, debugging and monitoring, see the <a href= |
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54 | "audio.htm">Reference Clock Audio Drivers</a> page.</p> |
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55 | |
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56 | <p>The WWV signal format is described in NIST Special Publication |
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57 | 432 (Revised 1990). It consists of three elements, a 5-ms, 1000-Hz |
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58 | pulse, which occurs at the beginning of each second, a 800-ms, |
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59 | 1000-Hz pulse, which occurs at the beginning of each minute, and a |
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60 | pulse-width modulated 100-Hz subcarrier for the data bits, one bit |
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61 | per second. The WWVH format is identical, except that the 1000-Hz |
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62 | pulses are sent at 1200 Hz. Each minute encodes nine BCD digits for |
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63 | the time of century plus seven bits for the daylight savings time |
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64 | (DST) indicator, leap warning indicator and DUT1 correction.</p> |
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65 | |
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66 | <h4>Program Architecture</h4> |
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67 | |
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68 | <p>As in the original program, the clock discipline is modelled as |
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69 | a Markov process, with probabilistic state transitions |
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70 | corresponding to a conventional clock and the probabilities of |
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71 | received decimal digits. The result is a performance level which |
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72 | results in very high accuracy and reliability, even under |
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73 | conditions when the minute beep of the signal, normally its most |
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74 | prominent feature, can barely be detected by ear with a shortwave |
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75 | receiver.</p> |
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76 | |
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77 | <p>The analog audio signal from the shortwave radio is sampled at |
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78 | 8000 Hz and converted to digital representation. The 1000/1200-Hz |
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79 | pulses and 100-Hz subcarrier are first separated using two IIR |
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80 | filters, a 600-Hz bandpass filter centered on 1100 Hz and a 150-Hz |
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81 | lowpass filter. The minute sync pulse is extracted using a 800-ms |
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82 | synchronous matched filter and pulse grooming logic which |
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83 | discriminates between WWV and WWVH signals and noise. The second |
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84 | sync pulse is extracted using a 5-ms FIR matched filter and |
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85 | 8000-stage comb filter.</p> |
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86 | |
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87 | <p>The phase of the 100-Hz subcarrier relative to the second sync |
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88 | pulse is fixed at the transmitter; however, the audio highpass |
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89 | filter in most radios affects the phase response at 100 Hz in |
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90 | unpredictable ways. The driver adjusts for each radio using two |
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91 | 170-ms synchronous matched filters. The I (in-phase) filter is used |
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92 | to demodulate the subcarrier envelope, while the Q |
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93 | (quadrature-phase) filter is used in a tracking loop to discipline |
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94 | the codec sample clock and thus the demodulator phase.</p> |
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95 | |
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96 | <p>The data bit probabilities are determined from the subcarrier |
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97 | envelope using a threshold-corrected slicer. The averaged envelope |
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98 | amplitude 30 ms from the beginning of the second establishes the |
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99 | minimum (noise floor) value, while the amplitude 200 ms from the |
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100 | beginning establishes the maximum (signal peak) value. The slice |
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101 | level is midway between these two values. The negative-going |
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102 | envelope transition at the slice level establishes the length of |
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103 | the data pulse, which in turn establish probabilities for binary |
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104 | zero (P0) or binary one (P1). The values are established by linear |
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105 | interpolation between the pulse lengths for P0 (300 ms) and P1 (500 |
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106 | ms) so that the sum is equal to one. If the driver has not |
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107 | synchronized to the minute pulse, or if the data bit amplitude, |
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108 | signal/noise ratio (SNR) or length are below thresholds, the bit is |
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109 | considered invalid and all three probabilities are set to zero.</p> |
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110 | |
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111 | <p>The difference between the P1 and P0 probabilities, or |
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112 | likelihood, for each data bit is exponentially averaged in a set of |
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113 | 60 accumulators, one for each second, to determine the semi-static |
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114 | miscellaneous bits, such as DST indicator, leap second warning and |
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115 | DUT1 correction. In this design, an average value larger than a |
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116 | positive threshold is interpreted as a hit on one and a value |
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117 | smaller than a negative threshold as a hit on zero. Values between |
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118 | the two thresholds, which can occur due to signal fades or loss of |
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119 | signal, are interpreted as a miss, and result in no change of |
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120 | indication.</p> |
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121 | |
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122 | <p>The BCD digit in each digit position of the timecode is |
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123 | represented as four data bits, all of which must be valid for the |
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124 | digit itself to be considered valid. If so, the bits are correlated |
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125 | with the bits corresponding to each of the valid decimal digits in |
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126 | this position. If the digit is invalid, the correlated value for |
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127 | all digits in this position is assumed zero. In either case, the |
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128 | values for all digits are exponentially averaged in a likelihood |
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129 | vector associated with this position. The digit associated with the |
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130 | maximum over all of the averaged values then becomes the maximum |
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131 | likelihood selection for this position and the ratio of the maximum |
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132 | over the next lower value becomes the likelihood ratio.</p> |
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133 | |
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134 | <p>The decoding matrix contains nine row vectors, one for each |
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135 | digit position. Each row vector includes the maximum likelihood |
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136 | digit, likelihood vector and other related data. The maximum |
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137 | likelihood digit for each of the nine digit positions becomes the |
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138 | maximum likelihood time of the century. A built-in transition |
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139 | function implements a conventional clock with decimal digits that |
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140 | count the minutes, hours, days and years, as corrected for leap |
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141 | seconds and leap years. The counting operation also rotates the |
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142 | likelihood vector corresponding to each digit as it advances. Thus, |
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143 | once the clock is set, each clock digit should correspond to the |
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144 | maximum likelihood digit as transmitted.</p> |
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145 | |
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146 | <p>Each row of the decoding matrix also includes a compare counter |
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147 | and the difference (modulo the radix) between the current clock |
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148 | digit and most recently determined maximum likelihood digit. If a |
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149 | digit likelihood exceeds the decision level and the difference is |
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150 | constant for a number of successive minutes in any row, the maximum |
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151 | likelihood digit replaces the clock digit in that row. When this |
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152 | condition is true for all rows and the second epoch has been |
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153 | reliably determined, the clock is set (or verified if it has |
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154 | already been set) and delivers correct time to the integral second. |
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155 | The fraction within the second is derived from the logical master |
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156 | clock, which runs at 8000 Hz and drives all system timing |
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157 | functions.</p> |
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158 | |
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159 | <p>The logical master clock is derived from the audio codec clock. |
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160 | Its frequency is disciplined by a frequency-lock loop (FLL) which |
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161 | operates independently of the data recovery functions. At averaging |
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162 | intervals determined by the measured jitter, the frequency error is |
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163 | calculated as the difference between the most recent and the |
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164 | current second epoch divided by the interval. The sample clock |
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165 | frequency is then corrected by this amount using an exponential |
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166 | average. When first started, the frequency averaging interval is |
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167 | eight seconds, in order to compensate for intrinsic codec clock |
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168 | frequency offsets up to 125 PPM. Under most conditions, the |
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169 | averaging interval doubles in stages from the initial value to over |
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170 | 1000 seconds, which results in an ultimate frequency precision of |
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171 | 0.125 PPM, or about 11 ms/day.</p> |
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172 | |
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173 | <p>It is important that the logical clock frequency is stable and |
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174 | accurately determined, since in most applications the shortwave |
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175 | radio will be tuned to a fixed frequency where WWV or WWVH signals |
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176 | are not available throughout the day. In addition, in some parts of |
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177 | the US, especially on the west coast, signals from either or both |
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178 | WWV and WWVH may be available at different times or even at the |
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179 | same time. Since the propagation times from either station are |
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180 | almost always different, each station must be reliably identified |
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181 | before attempting to set the clock.</p> |
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182 | |
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183 | <p>Station identification uses the 800-ms minute pulse transmitted |
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184 | by each station. In the acquisition phase the entire minute is |
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185 | searched using both the WWV and WWVH using matched filters and a |
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186 | pulse gate discriminator similar to that found in radar acquisition |
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187 | and tracking receivers. The peak amplitude found determines a range |
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188 | gate and window where the next pulse is expected to be found. The |
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189 | minute is scanned again to verify the peak is indeed in the window |
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190 | and with acceptable amplitude, SNR and jitter. At this point the |
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191 | receiver begins to track the second sync pulse and operate as above |
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192 | until the clock is set.</p> |
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193 | |
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194 | <p>Once the minute is synchronized, the range gate is fixed and |
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195 | only energy within the window is considered for the minute sync |
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196 | pulse. A compare counter increments by one if the minute pulse has |
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197 | acceptable amplitude, SNR and jitter and decrements otherwise. This |
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198 | is used as a quality indicator and reported in the timecode and |
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199 | also for the autotune function described below.</p> |
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200 | |
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201 | <h4>Performance</h4> |
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202 | |
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203 | <p>It is the intent of the design that the accuracy and stability |
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204 | of the indicated time be limited only by the characteristics of the |
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205 | propagation medium. Conventional wisdom is that synchronization via |
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206 | the HF medium is good only to a millisecond under the best |
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207 | propagation conditions. The performance of the NTP daemon |
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208 | disciplined by the driver is clearly better than this, even under |
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209 | marginal conditions. Ordinarily, with marginal to good signals and |
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210 | a frequency averaging interval of 1024 s, the frequency is |
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211 | stabilized within 0.1 PPM and the time within 125 <font face= |
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212 | "Symbol">m</font>s. The frequency stability characteristic is |
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213 | highly important, since the clock may have to free-run for several |
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214 | hours before reacquiring the WWV/H signal.</p> |
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215 | |
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216 | <p>The expected accuracy over a typical day was determined using |
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217 | the DSP93 and an oscilloscope and cesium oscillator calibrated with |
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218 | a GPS receiver. With marginal signals and allowing 15 minutes for |
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219 | initial synchronization and frequency compensation, the time |
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220 | accuracy determined from the WWV/H second sync pulse was reliably |
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221 | within 125 <font face="Symbol">m</font>s. In the particular DSP-93 |
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222 | used for program development, the uncorrected CPU clock frequency |
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223 | offset was 45.8±0.1 PPM. Over the first hour after initial |
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224 | synchronization, the clock frequency drifted about 1 PPM as the |
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225 | frequency averaging interval increased to the maximum 1024 s. Once |
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226 | reaching the maximum, the frequency wandered over the day up to 1 |
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227 | PPM, but it is not clear whether this is due to the stability of |
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228 | the DSP-93 clock oscillator or the changing height of the |
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229 | ionosphere. Once the frequency had stabilized and after loss of the |
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230 | WWV/H signal, the frequency drift was less than 0.5 PPM, which is |
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231 | equivalent to 1.8 ms/h or 43 ms/d. This resulted in a step phase |
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232 | correction up to several milliseconds when the signal returned.</p> |
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233 | |
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234 | <p>The measured propagation delay from the WWV transmitter at |
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235 | Boulder, CO, to the receiver at Newark, DE, is 23.5±0.1 ms. |
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236 | This is measured to the peak of the pulse after the second sync |
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237 | comb filter and includes components due to the ionospheric |
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238 | propagation delay, nominally 8.9 ms, communications receiver delay |
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239 | and program delay. The propagation delay can be expected to change |
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240 | about 0.2 ms over the day, as the result of changing ionosphere |
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241 | height. The DSP93 program delay was measured at 5.5 ms, most of |
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242 | which is due to the 400-Hz bandpass filter and 5-ms matched filter. |
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243 | Similar delays can be expected of this driver.</p> |
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244 | |
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245 | <h4>Program Operation</h4> |
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246 | |
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247 | The driver begins operation immediately upon startup. It first |
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248 | searches for one or both of the stations WWV and WWVH and attempts |
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249 | to acquire minute sync. This may take some fits and starts, as the |
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250 | driver expects to see three consecutive minutes with good signals |
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251 | and low jitter. If the autotune function is active, the driver will |
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252 | rotate over all five frequencies and both WWV and WWVH stations |
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253 | until three good minutes are found. |
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254 | |
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255 | <p>The driver then acquires second sync, which can take up to |
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256 | several minutes, depending on signal quality. At the same time the |
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257 | driver accumulates likelihood values for each of the nine digits of |
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258 | the clock, plus the seven miscellaneous bits included in the WWV/H |
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259 | transmission format. The minute units digit is decoded first and, |
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260 | when five repetitions have compared correctly, the remaining eight |
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261 | digits are decoded. When five repetitions of all nine digits have |
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262 | decoded correctly, which normally takes 15 minutes with good |
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263 | signals and up to an hour when buried in noise, and the second sync |
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264 | alarm has not been raised for two minutes, the clock is set (or |
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265 | verified) and is selectable to discipline the system clock.</p> |
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266 | |
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267 | <p>As long as the clock is set or verified, the system clock |
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268 | offsets are provided once each second to the reference clock |
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269 | interface, where they are saved in a buffer. At the end of each |
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270 | minute, the buffer samples are groomed by the median filter and |
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271 | trimmed-mean averaging functions. Using these functions, the system |
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272 | clock can in principle be disciplined to a much finer resolution |
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273 | than the 125-<font face="Symbol">m</font>s sample interval would |
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274 | suggest, although the ultimate accuracy is probably limited by |
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275 | propagation delay variations as the ionspheric height varies |
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276 | throughout the day and night.</p> |
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277 | |
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278 | <p>As long as signals are available, the clock frequency is |
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279 | disciplined for use during times when the signals are unavailable. |
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280 | The algorithm refines the frequency offset using increasingly |
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281 | longer averaging intervals to 1024 s, where the precision is about |
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282 | 0.1 PPM. With good signals, it takes well over two hours to reach |
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283 | this degree of precision; however, it can take many more hours than |
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284 | this in case of marginal signals. Once reaching the limit, the |
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285 | algorithm will follow frequency variations due to temperature |
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286 | fluctuations and ionospheric height variations.</p> |
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287 | |
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288 | <p>It may happen as the hours progress around the clock that WWV |
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289 | and WWVH signals may appear alone, together or not at all. When the |
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290 | driver is first started, the NTP reference identifier appears as |
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291 | <tt>NONE</tt>. When the driver has acquired one or both stations |
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292 | and mitigated which one is best, it sets the station identifier in |
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293 | the timecode as described below. In addition, the NTP reference |
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294 | identifier is set to the station callsign. If the propagation |
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295 | delays has been properly set with the <tt>fudge time1</tt> (WWV) |
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296 | and <tt>fudge time2</tt> (WWVH) commands in the configuration file, |
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297 | handover from one station to the other will be seamless.</p> |
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298 | |
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299 | <p>Once the clock has been set for the first time, it will appear |
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300 | reachable and selectable to discipline the system clock, even if |
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301 | the broadcast signal fades to obscurity. A consequence of this |
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302 | design is that, once the clock is set, the time and frequency are |
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303 | disciplined only by the second sync pulse and the clock digits |
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304 | themselves are driven by the clock state machine and ordinarily |
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305 | never changed. However, as long as the clock is set correctly, it |
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306 | will continue to read correctly after a period of signal loss, as |
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307 | long as it does not drift more than 500 ms from the correct time. |
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308 | Assuming the clock frequency can be disciplined within 1 PPM, the |
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309 | clock could coast without signals for some 5.8 days without |
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310 | exceeding that limit. If for some reason this did happen, the clock |
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311 | would be in the wrong second and would never resynchronize. To |
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312 | protect against this most unlikely situation, if after four days |
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313 | with no signals, the clock is considered unset and resumes the |
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314 | synchronization procedure from the beginning.</p> |
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315 | |
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316 | <p>To work well, the driver needs a communications receiver with |
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317 | good audio response at 100 Hz. Most shortwave and communications |
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318 | receivers roll off the audio response below 250 Hz, so this can be |
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319 | a problem, especially with receivers using DSP technology, since |
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320 | DSP filters can have very fast rolloff outside the passband. Some |
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321 | DSP transceivers, in particular the ICOM 775, have a programmable |
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322 | low frequency cutoff which can be set as low as 80 Hz. However, |
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323 | this particular radio has a strong low frequency buzz at about 10 |
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324 | Hz which appears in the audio output and can affect data recovery |
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325 | under marginal conditions. Although not tested, it would seem very |
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326 | likely that a cheap shortwave receiver could function just as well |
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327 | as an expensive communications receiver.</p> |
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328 | |
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329 | <h4>Autotune</h4> |
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330 | |
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331 | <p>The driver includes provisions to automatically tune the radio |
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332 | in response to changing radio propagation conditions throughout the |
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333 | day and night. The radio interface is compatible with the ICOM CI-V |
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334 | standard, which is a bidirectional serial bus operating at TTL |
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335 | levels. The bus can be connected to a serial port using a level |
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336 | converter such as the CT-17. The serial port speed is presently |
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337 | compiled in the program, but can be changed in the driver source |
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338 | file.</p> |
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339 | |
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340 | <p>Each ICOM radio is assigned a unique 8-bit ID select code, |
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341 | usually expressed in hex format. To activate the CI-V interface, |
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342 | the <tt>mode</tt> keyword of the <tt>server</tt> configuration |
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343 | command specifies a nonzero select code in decimal format. A table |
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344 | of ID select codes for the known ICOM radios is given below. Since |
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345 | all ICOM select codes are less than 128, the high order bit of the |
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346 | code is used by the driver to specify the baud rate. If this bit is |
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347 | not set, the rate is 9600 bps for the newer radios; if set, the |
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348 | rate is 1200 bps for the older radios. A missing <tt>mode</tt> |
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349 | keyword or a zero argument leaves the interface disabled.</p> |
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350 | |
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351 | <p>If specified, the driver will attempt to open the device <tt> |
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352 | /dev/icom</tt> and, if successful will activate the autotune |
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353 | function and tune the radio to each operating frequency in turn |
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354 | while attempting to acquire minute sync from either WWV or WWVH. |
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355 | However, the driver is liberal in what it assumes of the |
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356 | configuration. If the <tt>/dev/icom</tt> link is not present or the |
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357 | open fails or the CI-V bus or radio is inoperative, the driver |
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358 | quietly gives up with no harm done.</p> |
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359 | |
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360 | <p>Once acquiring minute sync, the driver operates as described |
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361 | above to set the clock. However, during seconds 59, 0 and 1 of each |
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362 | minute it tunes the radio to one of the five broadcast frequencies |
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363 | to measure the sync pulse and data pulse amplitudes and SNR and |
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364 | update the compare counter. Each of the five frequencies are probed |
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365 | in a five-minute rotation to build a database of current |
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366 | propagation conditions for all signals that can be heard at the |
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367 | time. At the end of each rotation, a mitigation procedure scans the |
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368 | database and retunes the radio to the best frequency and station |
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369 | found. For this to work well, the radio should be set for a fast |
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370 | AGC recovery time. This is most important while tracking a strong |
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371 | signal, which is normally the case, and then probing another |
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372 | frequency, which may have much weaker signals.</p> |
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373 | |
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374 | <p>Reception conditions for each frequency and station are |
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375 | evaluated according to a metric which considers the minute sync |
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376 | pulse amplitude, SNR and jitter, as well as, the data pulse |
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377 | amplitude and SNR. The minute pulse is evaluated at second 0, while |
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378 | the data pulses are evaluated at seconds 59 and 1. The results are |
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379 | summarized in a scoreboard of three bits</p> |
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380 | |
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381 | <dl> |
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382 | <dt><tt>0x0001</tt></dt> |
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383 | |
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384 | <dd>Jitter exceeded. The difference in epoches between the last |
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385 | minute sync pulse and the current one exceeds 50 ms (400 |
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386 | samples).</dd> |
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387 | |
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388 | <dt><tt>0x0002</tt></dt> |
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389 | |
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390 | <dd>Minute pulse error. For the minute sync pulse in second 0, |
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391 | either the amplitude or SNR is below threshold (2000 and 20 dB, |
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392 | respectively).</dd> |
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393 | |
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394 | <dt><tt>0x0004</tt></dt> |
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395 | |
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396 | <dd>Minute pulse error. For both of the data pulses in seocnds 59 |
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397 | and 1, either the amplitude or SNR is below threshold (1000 and 10 |
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398 | dB, respectively).</dd> |
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399 | </dl> |
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400 | |
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401 | <p>If none of the scoreboard bits are set, the compare counter is |
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402 | increased by one to a maximum of six. If any bits are set, the |
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403 | counter is decreased by one to a minimum of zero. At the end of |
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404 | each minute, the frequency and station with the maximum compare |
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405 | count is chosen, with ties going to the highest frequency.</p> |
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406 | |
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407 | <h4>Diagnostics</h4> |
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408 | |
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409 | <p>The autotune process produces diagnostic information along with |
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410 | the timecode. This is very useful for evaluating the performance of |
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411 | the algorithm, as well as radio propagation conditions in general. |
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412 | The message is produced once each minute for each frequency in turn |
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413 | after minute sync has been acquired.</p> |
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414 | |
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415 | <p><tt>wwv5 port agc wwv wwvh</tt></p> |
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416 | |
---|
417 | <p>where <tt>port</tt> and <tt>agc</tt> are the audio port and |
---|
418 | gain, respectively, for this frequency and <tt>wwv</tt> and <tt> |
---|
419 | wwvh</tt> are two sets of fields, one each for WWV and WWVH. Each |
---|
420 | of the two fields has the format</p> |
---|
421 | |
---|
422 | <p><tt>ident score comp sync/snr/jitr</tt></p> |
---|
423 | |
---|
424 | <p>where <tt>ident</tt>encodes the station (<tt>C</tt> for WWV, |
---|
425 | <tt>H</tt> for WWVH) and frequency (2, 5, 10, 15 and 20), <tt> |
---|
426 | score</tt> is the scoreboard described above, <tt>comp</tt> is the |
---|
427 | compare counter, <tt>sync</tt> is the minute sync pulse amplitude, |
---|
428 | <tt>snr</tt> the SNR of the pulse and <tt>jitr</tt> is the sample |
---|
429 | difference between the current epoch and the last epoch. An example |
---|
430 | is:</p> |
---|
431 | |
---|
432 | <p><tt>wwv5 2 111 C20 0100 6 8348/30.0/-3 H20 0203 0 |
---|
433 | 22/-12.4/8846</tt></p> |
---|
434 | |
---|
435 | <p>Here the radio is tuned to 20 MHz and the line-in port AGC is |
---|
436 | currently 111 at that frequency. The message contains a report for |
---|
437 | WWV (<tt>C20</tt>) and WWVH (<tt>H20</tt>). The WWV report |
---|
438 | scoreboard is 0100 and the compare count is 6, which suggests very |
---|
439 | good reception conditions, and the minute sync amplitude and SNR |
---|
440 | are well above thresholds (2000 and 20 dB, respectively). Probably |
---|
441 | the most sensitive indicator of reception quality is the jitter, -3 |
---|
442 | samples, which is well below threshold (50 ms or 400 samples). |
---|
443 | While the message shows solid reception conditions from WWV, this |
---|
444 | is not the case for WWVH. Both the minute sync amplitude and SNR |
---|
445 | are below thresholds and the jitter is above threshold.</p> |
---|
446 | |
---|
447 | <p>A sequence of five messages, one for each minute, might appear |
---|
448 | as follows:</p> |
---|
449 | |
---|
450 | <pre> |
---|
451 | wwv5 2 95 C2 0107 0 164/7.2/8100 H2 0207 0 80/-5.5/7754 |
---|
452 | wwv5 2 99 C5 0104 0 3995/21.8/395 H5 0207 0 27/-9.3/18826 |
---|
453 | wwv5 2 239 C10 0105 0 9994/30.0/2663 H10 0207 0 54/-16.1/-529 |
---|
454 | wwv5 2 155 C15 0103 3 3300/17.8/-1962 H15 0203 0 236/17.0/4873 |
---|
455 | wwv5 2 111 C20 0100 6 8348/30.0/-3 H20 0203 0 22/-12.4/8846 |
---|
456 | </pre> |
---|
457 | |
---|
458 | <p>Clearly, the only frequencies that are available are 15 MHz and |
---|
459 | 20 MHz and propagation may be failing for 15 MHz. However, minute |
---|
460 | sync pulses are being heard on 5 and 10 MHz, even though the data |
---|
461 | pulses are not. This is typical of late afternoon when the maximum |
---|
462 | usable frequency (MUF) is falling and the ionospheric loss at the |
---|
463 | lower frequencies is beginning to decrease.</p> |
---|
464 | |
---|
465 | <h4>Debugging Aids</h4> |
---|
466 | |
---|
467 | <p>The most convenient way to track the driver status is using the |
---|
468 | <tt>ntpq</tt> program and the <tt>clockvar</tt> command. This |
---|
469 | displays the last determined timecode and related status and error |
---|
470 | counters, even when the driver is not discipline the system clock. |
---|
471 | If the debugging trace feature (<tt>-d</tt> on the <tt>ntpd</tt> |
---|
472 | command line)is enabled, the driver produces detailed status |
---|
473 | messages as it operates. If the <tt>fudge flag 4</tt> is set, these |
---|
474 | messages are written to the <tt>clockstats</tt> file. All messages |
---|
475 | produced by this driver have the prefix <tt>chu</tt> for convenient |
---|
476 | filtering with the Unix <tt>grep</tt> command.</p> |
---|
477 | |
---|
478 | <p>In the following descriptions the units of amplitude, phase, |
---|
479 | probability and likelihood are normalized to the range 0-6000 for |
---|
480 | convenience. In addition, the signal/noise ratio (SNR) and |
---|
481 | likelihood ratio are measured in decibels and the words with bit |
---|
482 | fields are in hex. Most messages begin with a leader in the |
---|
483 | following format:</p> |
---|
484 | |
---|
485 | <p><tt>wwvn ss stat sigl</tt></p> |
---|
486 | |
---|
487 | <p>where <tt>wwvn</tt> is the message code, <tt>ss</tt> the second |
---|
488 | of minute, <tt>stat</tt> the driver status word and <tt>sigl</tt> |
---|
489 | the second sync pulse amplitude. A full explanation of the status |
---|
490 | bits is contained in the driver source listing; however, the |
---|
491 | following are the most useful for debugging.</p> |
---|
492 | |
---|
493 | <dl> |
---|
494 | <dt><tt>0x0001</tt></dt> |
---|
495 | |
---|
496 | <dd>Minute sync. Set when the decoder has identified a station and |
---|
497 | acquired the minute sync pulse.</dd> |
---|
498 | |
---|
499 | <dt><tt>0x0002</tt></dt> |
---|
500 | |
---|
501 | <dd>Second sync. Set when the decoder has acquired the second sync |
---|
502 | pulse and within 125 <font face="Symbol">m</font>s of the correct |
---|
503 | phase.</dd> |
---|
504 | |
---|
505 | <dt><tt>0x0004</tt></dt> |
---|
506 | |
---|
507 | <dd>Minute unit sync. Set when the decoder has reliably determined |
---|
508 | the unit digit of the minute.</dd> |
---|
509 | |
---|
510 | <dt><tt>0x0008</tt></dt> |
---|
511 | |
---|
512 | <dd>Clock set. Set when the decoder has reliably determined all |
---|
513 | nine digits of the timecode and is selectable to discipline the |
---|
514 | system clock.</dd> |
---|
515 | </dl> |
---|
516 | |
---|
517 | <p>With debugging enabled the driver produces messages in the |
---|
518 | following formats:</p> |
---|
519 | |
---|
520 | <p>Format <tt>wwv8</tt> messages are produced once per minute by |
---|
521 | the WWV and WWVH station processes before minute sync has been |
---|
522 | acquired. They show the progress of identifying and tracking the |
---|
523 | minute pulse of each station.</p> |
---|
524 | |
---|
525 | <p><tt>wwv8 port agc ident comp ampl snr epoch jitr offs</tt></p> |
---|
526 | |
---|
527 | <p>where <tt>port</tt> and <tt>agc</tt> are the audio port and |
---|
528 | gain, respectively. The <tt>ident</tt>encodes the station |
---|
529 | (<tt>C</tt> for WWV, <tt>H</tt> for WWVH) and frequency (2, 5, 10, |
---|
530 | 15 and 20). For the encoded frequency, <tt>comp</tt> is the compare |
---|
531 | counter, <tt>ampl</tt> the pulse amplitude, <tt>snr</tt> the SNR, |
---|
532 | <tt>epoch</tt> the sample number of the minute pulse in the minute, |
---|
533 | <tt>jitr</tt> the change since the last <tt>epoch</tt> and <tt> |
---|
534 | offs</tt> the minute pulse offset relative to the second pulse. An |
---|
535 | example is:</p> |
---|
536 | |
---|
537 | <p><tt>wwv8 2 127 C15 2 9247 30.0 18843 -1 1</tt><br> |
---|
538 | <tt>wwv8 2 127 H15 0 134 -2.9 19016 193 174</tt></p> |
---|
539 | |
---|
540 | <p>Here the radio is tuned to 15 MHz and the line-in port AGC is |
---|
541 | currently 127 at that frequency. The driver has not yet acquired |
---|
542 | minute sync, WWV has been heard for at least two minutes, and WWVH |
---|
543 | is in the noise. The WWV minute pulse amplitude and SNR are well |
---|
544 | above the threshold (2000 and 6 dB, respectively) and the minute |
---|
545 | epoch has been determined -1 sample relative to the last one and 1 |
---|
546 | sample relative to the second sync pulse. The compare counter has |
---|
547 | incrmented to two; when it gets to three, minute sync has been |
---|
548 | acquired.</p> |
---|
549 | |
---|
550 | <p>Format <tt>wwv3</tt> messages are produced after minute sync has |
---|
551 | been acquired and until the seconds unit digit is determined. They |
---|
552 | show the results of decoding each bit of the transmitted |
---|
553 | timecode.</p> |
---|
554 | |
---|
555 | <p><tt>wwv3 ss stat sigl ampl phas snr prob like</tt></p> |
---|
556 | |
---|
557 | <p>where <tt>ss</tt>, <tt>stat</tt> and <tt>sigl</tt> are as above, |
---|
558 | <tt>ampl</tt> is the subcarrier amplitude, <tt>phas</tt> the |
---|
559 | subcarrier phase, <tt>snr</tt> the subcarrier SNR, <tt>prob</tt> |
---|
560 | the bit probability and <tt>like</tt> the bit likelihood. An |
---|
561 | example is:</p> |
---|
562 | |
---|
563 | <p><tt>wwv3 28 0123 4122 4286 0 24.8 -5545 -1735</tt></p> |
---|
564 | |
---|
565 | <p>Here the driver has acquired minute and second sync, but has not |
---|
566 | yet determined the seconds unit digit. However, it has just decoded |
---|
567 | bit 28 of the minute. The results show the second sync pulse |
---|
568 | amplitude well over the threshold (500), subcarrier amplitude well |
---|
569 | above the threshold (1000), good subcarrier tracking phase and SNR |
---|
570 | well above the threshold (10 dB). The bit is almost certainly a |
---|
571 | zero and the likelihood of a zero in this second is very high.</p> |
---|
572 | |
---|
573 | <p>Format <tt>wwv4</tt> messages are produced for each of the nine |
---|
574 | BCD timecode digits until the clock has been set or verified. They |
---|
575 | show the results of decoding each digit of the transmitted |
---|
576 | timecode.</p> |
---|
577 | |
---|
578 | <p><tt>wwv4 ss stat sigl radx ckdig mldig diff cnt like |
---|
579 | snr</tt></p> |
---|
580 | |
---|
581 | <p>where <tt>ss</tt>, <tt>stat</tt> and <tt>sigl</tt> are as above, |
---|
582 | <tt>radx</tt> is the digit radix (3, 4, 6, 10), <tt>ckdig</tt> the |
---|
583 | current clock digit, <tt>mldig</tt> the maximum likelihood digit, |
---|
584 | <tt>diff</tt> the difference between these two digits modulo the |
---|
585 | radix, <tt>cnt</tt> the compare counter, <tt>like</tt> the digit |
---|
586 | likelihood and <tt>snr</tt> the likelihood ratio. An example |
---|
587 | is:</p> |
---|
588 | |
---|
589 | <p><tt>wwv4 8 010f 5772 10 9 9 0 6 4615 6.1</tt></p> |
---|
590 | |
---|
591 | <p>Here the driver has previousl set or verified the clock. It has |
---|
592 | just decoded the digit preceding second 8 of the minute. The digit |
---|
593 | radix is 10, the current clock and maximum likelihood digits are |
---|
594 | both 9, the likelihood is well above the threshold (1000) and the |
---|
595 | likelihood function well above threshold (3.0 dB). Short of a |
---|
596 | hugely unlikely probability conspiracy, the clock digit is most |
---|
597 | certainly a 9.</p> |
---|
598 | |
---|
599 | <p>Format <tt>wwv2</tt> messages are produced at each master |
---|
600 | oscillator frequency update, which starts at 8 s, but eventually |
---|
601 | climbs to 1024 s. They show the progress of the algorithm as it |
---|
602 | refines the frequency measurement to a precision of 0.1 PPM.</p> |
---|
603 | |
---|
604 | <p><tt>wwv2 ss stat sigl avint avcnt avinc jitr delt freq</tt></p> |
---|
605 | |
---|
606 | <p>where <tt>ss</tt>, <tt>stat</tt> and <tt>sigl</tt> are as above, |
---|
607 | <tt>avint</tt> is the averaging interval, <tt>avcnt</tt> the |
---|
608 | averaging interval counter, <tt>avinc</tt> the interval increment, |
---|
609 | <tt>jitr</tt> the sample change between the beginning and end of |
---|
610 | the interval, <tt>delt</tt> the computed frequency change and <tt> |
---|
611 | freq</tt> the current frequency (PPM). An example is:</p> |
---|
612 | |
---|
613 | <p><tt>wwv2 22 030f 5795 256 256 4 0 0.0 66.7</tt></p> |
---|
614 | |
---|
615 | <p>Here the driver has acquired minute and second sync and set the |
---|
616 | clock. The averaging interval has increased to 256 s on the way to |
---|
617 | 1024 s, has stayed at that interval for 4 averaging intervals, has |
---|
618 | measured no change in frequency and the current frequency is 66.7 |
---|
619 | PPM.</p> |
---|
620 | |
---|
621 | <p>If the CI-V interface for ICOM radios is active, a debug level |
---|
622 | greater than 1 will produce a trace of the CI-V command and |
---|
623 | response messages. Interpretation of these messages requires |
---|
624 | knowledge of the CI-V protocol, which is beyond the scope of this |
---|
625 | document.</p> |
---|
626 | |
---|
627 | <h4>Monitor Data</h4> |
---|
628 | |
---|
629 | When enabled by the <tt>filegen</tt> facility, every received |
---|
630 | timecode is written to the <tt>clockstats</tt> file in the |
---|
631 | following format: |
---|
632 | |
---|
633 | <pre> |
---|
634 | sq yy ddd hh:mm:ss.fff ld du lset agc stn rfrq errs freq cons |
---|
635 | |
---|
636 | s sync indicator |
---|
637 | q quality character |
---|
638 | yyyy Gregorian year |
---|
639 | ddd day of year |
---|
640 | hh hour of day |
---|
641 | mm minute of hour |
---|
642 | fff millisecond of second |
---|
643 | l leap second warning |
---|
644 | d DST state |
---|
645 | dut DUT sign and magnitude |
---|
646 | lset minutes since last set |
---|
647 | agc audio gain |
---|
648 | ident station identifier and frequency |
---|
649 | comp minute sync compare counter |
---|
650 | errs bit error counter |
---|
651 | freq frequency offset |
---|
652 | avgt averaging time |
---|
653 | </pre> |
---|
654 | |
---|
655 | The fields beginning with <tt>year</tt> and extending through <tt> |
---|
656 | dut</tt> are decoded from the received data and are in fixed-length |
---|
657 | format. The <tt>agc</tt> and <tt>lset</tt> fields, as well as the |
---|
658 | following driver-dependent fields, are in variable-length format. |
---|
659 | |
---|
660 | <dl> |
---|
661 | <dt><tt>s</tt></dt> |
---|
662 | |
---|
663 | <dd>The sync indicator is initially <tt>?</tt> before the clock is |
---|
664 | set, but turns to space when all nine digits of the timecode are |
---|
665 | correctly set.</dd> |
---|
666 | |
---|
667 | <dt><tt>q</tt></dt> |
---|
668 | |
---|
669 | <dd>The quality character is a four-bit hexadecimal code showing |
---|
670 | which alarms have been raised. Each bit is associated with a |
---|
671 | specific alarm condition according to the following: |
---|
672 | |
---|
673 | <dl> |
---|
674 | <dt><tt>0x8</tt></dt> |
---|
675 | |
---|
676 | <dd>Sync alarm. The decoder may not be in correct second or minute |
---|
677 | phase relative to the transmitter.</dd> |
---|
678 | |
---|
679 | <dt><tt>0x4</tt></dt> |
---|
680 | |
---|
681 | <dd>Error alarm. More than 30 data bit errors occurred in the last |
---|
682 | minute.</dd> |
---|
683 | |
---|
684 | <dt><tt>0x2</tt></dt> |
---|
685 | |
---|
686 | <dd>Symbol alarm. The probability of correct decoding for a digit |
---|
687 | or miscellaneous bit has fallen below the threshold.</dd> |
---|
688 | |
---|
689 | <dt><tt>0x1</tt></dt> |
---|
690 | |
---|
691 | <dd>Decoding alarm. A maximum likelihood digit fails to agree with |
---|
692 | the current associated clock digit.</dd> |
---|
693 | </dl> |
---|
694 | |
---|
695 | It is important to note that one or more of the above alarms does |
---|
696 | not necessarily indicate a clock error, but only that the decoder |
---|
697 | has detected a condition that may in future result in an |
---|
698 | error.</dd> |
---|
699 | |
---|
700 | <dt><tt>yyyy ddd hh:mm:ss.fff</tt></dt> |
---|
701 | |
---|
702 | <dd>The timecode format itself is self explanatory. Since the |
---|
703 | driver latches the on-time epoch directly from the second sync |
---|
704 | pulse, the fraction <tt>fff</tt>is always zero. Although the |
---|
705 | transmitted timecode includes only the year of century, the |
---|
706 | Gregorian year is augmented 2000 if the indicated year is less than |
---|
707 | 72 and 1900 otherwise.</dd> |
---|
708 | |
---|
709 | <dt><tt>l</tt></dt> |
---|
710 | |
---|
711 | <dd>The leap second warning is normally space, but changes to <tt> |
---|
712 | L</tt> if a leap second is to occur at the end of the month of June |
---|
713 | or December.</dd> |
---|
714 | |
---|
715 | <dt><tt>d</tt></dt> |
---|
716 | |
---|
717 | <dd>The DST state is <tt>S</tt> or <tt>D</tt> when standard time or |
---|
718 | daylight time is in effect, respectively. The state is <tt>I</tt> |
---|
719 | or <tt>O</tt> when daylight time is about to go into effect or out |
---|
720 | of effect, respectively.</dd> |
---|
721 | |
---|
722 | <dt><tt>dut</tt></dt> |
---|
723 | |
---|
724 | <dd>The DUT sign and magnitude shows the current UT1 offset |
---|
725 | relative to the displayed UTC time, in deciseconds.</dd> |
---|
726 | |
---|
727 | <dt><tt>lset</tt></dt> |
---|
728 | |
---|
729 | <dd>Before the clock is set, the interval since last set is the |
---|
730 | number of minutes since the driver was started; after the clock is |
---|
731 | set, this is number of minutes since the time was last verified |
---|
732 | relative to the broadcast signal.</dd> |
---|
733 | |
---|
734 | <dt><tt>agc</tt></dt> |
---|
735 | |
---|
736 | <dd>The audio gain shows the current codec gain setting in the |
---|
737 | range 0 to 255. Ordinarily, the receiver audio gain control or IRIG |
---|
738 | level control should be set for a value midway in this range.</dd> |
---|
739 | |
---|
740 | <dt><tt>ident</tt></dt> |
---|
741 | |
---|
742 | <dd>The station identifier shows the station, <tt>C</tt> for WWV or |
---|
743 | <tt>H</tt> for WWVH, and frequency being tracked. If neither |
---|
744 | station is heard on any frequency, the station identifier shows |
---|
745 | <tt>X</tt>.</dd> |
---|
746 | |
---|
747 | <dt><tt>comp</tt></dt> |
---|
748 | |
---|
749 | <dd>The minute sync compare counter is useful to determine the |
---|
750 | quality of the minute sync signal and can range from 0 (no signal) |
---|
751 | to 5 (best).</dd> |
---|
752 | |
---|
753 | <dt><tt>errs</tt></dt> |
---|
754 | |
---|
755 | <dd>The bit error counter is useful to determine the quality of the |
---|
756 | data signal received in the most recent minute. It is normal to |
---|
757 | drop a couple of data bits under good signal conditions and |
---|
758 | increasing numbers as conditions worsen. While the decoder performs |
---|
759 | moderately well even with half the bits are in error in any minute, |
---|
760 | usually by that point the sync signals are lost and the decoder |
---|
761 | reverts to free-run anyway.</dd> |
---|
762 | |
---|
763 | <dt><tt>freq</tt></dt> |
---|
764 | |
---|
765 | <dd>The frequency offset is the current estimate of the codec |
---|
766 | frequency offset to within 0.1 PPM. This may wander a bit over the |
---|
767 | day due to local temperature fluctuations and propagation |
---|
768 | conditions.</dd> |
---|
769 | |
---|
770 | <dt><tt>avgt</tt></dt> |
---|
771 | |
---|
772 | <dd>The averaging time is the interval between frequency updates in |
---|
773 | powers of two to a maximum of 1024 s. Attainment of the maximum |
---|
774 | indicates the driver is operating at the best possible resolution |
---|
775 | in time and frequency.</dd> |
---|
776 | </dl> |
---|
777 | |
---|
778 | <p>An example timecode is:</p> |
---|
779 | |
---|
780 | <p><tt>0 2000 006 22:36:00.000 S +3 1 115 C20 6 5 66.4 |
---|
781 | 1024</tt></p> |
---|
782 | |
---|
783 | <p>Here the clock has been set and no alarms are raised. The year, |
---|
784 | day and time are displayed along with no leap warning, standard |
---|
785 | time and DUT +0.3 s. The clock was set on the last minute, the AGC |
---|
786 | is safely in the middle ot the range 0-255, and the receiver is |
---|
787 | tracking WWV on 20 MHz. Excellent reeiving conditions prevail, as |
---|
788 | indicated by the compare count 6 and 5 bit errors during the last |
---|
789 | minute. The current frequency is 66.4 PPM and the averaging |
---|
790 | interval is 1024 s, indicating the maximum precision available.</p> |
---|
791 | |
---|
792 | <h4>Modes</h4> |
---|
793 | |
---|
794 | <p>The <tt>mode</tt> keyword of the <tt>server</tt> configuration |
---|
795 | command specifies the ICOM ID select code. A missing or zero |
---|
796 | argument disables the CI-V interface. Following are the ID select |
---|
797 | codes for the known radios.</p> |
---|
798 | |
---|
799 | <table cols="6" width="100%"> |
---|
800 | <tr> |
---|
801 | <td>Radio</td> |
---|
802 | <td>Hex</td> |
---|
803 | <td>Decimal</td> |
---|
804 | <td>Radio</td> |
---|
805 | <td>Hex</td> |
---|
806 | <td>Decimal</td> |
---|
807 | </tr> |
---|
808 | |
---|
809 | <tr> |
---|
810 | <td>IC725</td> |
---|
811 | <td>0x28</td> |
---|
812 | <td>40</td> |
---|
813 | <td>IC781</td> |
---|
814 | <td>0x26</td> |
---|
815 | <td>38</td> |
---|
816 | </tr> |
---|
817 | |
---|
818 | <tr> |
---|
819 | <td>IC726</td> |
---|
820 | <td>0x30</td> |
---|
821 | <td>48</td> |
---|
822 | <td>R7000</td> |
---|
823 | <td>0x08</td> |
---|
824 | <td>8</td> |
---|
825 | </tr> |
---|
826 | |
---|
827 | <tr> |
---|
828 | <td>IC735</td> |
---|
829 | <td>0x04</td> |
---|
830 | <td>4</td> |
---|
831 | <td>R71</td> |
---|
832 | <td>0x1A</td> |
---|
833 | <td>26</td> |
---|
834 | </tr> |
---|
835 | |
---|
836 | <tr> |
---|
837 | <td>IC751</td> |
---|
838 | <td>0x1c</td> |
---|
839 | <td>28</td> |
---|
840 | <td>R7100</td> |
---|
841 | <td>0x34</td> |
---|
842 | <td>52</td> |
---|
843 | </tr> |
---|
844 | |
---|
845 | <tr> |
---|
846 | <td>IC761</td> |
---|
847 | <td>0x1e</td> |
---|
848 | <td>30</td> |
---|
849 | <td>R72</td> |
---|
850 | <td>0x32</td> |
---|
851 | <td>50</td> |
---|
852 | </tr> |
---|
853 | |
---|
854 | <tr> |
---|
855 | <td>IC765</td> |
---|
856 | <td>0x2c</td> |
---|
857 | <td>44</td> |
---|
858 | <td>R8500</td> |
---|
859 | <td>0x4a</td> |
---|
860 | <td>74</td> |
---|
861 | </tr> |
---|
862 | |
---|
863 | <tr> |
---|
864 | <td>IC775</td> |
---|
865 | <td>0x46</td> |
---|
866 | <td>68</td> |
---|
867 | <td>R9000</td> |
---|
868 | <td>0x2a</td> |
---|
869 | <td>42</td> |
---|
870 | </tr> |
---|
871 | </table> |
---|
872 | |
---|
873 | <h4>Fudge Factors</h4> |
---|
874 | |
---|
875 | <dl> |
---|
876 | <dt><tt>time1 <i>time</i></tt></dt> |
---|
877 | |
---|
878 | <dd>Specifies the propagation delay for WWV (40:40:49.0N |
---|
879 | 105:02:27.0W), in seconds and fraction, with default 0.0.</dd> |
---|
880 | |
---|
881 | <dt><tt>time2 <i>time</i></tt></dt> |
---|
882 | |
---|
883 | <dd>Specifies the propagation delay for WWVH (21:59:26.0N |
---|
884 | 159:46:00.0W), in seconds and fraction, with default 0.0.</dd> |
---|
885 | |
---|
886 | <dt><tt>stratum <i>number</i></tt></dt> |
---|
887 | |
---|
888 | <dd>Specifies the driver stratum, in decimal from 0 to 15, with |
---|
889 | default 0.</dd> |
---|
890 | |
---|
891 | <dt><tt>refid <i>string</i></tt></dt> |
---|
892 | |
---|
893 | <dd>Ordinarily, this field specifies the driver reference |
---|
894 | identifier; however, the driver sets the reference identifier |
---|
895 | automatically as described above.</dd> |
---|
896 | |
---|
897 | <dt><tt>flag1 0 | 1</tt></dt> |
---|
898 | |
---|
899 | <dd>Not used by this driver.</dd> |
---|
900 | |
---|
901 | <dt><tt>flag2 0 | 1</tt></dt> |
---|
902 | |
---|
903 | <dd>Specifies the microphone port if set to zero or the line-in |
---|
904 | port if set to one. It does not seem useful to specify the compact |
---|
905 | disc player port.</dd> |
---|
906 | |
---|
907 | <dt><tt>flag3 0 | 1</tt></dt> |
---|
908 | |
---|
909 | <dd>Enables audio monitoring of the input signal. For this purpose, |
---|
910 | the speaker volume must be set before the driver is started.</dd> |
---|
911 | |
---|
912 | <dt><tt>flag4 0 | 1</tt></dt> |
---|
913 | |
---|
914 | <dd>Enable verbose <tt>clockstats</tt> recording if set.</dd> |
---|
915 | </dl> |
---|
916 | |
---|
917 | <h4>Additional Information</h4> |
---|
918 | |
---|
919 | <a href="refclock.htm">Reference Clock Drivers</a> <br> |
---|
920 | <a href="audio.htm">Reference Clock Audio Drivers</a> |
---|
921 | |
---|
922 | <hr> |
---|
923 | <a href="index.htm"><img align="left" src="pic/home.gif" alt= |
---|
924 | "gif"></a> |
---|
925 | |
---|
926 | <address><a href="mailto:mills@udel.edu">David L. Mills |
---|
927 | <mills@udel.edu></a></address> |
---|
928 | </body> |
---|
929 | </html> |
---|
930 | |
---|