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5 | <title>Reference Clock Audio Drivers</title> |
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7 | <body> |
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8 | <h3>Reference Clock Audio Drivers</h3> |
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9 | |
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10 | <img align="left" src="pic/radio2.jpg" alt="gif"> |
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11 | |
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12 | <p>Make a little noise here.<br clear="left"> |
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13 | </p> |
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14 | |
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15 | <hr> |
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16 | <p>There are some applications in which the computer time can be |
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17 | disciplined to an audio signal, rather than a serial timecode and |
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18 | communications port or special purpose bus peripheral. This is |
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19 | useful in such cases where the audio signal is sent over a |
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20 | telephone circuit, for example, or received directly from a |
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21 | shortwave receiver. In such cases the audio signal can be connected |
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22 | via an ordinary sound card or baseboard audio codec. The suite of |
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23 | NTP reference clock drivers currently includes three drivers |
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24 | suitable for these applications. They include a driver for the |
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25 | Inter Range Instrumentation Group (IRIG) signals produced by most |
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26 | radio clocks and timing devices, another for the Canadian |
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27 | time/frequency radio station CHU and a third for the NIST |
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28 | time/frequency radio stations WWV and WWVH. The radio drivers are |
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29 | designed to work with ordinary inexpensive shortwave radios and may |
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30 | be one of the least expensive ways to build a good primary time |
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31 | server.</p> |
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32 | |
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33 | <p>All three drivers make ample use of sophisticated digital signal |
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34 | processing algorithms designed to efficiently extract timing |
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35 | signals from noise and interference. The radio station drivers in |
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36 | particular implement optimum linear demodulation and decoding |
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37 | techniques, including maximum likelihood and soft-decision methods. |
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38 | The documentation page for each driver contains an in-depth |
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39 | discussion on the algorithms and performance expectations. In some |
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40 | cases the algorithms are further analyzed, modelled and evaluated |
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41 | in a technical report.</p> |
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42 | |
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43 | <p>Currently, the audio drivers are compatible with Sun operating |
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44 | systems, including Solaris and SunOS, and the native audio codec |
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45 | interface supported by these systems. In fact, the interface is |
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46 | quite generic and support for other systems, in particular the |
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47 | various Unix generics, should not be difficult. Volunteers are |
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48 | solicited.</p> |
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49 | |
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50 | <p>The audio drivers include a number of common features designed |
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51 | to groom input signals, suppress spikes and normalize signal |
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52 | levels. An automatic gain control (AGC) feature provides protection |
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53 | against overdriven or underdriven input signals. It is designed to |
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54 | maintain adequate demodulator signal amplitude while avoiding |
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55 | occasional noise spikes. In order to assure reliable operation, the |
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56 | signal level must be in the range where the audio gain control is |
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57 | effective. In general, this means the input signal level must be |
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58 | such as to cause the AGC to set the gain somewhere in the middle of |
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59 | the range from 0 to 255, as indicated in the timecode displayed by |
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60 | the <tt>ntpq</tt> program.</p> |
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61 | |
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62 | <p>The drivers operate by disciplining a logical clock based on the |
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63 | codec sample clock to the audio signal as received. This is done by |
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64 | stuffing or slipping samples as required to maintain exact |
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65 | frequency to the order of 0.1 PPM. In order for the driver to |
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66 | reliably lock on the audio signal, the sample clock frequency |
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67 | tolerance must be less than 250 PPM (.025 percent) for the IRIG |
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68 | driver and half that for the radio drivers. The largest error |
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69 | observed so far is about 60 PPM, but it is possible some sound |
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70 | cards or codecs may exceed that value.</p> |
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71 | |
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72 | <p>The drivers include provisions to select the input port and to |
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73 | monitor the input signal. The <tt>fudge flag 2</tt> selects the |
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74 | microphone port if set to zero or the line-in port if set to one. |
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75 | It does not seem useful to specify the compact disc player port. |
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76 | The <tt>fudge flag 3</tt> enables the input signal monitor using |
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77 | the previously selected output port and output gain. Both of these |
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78 | flags can be set in the configuration file or remotely using the |
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79 | <tt>ntpdc</tt> utility program.</p> |
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80 | |
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81 | <h4>Shortwave Radio Drivers</h4> |
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82 | |
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83 | <p>The WWV/H and CHU audio drivers require an external shortwave |
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84 | radio with the radio output - speaker or headphone jack - connected |
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85 | to either the microphone or line-in port on the computer. There is |
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86 | some degree of art in setting up the radio and antenna and getting |
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87 | the setup to work. While the drivers are highly sophisticated and |
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88 | efficient in extracting timing signals from noise and interference, |
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89 | it always helps to have as clear a signal as possible.</p> |
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90 | |
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91 | <p>The most important factor affecting the radio signal is the |
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92 | antenna. It need not be long - even 15 feet is enough if it is |
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93 | located outside of a metal frame building, preferably on the roof, |
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94 | and away from metallic objects. An ordinary CB whip mounted on a |
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95 | PVC pipe and wooden X-frame on the roof should work well with most |
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96 | portable radios, as they are optimized for small antennas.</p> |
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97 | |
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98 | <p>The radio need not be located near the computer; in fact, it |
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99 | generally works better if the radio is outside the near field of |
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100 | computers and other electromagnetic noisemakers. It can be in the |
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101 | elevator penthouse connected by house wiring, which can also be |
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102 | used to power the radio. A couple of center-tapped audio |
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103 | transformers will minimize noise pickup and provide phantom power |
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104 | to the radio with return via the AC neutral wire.</p> |
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105 | |
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106 | <p>The WWV/H and CHU transmitters operate on several frequencies |
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107 | simultaneously, so that in most parts of North America at least one |
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108 | frequency supports propagation to the receiver location at any |
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109 | given hour. While both drivers support the ICOM CI-V radio |
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110 | interface and can tune the radio automatically, computer-tunable |
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111 | radios are expensive and probably not cost effective compared to a |
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112 | GPS receiver. So, the radio frequency must usually be fixed and |
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113 | chosen by compromise.</p> |
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114 | |
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115 | <p>Shortwave (3-30 MHz) radio propagation phenomena are well known |
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116 | to shortwave enthusiasts. The phenomena generally obey the |
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117 | following rules:</p> |
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118 | |
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119 | <ul> |
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120 | <li>The optimum frequency is higher in daytime than nighttime, |
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121 | stays high longer on summer days and low longer on winter |
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122 | nights.</li> |
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123 | |
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124 | <li>Transitions between daytime and nightime conditions generally |
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125 | occur somewhat after sunrise and sunset at the midpoint of the path |
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126 | from transmitter to receiver.</li> |
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127 | |
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128 | <li>Ambient noise (static) on the lower frequencies follows the |
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129 | thunderstorm season, so is higher on summer afternoons and |
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130 | evenings.</li> |
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131 | |
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132 | <li>The lower frequency bands are best for shorter distances, while |
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133 | the higher bands are best for longer distances.</li> |
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134 | |
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135 | <li>The optimum frequencies are higher at the peak of the 11-year |
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136 | sunspot cycle and lower at the trough. The current sunspot cycle |
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137 | should peak in the first couple of years beginning the |
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138 | century.</li> |
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139 | </ul> |
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140 | |
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141 | The best way to choose a frequency is to listen at various times |
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142 | over the day and determine the best highest (daytime) and lowest |
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143 | (nighttime) frequencies. Then, assuming one is available, choose |
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144 | the highest frequency between these frequencies. This strategy |
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145 | assumes that the high frequency is more problematic than the low, |
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146 | that the low frequency probably comes with severe multipath and |
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147 | static, and insures that probably twice a day the chosen frequency |
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148 | will work. For instance, on the east coast the best compromise CHU |
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149 | frequency is probably 7335 kHz and the best WWV frequency is |
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150 | probably 15 MHz. |
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151 | |
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152 | <h4>Debugging Aids</h4> |
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153 | |
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154 | <p>The audio drivers include extensive debugging support to help |
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155 | hook up the audio signals and monitor the driver operations. The |
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156 | documentation page for each driver describes the various messages |
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157 | that can be produced either in real-time or written to the <tt> |
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158 | clockstats</tt> file for later analysis. Of particular help in |
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159 | verifying signal connections and compatibility is a provision to |
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160 | monitor the signal via headphones or speaker.</p> |
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161 | |
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162 | <p>The drivers write a synthesized timecode to the <tt> |
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163 | clockstats</tt> file each time the clock is set or verified and at |
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164 | other times if verbose monitoring is enabled. The format includes |
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165 | several fixed-length fields defining the Gregorian time to the |
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166 | millisecond, together with additional variable-length fields |
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167 | specific to each driver. The data include the intervals since the |
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168 | clock was last set or verified, the audio gain and various state |
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169 | variables and counters specific to each driver.</p> |
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170 | |
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171 | <h4>Additional Information</h4> |
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172 | |
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173 | <a href="refclock.htm">Reference Clock Drivers</a> <br> |
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174 | <a href="driver7.htm">Radio CHU Audio Demodulator/Decoder</a> <br> |
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175 | <a href="driver36.htm">Radio WWV/H Audio Demodulator/Decoder</a> |
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176 | <br> |
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177 | <a href="driver6.htm">IRIG Audio Decoder</a> |
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178 | |
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179 | <hr> |
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180 | <a href="index.htm"><img align="left" src="pic/home.gif" alt= |
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181 | "gif"></a> |
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182 | |
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183 | <address><a href="mailto:mills@udel.edu">David L. Mills |
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184 | <mills@udel.edu></a></address> |
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185 | </body> |
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186 | </html> |
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187 | |
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