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diff --git a/contrib/ntp/html/driver36.htm b/contrib/ntp/html/driver36.htm deleted file mode 100644 index 2c74646..0000000 --- a/contrib/ntp/html/driver36.htm +++ /dev/null @@ -1,930 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"> -<html> -<head> -<meta name="generator" content="HTML Tidy, see www.w3.org"> -<title>Radio WWV/H Audio Demodulator/Decoder</title> -</head> -<body> -<h3>Radio WWV/H Audio Demodulator/Decoder</h3> - -<hr> -<h4>Synopsis</h4> - -Address: 127.127.36.<i>u</i> <br> -Reference ID: <tt>WWV</tt> or <tt>WWVH</tt> <br> -Driver ID: <tt>WWV_AUDIO</tt> <br> -Autotune Port: <tt>/dev/icom</tt>; 1200/9600 baud, 8-bits, no -parity <br> -Audio Device: <tt>/dev/audio</tt> and <tt>/dev/audioctl</tt> - -<h4>Description</h4> - -This driver synchronizes the computer time using data encoded in -shortwave radio transmissions from NIST time/frequency stations WWV -in Ft. Collins, CO, and WWVH in Kauai, HI. Transmissions are made -continuously on 2.5, 5, 10, 15 and 20 MHz. An ordinary shortwave -receiver can be tuned manually to one of these frequencies or, in -the case of ICOM receivers, the receiver can be tuned automatically -by the driver as propagation conditions change throughout the day -and night. The performance of this driver when tracking one of the -stations is ordinarily better than 1 ms in time with frequency -drift less than 0.5 PPM when not tracking either station. - -<p>The demodulation and decoding algorithms used by this driver are -based on a machine language program developed for the TAPR DSP93 -DSP unit, which uses the TI 320C25 DSP chip. The analysis, design -and performance of the program running on this unit is described -in: Mills, D.L. A precision radio clock for WWV transmissions. -Electrical Engineering Report 97-8-1, University of Delaware, -August 1997, 25 pp. Available from <a href= -"http://www.eecis.udel.edu/~mills/reports.htm"> -www.eecis.udel.edu/~mills/reports.htm</a>. For use in this driver, -the original program was rebuilt in the C language and adapted to -the NTP driver interface. The algorithms have been modified -somewhat to improve performance under weak signal conditions and to -provide an automatic station identification feature.</p> - -<p>This driver incorporates several features in common with other -audio drivers such as described in the <a href="driver7.htm">Radio -CHU Audio Demodulator/Decoder</a> and the <a href="driver6.htm"> -IRIG Audio Decoder</a> pages. They include automatic gain control -(AGC), selectable audio codec port and signal monitoring -capabilities. For a discussion of these common features, as well as -a guide to hookup, debugging and monitoring, see the <a href= -"audio.htm">Reference Clock Audio Drivers</a> page.</p> - -<p>The WWV signal format is described in NIST Special Publication -432 (Revised 1990). It consists of three elements, a 5-ms, 1000-Hz -pulse, which occurs at the beginning of each second, a 800-ms, -1000-Hz pulse, which occurs at the beginning of each minute, and a -pulse-width modulated 100-Hz subcarrier for the data bits, one bit -per second. The WWVH format is identical, except that the 1000-Hz -pulses are sent at 1200 Hz. Each minute encodes nine BCD digits for -the time of century plus seven bits for the daylight savings time -(DST) indicator, leap warning indicator and DUT1 correction.</p> - -<h4>Program Architecture</h4> - -<p>As in the original program, the clock discipline is modelled as -a Markov process, with probabilistic state transitions -corresponding to a conventional clock and the probabilities of -received decimal digits. The result is a performance level which -results in very high accuracy and reliability, even under -conditions when the minute beep of the signal, normally its most -prominent feature, can barely be detected by ear with a shortwave -receiver.</p> - -<p>The analog audio signal from the shortwave radio is sampled at -8000 Hz and converted to digital representation. The 1000/1200-Hz -pulses and 100-Hz subcarrier are first separated using two IIR -filters, a 600-Hz bandpass filter centered on 1100 Hz and a 150-Hz -lowpass filter. The minute sync pulse is extracted using a 800-ms -synchronous matched filter and pulse grooming logic which -discriminates between WWV and WWVH signals and noise. The second -sync pulse is extracted using a 5-ms FIR matched filter and -8000-stage comb filter.</p> - -<p>The phase of the 100-Hz subcarrier relative to the second sync -pulse is fixed at the transmitter; however, the audio highpass -filter in most radios affects the phase response at 100 Hz in -unpredictable ways. The driver adjusts for each radio using two -170-ms synchronous matched filters. The I (in-phase) filter is used -to demodulate the subcarrier envelope, while the Q -(quadrature-phase) filter is used in a tracking loop to discipline -the codec sample clock and thus the demodulator phase.</p> - -<p>The data bit probabilities are determined from the subcarrier -envelope using a threshold-corrected slicer. The averaged envelope -amplitude 30 ms from the beginning of the second establishes the -minimum (noise floor) value, while the amplitude 200 ms from the -beginning establishes the maximum (signal peak) value. The slice -level is midway between these two values. The negative-going -envelope transition at the slice level establishes the length of -the data pulse, which in turn establish probabilities for binary -zero (P0) or binary one (P1). The values are established by linear -interpolation between the pulse lengths for P0 (300 ms) and P1 (500 -ms) so that the sum is equal to one. If the driver has not -synchronized to the minute pulse, or if the data bit amplitude, -signal/noise ratio (SNR) or length are below thresholds, the bit is -considered invalid and all three probabilities are set to zero.</p> - -<p>The difference between the P1 and P0 probabilities, or -likelihood, for each data bit is exponentially averaged in a set of -60 accumulators, one for each second, to determine the semi-static -miscellaneous bits, such as DST indicator, leap second warning and -DUT1 correction. In this design, an average value larger than a -positive threshold is interpreted as a hit on one and a value -smaller than a negative threshold as a hit on zero. Values between -the two thresholds, which can occur due to signal fades or loss of -signal, are interpreted as a miss, and result in no change of -indication.</p> - -<p>The BCD digit in each digit position of the timecode is -represented as four data bits, all of which must be valid for the -digit itself to be considered valid. If so, the bits are correlated -with the bits corresponding to each of the valid decimal digits in -this position. If the digit is invalid, the correlated value for -all digits in this position is assumed zero. In either case, the -values for all digits are exponentially averaged in a likelihood -vector associated with this position. The digit associated with the -maximum over all of the averaged values then becomes the maximum -likelihood selection for this position and the ratio of the maximum -over the next lower value becomes the likelihood ratio.</p> - -<p>The decoding matrix contains nine row vectors, one for each -digit position. Each row vector includes the maximum likelihood -digit, likelihood vector and other related data. The maximum -likelihood digit for each of the nine digit positions becomes the -maximum likelihood time of the century. A built-in transition -function implements a conventional clock with decimal digits that -count the minutes, hours, days and years, as corrected for leap -seconds and leap years. The counting operation also rotates the -likelihood vector corresponding to each digit as it advances. Thus, -once the clock is set, each clock digit should correspond to the -maximum likelihood digit as transmitted.</p> - -<p>Each row of the decoding matrix also includes a compare counter -and the difference (modulo the radix) between the current clock -digit and most recently determined maximum likelihood digit. If a -digit likelihood exceeds the decision level and the difference is -constant for a number of successive minutes in any row, the maximum -likelihood digit replaces the clock digit in that row. When this -condition is true for all rows and the second epoch has been -reliably determined, the clock is set (or verified if it has -already been set) and delivers correct time to the integral second. -The fraction within the second is derived from the logical master -clock, which runs at 8000 Hz and drives all system timing -functions.</p> - -<p>The logical master clock is derived from the audio codec clock. -Its frequency is disciplined by a frequency-lock loop (FLL) which -operates independently of the data recovery functions. At averaging -intervals determined by the measured jitter, the frequency error is -calculated as the difference between the most recent and the -current second epoch divided by the interval. The sample clock -frequency is then corrected by this amount using an exponential -average. When first started, the frequency averaging interval is -eight seconds, in order to compensate for intrinsic codec clock -frequency offsets up to 125 PPM. Under most conditions, the -averaging interval doubles in stages from the initial value to over -1000 seconds, which results in an ultimate frequency precision of -0.125 PPM, or about 11 ms/day.</p> - -<p>It is important that the logical clock frequency is stable and -accurately determined, since in most applications the shortwave -radio will be tuned to a fixed frequency where WWV or WWVH signals -are not available throughout the day. In addition, in some parts of -the US, especially on the west coast, signals from either or both -WWV and WWVH may be available at different times or even at the -same time. Since the propagation times from either station are -almost always different, each station must be reliably identified -before attempting to set the clock.</p> - -<p>Station identification uses the 800-ms minute pulse transmitted -by each station. In the acquisition phase the entire minute is -searched using both the WWV and WWVH using matched filters and a -pulse gate discriminator similar to that found in radar acquisition -and tracking receivers. The peak amplitude found determines a range -gate and window where the next pulse is expected to be found. The -minute is scanned again to verify the peak is indeed in the window -and with acceptable amplitude, SNR and jitter. At this point the -receiver begins to track the second sync pulse and operate as above -until the clock is set.</p> - -<p>Once the minute is synchronized, the range gate is fixed and -only energy within the window is considered for the minute sync -pulse. A compare counter increments by one if the minute pulse has -acceptable amplitude, SNR and jitter and decrements otherwise. This -is used as a quality indicator and reported in the timecode and -also for the autotune function described below.</p> - -<h4>Performance</h4> - -<p>It is the intent of the design that the accuracy and stability -of the indicated time be limited only by the characteristics of the -propagation medium. Conventional wisdom is that synchronization via -the HF medium is good only to a millisecond under the best -propagation conditions. The performance of the NTP daemon -disciplined by the driver is clearly better than this, even under -marginal conditions. Ordinarily, with marginal to good signals and -a frequency averaging interval of 1024 s, the frequency is -stabilized within 0.1 PPM and the time within 125 <font face= -"Symbol">m</font>s. The frequency stability characteristic is -highly important, since the clock may have to free-run for several -hours before reacquiring the WWV/H signal.</p> - -<p>The expected accuracy over a typical day was determined using -the DSP93 and an oscilloscope and cesium oscillator calibrated with -a GPS receiver. With marginal signals and allowing 15 minutes for -initial synchronization and frequency compensation, the time -accuracy determined from the WWV/H second sync pulse was reliably -within 125 <font face="Symbol">m</font>s. In the particular DSP-93 -used for program development, the uncorrected CPU clock frequency -offset was 45.8±0.1 PPM. Over the first hour after initial -synchronization, the clock frequency drifted about 1 PPM as the -frequency averaging interval increased to the maximum 1024 s. Once -reaching the maximum, the frequency wandered over the day up to 1 -PPM, but it is not clear whether this is due to the stability of -the DSP-93 clock oscillator or the changing height of the -ionosphere. Once the frequency had stabilized and after loss of the -WWV/H signal, the frequency drift was less than 0.5 PPM, which is -equivalent to 1.8 ms/h or 43 ms/d. This resulted in a step phase -correction up to several milliseconds when the signal returned.</p> - -<p>The measured propagation delay from the WWV transmitter at -Boulder, CO, to the receiver at Newark, DE, is 23.5±0.1 ms. -This is measured to the peak of the pulse after the second sync -comb filter and includes components due to the ionospheric -propagation delay, nominally 8.9 ms, communications receiver delay -and program delay. The propagation delay can be expected to change -about 0.2 ms over the day, as the result of changing ionosphere -height. The DSP93 program delay was measured at 5.5 ms, most of -which is due to the 400-Hz bandpass filter and 5-ms matched filter. -Similar delays can be expected of this driver.</p> - -<h4>Program Operation</h4> - -The driver begins operation immediately upon startup. It first -searches for one or both of the stations WWV and WWVH and attempts -to acquire minute sync. This may take some fits and starts, as the -driver expects to see three consecutive minutes with good signals -and low jitter. If the autotune function is active, the driver will -rotate over all five frequencies and both WWV and WWVH stations -until three good minutes are found. - -<p>The driver then acquires second sync, which can take up to -several minutes, depending on signal quality. At the same time the -driver accumulates likelihood values for each of the nine digits of -the clock, plus the seven miscellaneous bits included in the WWV/H -transmission format. The minute units digit is decoded first and, -when five repetitions have compared correctly, the remaining eight -digits are decoded. When five repetitions of all nine digits have -decoded correctly, which normally takes 15 minutes with good -signals and up to an hour when buried in noise, and the second sync -alarm has not been raised for two minutes, the clock is set (or -verified) and is selectable to discipline the system clock.</p> - -<p>As long as the clock is set or verified, the system clock -offsets are provided once each second to the reference clock -interface, where they are saved in a buffer. At the end of each -minute, the buffer samples are groomed by the median filter and -trimmed-mean averaging functions. Using these functions, the system -clock can in principle be disciplined to a much finer resolution -than the 125-<font face="Symbol">m</font>s sample interval would -suggest, although the ultimate accuracy is probably limited by -propagation delay variations as the ionspheric height varies -throughout the day and night.</p> - -<p>As long as signals are available, the clock frequency is -disciplined for use during times when the signals are unavailable. -The algorithm refines the frequency offset using increasingly -longer averaging intervals to 1024 s, where the precision is about -0.1 PPM. With good signals, it takes well over two hours to reach -this degree of precision; however, it can take many more hours than -this in case of marginal signals. Once reaching the limit, the -algorithm will follow frequency variations due to temperature -fluctuations and ionospheric height variations.</p> - -<p>It may happen as the hours progress around the clock that WWV -and WWVH signals may appear alone, together or not at all. When the -driver is first started, the NTP reference identifier appears as -<tt>NONE</tt>. When the driver has acquired one or both stations -and mitigated which one is best, it sets the station identifier in -the timecode as described below. In addition, the NTP reference -identifier is set to the station callsign. If the propagation -delays has been properly set with the <tt>fudge time1</tt> (WWV) -and <tt>fudge time2</tt> (WWVH) commands in the configuration file, -handover from one station to the other will be seamless.</p> - -<p>Once the clock has been set for the first time, it will appear -reachable and selectable to discipline the system clock, even if -the broadcast signal fades to obscurity. A consequence of this -design is that, once the clock is set, the time and frequency are -disciplined only by the second sync pulse and the clock digits -themselves are driven by the clock state machine and ordinarily -never changed. However, as long as the clock is set correctly, it -will continue to read correctly after a period of signal loss, as -long as it does not drift more than 500 ms from the correct time. -Assuming the clock frequency can be disciplined within 1 PPM, the -clock could coast without signals for some 5.8 days without -exceeding that limit. If for some reason this did happen, the clock -would be in the wrong second and would never resynchronize. To -protect against this most unlikely situation, if after four days -with no signals, the clock is considered unset and resumes the -synchronization procedure from the beginning.</p> - -<p>To work well, the driver needs a communications receiver with -good audio response at 100 Hz. Most shortwave and communications -receivers roll off the audio response below 250 Hz, so this can be -a problem, especially with receivers using DSP technology, since -DSP filters can have very fast rolloff outside the passband. Some -DSP transceivers, in particular the ICOM 775, have a programmable -low frequency cutoff which can be set as low as 80 Hz. However, -this particular radio has a strong low frequency buzz at about 10 -Hz which appears in the audio output and can affect data recovery -under marginal conditions. Although not tested, it would seem very -likely that a cheap shortwave receiver could function just as well -as an expensive communications receiver.</p> - -<h4>Autotune</h4> - -<p>The driver includes provisions to automatically tune the radio -in response to changing radio propagation conditions throughout the -day and night. The radio interface is compatible with the ICOM CI-V -standard, which is a bidirectional serial bus operating at TTL -levels. The bus can be connected to a serial port using a level -converter such as the CT-17. The serial port speed is presently -compiled in the program, but can be changed in the driver source -file.</p> - -<p>Each ICOM radio is assigned a unique 8-bit ID select code, -usually expressed in hex format. To activate the CI-V interface, -the <tt>mode</tt> keyword of the <tt>server</tt> configuration -command specifies a nonzero select code in decimal format. A table -of ID select codes for the known ICOM radios is given below. Since -all ICOM select codes are less than 128, the high order bit of the -code is used by the driver to specify the baud rate. If this bit is -not set, the rate is 9600 bps for the newer radios; if set, the -rate is 1200 bps for the older radios. A missing <tt>mode</tt> -keyword or a zero argument leaves the interface disabled.</p> - -<p>If specified, the driver will attempt to open the device <tt> -/dev/icom</tt> and, if successful will activate the autotune -function and tune the radio to each operating frequency in turn -while attempting to acquire minute sync from either WWV or WWVH. -However, the driver is liberal in what it assumes of the -configuration. If the <tt>/dev/icom</tt> link is not present or the -open fails or the CI-V bus or radio is inoperative, the driver -quietly gives up with no harm done.</p> - -<p>Once acquiring minute sync, the driver operates as described -above to set the clock. However, during seconds 59, 0 and 1 of each -minute it tunes the radio to one of the five broadcast frequencies -to measure the sync pulse and data pulse amplitudes and SNR and -update the compare counter. Each of the five frequencies are probed -in a five-minute rotation to build a database of current -propagation conditions for all signals that can be heard at the -time. At the end of each rotation, a mitigation procedure scans the -database and retunes the radio to the best frequency and station -found. For this to work well, the radio should be set for a fast -AGC recovery time. This is most important while tracking a strong -signal, which is normally the case, and then probing another -frequency, which may have much weaker signals.</p> - -<p>Reception conditions for each frequency and station are -evaluated according to a metric which considers the minute sync -pulse amplitude, SNR and jitter, as well as, the data pulse -amplitude and SNR. The minute pulse is evaluated at second 0, while -the data pulses are evaluated at seconds 59 and 1. The results are -summarized in a scoreboard of three bits</p> - -<dl> -<dt><tt>0x0001</tt></dt> - -<dd>Jitter exceeded. The difference in epoches between the last -minute sync pulse and the current one exceeds 50 ms (400 -samples).</dd> - -<dt><tt>0x0002</tt></dt> - -<dd>Minute pulse error. For the minute sync pulse in second 0, -either the amplitude or SNR is below threshold (2000 and 20 dB, -respectively).</dd> - -<dt><tt>0x0004</tt></dt> - -<dd>Minute pulse error. For both of the data pulses in seocnds 59 -and 1, either the amplitude or SNR is below threshold (1000 and 10 -dB, respectively).</dd> -</dl> - -<p>If none of the scoreboard bits are set, the compare counter is -increased by one to a maximum of six. If any bits are set, the -counter is decreased by one to a minimum of zero. At the end of -each minute, the frequency and station with the maximum compare -count is chosen, with ties going to the highest frequency.</p> - -<h4>Diagnostics</h4> - -<p>The autotune process produces diagnostic information along with -the timecode. This is very useful for evaluating the performance of -the algorithm, as well as radio propagation conditions in general. -The message is produced once each minute for each frequency in turn -after minute sync has been acquired.</p> - -<p><tt>wwv5 port agc wwv wwvh</tt></p> - -<p>where <tt>port</tt> and <tt>agc</tt> are the audio port and -gain, respectively, for this frequency and <tt>wwv</tt> and <tt> -wwvh</tt> are two sets of fields, one each for WWV and WWVH. Each -of the two fields has the format</p> - -<p><tt>ident score comp sync/snr/jitr</tt></p> - -<p>where <tt>ident</tt>encodes the station (<tt>C</tt> for WWV, -<tt>H</tt> for WWVH) and frequency (2, 5, 10, 15 and 20), <tt> -score</tt> is the scoreboard described above, <tt>comp</tt> is the -compare counter, <tt>sync</tt> is the minute sync pulse amplitude, -<tt>snr</tt> the SNR of the pulse and <tt>jitr</tt> is the sample -difference between the current epoch and the last epoch. An example -is:</p> - -<p><tt>wwv5 2 111 C20 0100 6 8348/30.0/-3 H20 0203 0 -22/-12.4/8846</tt></p> - -<p>Here the radio is tuned to 20 MHz and the line-in port AGC is -currently 111 at that frequency. The message contains a report for -WWV (<tt>C20</tt>) and WWVH (<tt>H20</tt>). The WWV report -scoreboard is 0100 and the compare count is 6, which suggests very -good reception conditions, and the minute sync amplitude and SNR -are well above thresholds (2000 and 20 dB, respectively). Probably -the most sensitive indicator of reception quality is the jitter, -3 -samples, which is well below threshold (50 ms or 400 samples). -While the message shows solid reception conditions from WWV, this -is not the case for WWVH. Both the minute sync amplitude and SNR -are below thresholds and the jitter is above threshold.</p> - -<p>A sequence of five messages, one for each minute, might appear -as follows:</p> - -<pre> -wwv5 2 95 C2 0107 0 164/7.2/8100 H2 0207 0 80/-5.5/7754 -wwv5 2 99 C5 0104 0 3995/21.8/395 H5 0207 0 27/-9.3/18826 -wwv5 2 239 C10 0105 0 9994/30.0/2663 H10 0207 0 54/-16.1/-529 -wwv5 2 155 C15 0103 3 3300/17.8/-1962 H15 0203 0 236/17.0/4873 -wwv5 2 111 C20 0100 6 8348/30.0/-3 H20 0203 0 22/-12.4/8846 -</pre> - -<p>Clearly, the only frequencies that are available are 15 MHz and -20 MHz and propagation may be failing for 15 MHz. However, minute -sync pulses are being heard on 5 and 10 MHz, even though the data -pulses are not. This is typical of late afternoon when the maximum -usable frequency (MUF) is falling and the ionospheric loss at the -lower frequencies is beginning to decrease.</p> - -<h4>Debugging Aids</h4> - -<p>The most convenient way to track the driver status is using the -<tt>ntpq</tt> program and the <tt>clockvar</tt> command. This -displays the last determined timecode and related status and error -counters, even when the driver is not discipline the system clock. -If the debugging trace feature (<tt>-d</tt> on the <tt>ntpd</tt> -command line)is enabled, the driver produces detailed status -messages as it operates. If the <tt>fudge flag 4</tt> is set, these -messages are written to the <tt>clockstats</tt> file. All messages -produced by this driver have the prefix <tt>chu</tt> for convenient -filtering with the Unix <tt>grep</tt> command.</p> - -<p>In the following descriptions the units of amplitude, phase, -probability and likelihood are normalized to the range 0-6000 for -convenience. In addition, the signal/noise ratio (SNR) and -likelihood ratio are measured in decibels and the words with bit -fields are in hex. Most messages begin with a leader in the -following format:</p> - -<p><tt>wwvn ss stat sigl</tt></p> - -<p>where <tt>wwvn</tt> is the message code, <tt>ss</tt> the second -of minute, <tt>stat</tt> the driver status word and <tt>sigl</tt> -the second sync pulse amplitude. A full explanation of the status -bits is contained in the driver source listing; however, the -following are the most useful for debugging.</p> - -<dl> -<dt><tt>0x0001</tt></dt> - -<dd>Minute sync. Set when the decoder has identified a station and -acquired the minute sync pulse.</dd> - -<dt><tt>0x0002</tt></dt> - -<dd>Second sync. Set when the decoder has acquired the second sync -pulse and within 125 <font face="Symbol">m</font>s of the correct -phase.</dd> - -<dt><tt>0x0004</tt></dt> - -<dd>Minute unit sync. Set when the decoder has reliably determined -the unit digit of the minute.</dd> - -<dt><tt>0x0008</tt></dt> - -<dd>Clock set. Set when the decoder has reliably determined all -nine digits of the timecode and is selectable to discipline the -system clock.</dd> -</dl> - -<p>With debugging enabled the driver produces messages in the -following formats:</p> - -<p>Format <tt>wwv8</tt> messages are produced once per minute by -the WWV and WWVH station processes before minute sync has been -acquired. They show the progress of identifying and tracking the -minute pulse of each station.</p> - -<p><tt>wwv8 port agc ident comp ampl snr epoch jitr offs</tt></p> - -<p>where <tt>port</tt> and <tt>agc</tt> are the audio port and -gain, respectively. The <tt>ident</tt>encodes the station -(<tt>C</tt> for WWV, <tt>H</tt> for WWVH) and frequency (2, 5, 10, -15 and 20). For the encoded frequency, <tt>comp</tt> is the compare -counter, <tt>ampl</tt> the pulse amplitude, <tt>snr</tt> the SNR, -<tt>epoch</tt> the sample number of the minute pulse in the minute, -<tt>jitr</tt> the change since the last <tt>epoch</tt> and <tt> -offs</tt> the minute pulse offset relative to the second pulse. An -example is:</p> - -<p><tt>wwv8 2 127 C15 2 9247 30.0 18843 -1 1</tt><br> -<tt>wwv8 2 127 H15 0 134 -2.9 19016 193 174</tt></p> - -<p>Here the radio is tuned to 15 MHz and the line-in port AGC is -currently 127 at that frequency. The driver has not yet acquired -minute sync, WWV has been heard for at least two minutes, and WWVH -is in the noise. The WWV minute pulse amplitude and SNR are well -above the threshold (2000 and 6 dB, respectively) and the minute -epoch has been determined -1 sample relative to the last one and 1 -sample relative to the second sync pulse. The compare counter has -incrmented to two; when it gets to three, minute sync has been -acquired.</p> - -<p>Format <tt>wwv3</tt> messages are produced after minute sync has -been acquired and until the seconds unit digit is determined. They -show the results of decoding each bit of the transmitted -timecode.</p> - -<p><tt>wwv3 ss stat sigl ampl phas snr prob like</tt></p> - -<p>where <tt>ss</tt>, <tt>stat</tt> and <tt>sigl</tt> are as above, -<tt>ampl</tt> is the subcarrier amplitude, <tt>phas</tt> the -subcarrier phase, <tt>snr</tt> the subcarrier SNR, <tt>prob</tt> -the bit probability and <tt>like</tt> the bit likelihood. An -example is:</p> - -<p><tt>wwv3 28 0123 4122 4286 0 24.8 -5545 -1735</tt></p> - -<p>Here the driver has acquired minute and second sync, but has not -yet determined the seconds unit digit. However, it has just decoded -bit 28 of the minute. The results show the second sync pulse -amplitude well over the threshold (500), subcarrier amplitude well -above the threshold (1000), good subcarrier tracking phase and SNR -well above the threshold (10 dB). The bit is almost certainly a -zero and the likelihood of a zero in this second is very high.</p> - -<p>Format <tt>wwv4</tt> messages are produced for each of the nine -BCD timecode digits until the clock has been set or verified. They -show the results of decoding each digit of the transmitted -timecode.</p> - -<p><tt>wwv4 ss stat sigl radx ckdig mldig diff cnt like -snr</tt></p> - -<p>where <tt>ss</tt>, <tt>stat</tt> and <tt>sigl</tt> are as above, -<tt>radx</tt> is the digit radix (3, 4, 6, 10), <tt>ckdig</tt> the -current clock digit, <tt>mldig</tt> the maximum likelihood digit, -<tt>diff</tt> the difference between these two digits modulo the -radix, <tt>cnt</tt> the compare counter, <tt>like</tt> the digit -likelihood and <tt>snr</tt> the likelihood ratio. An example -is:</p> - -<p><tt>wwv4 8 010f 5772 10 9 9 0 6 4615 6.1</tt></p> - -<p>Here the driver has previousl set or verified the clock. It has -just decoded the digit preceding second 8 of the minute. The digit -radix is 10, the current clock and maximum likelihood digits are -both 9, the likelihood is well above the threshold (1000) and the -likelihood function well above threshold (3.0 dB). Short of a -hugely unlikely probability conspiracy, the clock digit is most -certainly a 9.</p> - -<p>Format <tt>wwv2</tt> messages are produced at each master -oscillator frequency update, which starts at 8 s, but eventually -climbs to 1024 s. They show the progress of the algorithm as it -refines the frequency measurement to a precision of 0.1 PPM.</p> - -<p><tt>wwv2 ss stat sigl avint avcnt avinc jitr delt freq</tt></p> - -<p>where <tt>ss</tt>, <tt>stat</tt> and <tt>sigl</tt> are as above, -<tt>avint</tt> is the averaging interval, <tt>avcnt</tt> the -averaging interval counter, <tt>avinc</tt> the interval increment, -<tt>jitr</tt> the sample change between the beginning and end of -the interval, <tt>delt</tt> the computed frequency change and <tt> -freq</tt> the current frequency (PPM). An example is:</p> - -<p><tt>wwv2 22 030f 5795 256 256 4 0 0.0 66.7</tt></p> - -<p>Here the driver has acquired minute and second sync and set the -clock. The averaging interval has increased to 256 s on the way to -1024 s, has stayed at that interval for 4 averaging intervals, has -measured no change in frequency and the current frequency is 66.7 -PPM.</p> - -<p>If the CI-V interface for ICOM radios is active, a debug level -greater than 1 will produce a trace of the CI-V command and -response messages. Interpretation of these messages requires -knowledge of the CI-V protocol, which is beyond the scope of this -document.</p> - -<h4>Monitor Data</h4> - -When enabled by the <tt>filegen</tt> facility, every received -timecode is written to the <tt>clockstats</tt> file in the -following format: - -<pre> - sq yy ddd hh:mm:ss.fff ld du lset agc stn rfrq errs freq cons - - s sync indicator - q quality character - yyyy Gregorian year - ddd day of year - hh hour of day - mm minute of hour - fff millisecond of second - l leap second warning - d DST state - dut DUT sign and magnitude - lset minutes since last set - agc audio gain - ident station identifier and frequency - comp minute sync compare counter - errs bit error counter - freq frequency offset - avgt averaging time -</pre> - -The fields beginning with <tt>year</tt> and extending through <tt> -dut</tt> are decoded from the received data and are in fixed-length -format. The <tt>agc</tt> and <tt>lset</tt> fields, as well as the -following driver-dependent fields, are in variable-length format. - -<dl> -<dt><tt>s</tt></dt> - -<dd>The sync indicator is initially <tt>?</tt> before the clock is -set, but turns to space when all nine digits of the timecode are -correctly set.</dd> - -<dt><tt>q</tt></dt> - -<dd>The quality character is a four-bit hexadecimal code showing -which alarms have been raised. Each bit is associated with a -specific alarm condition according to the following: - -<dl> -<dt><tt>0x8</tt></dt> - -<dd>Sync alarm. The decoder may not be in correct second or minute -phase relative to the transmitter.</dd> - -<dt><tt>0x4</tt></dt> - -<dd>Error alarm. More than 30 data bit errors occurred in the last -minute.</dd> - -<dt><tt>0x2</tt></dt> - -<dd>Symbol alarm. The probability of correct decoding for a digit -or miscellaneous bit has fallen below the threshold.</dd> - -<dt><tt>0x1</tt></dt> - -<dd>Decoding alarm. A maximum likelihood digit fails to agree with -the current associated clock digit.</dd> -</dl> - -It is important to note that one or more of the above alarms does -not necessarily indicate a clock error, but only that the decoder -has detected a condition that may in future result in an -error.</dd> - -<dt><tt>yyyy ddd hh:mm:ss.fff</tt></dt> - -<dd>The timecode format itself is self explanatory. Since the -driver latches the on-time epoch directly from the second sync -pulse, the fraction <tt>fff</tt>is always zero. Although the -transmitted timecode includes only the year of century, the -Gregorian year is augmented 2000 if the indicated year is less than -72 and 1900 otherwise.</dd> - -<dt><tt>l</tt></dt> - -<dd>The leap second warning is normally space, but changes to <tt> -L</tt> if a leap second is to occur at the end of the month of June -or December.</dd> - -<dt><tt>d</tt></dt> - -<dd>The DST state is <tt>S</tt> or <tt>D</tt> when standard time or -daylight time is in effect, respectively. The state is <tt>I</tt> -or <tt>O</tt> when daylight time is about to go into effect or out -of effect, respectively.</dd> - -<dt><tt>dut</tt></dt> - -<dd>The DUT sign and magnitude shows the current UT1 offset -relative to the displayed UTC time, in deciseconds.</dd> - -<dt><tt>lset</tt></dt> - -<dd>Before the clock is set, the interval since last set is the -number of minutes since the driver was started; after the clock is -set, this is number of minutes since the time was last verified -relative to the broadcast signal.</dd> - -<dt><tt>agc</tt></dt> - -<dd>The audio gain shows the current codec gain setting in the -range 0 to 255. Ordinarily, the receiver audio gain control or IRIG -level control should be set for a value midway in this range.</dd> - -<dt><tt>ident</tt></dt> - -<dd>The station identifier shows the station, <tt>C</tt> for WWV or -<tt>H</tt> for WWVH, and frequency being tracked. If neither -station is heard on any frequency, the station identifier shows -<tt>X</tt>.</dd> - -<dt><tt>comp</tt></dt> - -<dd>The minute sync compare counter is useful to determine the -quality of the minute sync signal and can range from 0 (no signal) -to 5 (best).</dd> - -<dt><tt>errs</tt></dt> - -<dd>The bit error counter is useful to determine the quality of the -data signal received in the most recent minute. It is normal to -drop a couple of data bits under good signal conditions and -increasing numbers as conditions worsen. While the decoder performs -moderately well even with half the bits are in error in any minute, -usually by that point the sync signals are lost and the decoder -reverts to free-run anyway.</dd> - -<dt><tt>freq</tt></dt> - -<dd>The frequency offset is the current estimate of the codec -frequency offset to within 0.1 PPM. This may wander a bit over the -day due to local temperature fluctuations and propagation -conditions.</dd> - -<dt><tt>avgt</tt></dt> - -<dd>The averaging time is the interval between frequency updates in -powers of two to a maximum of 1024 s. Attainment of the maximum -indicates the driver is operating at the best possible resolution -in time and frequency.</dd> -</dl> - -<p>An example timecode is:</p> - -<p><tt>0 2000 006 22:36:00.000 S +3 1 115 C20 6 5 66.4 -1024</tt></p> - -<p>Here the clock has been set and no alarms are raised. The year, -day and time are displayed along with no leap warning, standard -time and DUT +0.3 s. The clock was set on the last minute, the AGC -is safely in the middle ot the range 0-255, and the receiver is -tracking WWV on 20 MHz. Excellent reeiving conditions prevail, as -indicated by the compare count 6 and 5 bit errors during the last -minute. The current frequency is 66.4 PPM and the averaging -interval is 1024 s, indicating the maximum precision available.</p> - -<h4>Modes</h4> - -<p>The <tt>mode</tt> keyword of the <tt>server</tt> configuration -command specifies the ICOM ID select code. A missing or zero -argument disables the CI-V interface. Following are the ID select -codes for the known radios.</p> - -<table cols="6" width="100%"> -<tr> -<td>Radio</td> -<td>Hex</td> -<td>Decimal</td> -<td>Radio</td> -<td>Hex</td> -<td>Decimal</td> -</tr> - -<tr> -<td>IC725</td> -<td>0x28</td> -<td>40</td> -<td>IC781</td> -<td>0x26</td> -<td>38</td> -</tr> - -<tr> -<td>IC726</td> -<td>0x30</td> -<td>48</td> -<td>R7000</td> -<td>0x08</td> -<td>8</td> -</tr> - -<tr> -<td>IC735</td> -<td>0x04</td> -<td>4</td> -<td>R71</td> -<td>0x1A</td> -<td>26</td> -</tr> - -<tr> -<td>IC751</td> -<td>0x1c</td> -<td>28</td> -<td>R7100</td> -<td>0x34</td> -<td>52</td> -</tr> - -<tr> -<td>IC761</td> -<td>0x1e</td> -<td>30</td> -<td>R72</td> -<td>0x32</td> -<td>50</td> -</tr> - -<tr> -<td>IC765</td> -<td>0x2c</td> -<td>44</td> -<td>R8500</td> -<td>0x4a</td> -<td>74</td> -</tr> - -<tr> -<td>IC775</td> -<td>0x46</td> -<td>68</td> -<td>R9000</td> -<td>0x2a</td> -<td>42</td> -</tr> -</table> - -<h4>Fudge Factors</h4> - -<dl> -<dt><tt>time1 <i>time</i></tt></dt> - -<dd>Specifies the propagation delay for WWV (40:40:49.0N -105:02:27.0W), in seconds and fraction, with default 0.0.</dd> - -<dt><tt>time2 <i>time</i></tt></dt> - -<dd>Specifies the propagation delay for WWVH (21:59:26.0N -159:46:00.0W), in seconds and fraction, with default 0.0.</dd> - -<dt><tt>stratum <i>number</i></tt></dt> - -<dd>Specifies the driver stratum, in decimal from 0 to 15, with -default 0.</dd> - -<dt><tt>refid <i>string</i></tt></dt> - -<dd>Ordinarily, this field specifies the driver reference -identifier; however, the driver sets the reference identifier -automatically as described above.</dd> - -<dt><tt>flag1 0 | 1</tt></dt> - -<dd>Not used by this driver.</dd> - -<dt><tt>flag2 0 | 1</tt></dt> - -<dd>Specifies the microphone port if set to zero or the line-in -port if set to one. It does not seem useful to specify the compact -disc player port.</dd> - -<dt><tt>flag3 0 | 1</tt></dt> - -<dd>Enables audio monitoring of the input signal. For this purpose, -the speaker volume must be set before the driver is started.</dd> - -<dt><tt>flag4 0 | 1</tt></dt> - -<dd>Enable verbose <tt>clockstats</tt> recording if set.</dd> -</dl> - -<h4>Additional Information</h4> - -<a href="refclock.htm">Reference Clock Drivers</a> <br> -<a href="audio.htm">Reference Clock Audio Drivers</a> - -<hr> -<a href="index.htm"><img align="left" src="pic/home.gif" alt= -"gif"></a> - -<address><a href="mailto:mills@udel.edu">David L. Mills -<mills@udel.edu></a></address> -</body> -</html> - |
