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diff --git a/contrib/ntp/html/driver36.htm b/contrib/ntp/html/driver36.htm new file mode 100644 index 000000000000..6f70dcc7ea9c --- /dev/null +++ b/contrib/ntp/html/driver36.htm @@ -0,0 +1,839 @@ +<html><head><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>; 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>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>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. + +<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>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>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>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>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>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>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>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>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>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>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>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. + +<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>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>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. + +<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>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>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>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>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>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. + +<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>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. A missing <tt>mode</tt> +keyword or a zero argument leaves the interface disabled. 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>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>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 + +<dl> + +<p><dt><tt>0x0001</tt> +<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> +<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> +<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. + +<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><tt>wwv5 port agc wwv wwvh</tt> + +<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><tt>ident score comp sync/snr/jitr</tt> + +<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><tt>wwv5 2 111 C20 0100 6 8348/30.0/-3 H20 0203 0 22/-12.4/8846</tt> + +<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>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. + +<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>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><tt>wwvn ss stat sigl</tt> + +<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. + +<dl> + +<p><dt><tt>0x0001</tt> +<dd>Minute sync. Set when the decoder has identified a station and +acquired the minute sync pulse.</dd> +<p><dt><tt>0x0002</tt> +<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> + +<p><dt><tt>0x0004</tt> +<dd>Minute unit sync. Set when the decoder has reliably determined the +unit digit of the minute.</dd> + +<p><dt><tt>0x0008</tt> +<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>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><tt>wwv8 port agc ident comp ampl snr epoch jitr offs</tt> + +<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><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>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>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><tt>wwv3 ss stat sigl ampl phas snr prob like</tt> + +<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><tt>wwv3 28 0123 4122 4286 0 24.8 -5545 -1735</tt> + +<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>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><tt>wwv4 ss stat sigl radx ckdig mldig diff cnt like snr</tt> + +<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><tt>wwv4 8 010f 5772 10 9 9 0 6 4615 6.1</tt> + +<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>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><tt>wwv2 ss stat sigl avint avcnt avinc jitr delt freq</tt> + +<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><tt>wwv2 22 030f 5795 256 256 4 0 0.0 66.7</tt> + +<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>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. + +<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> +<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> +<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> +<dd>Sync alarm. The decoder may not be in correct second or minute phase +relative to the transmitter.</dd> + +<dt><tt>0x4</tt> +<dd>Error alarm. More than 30 data bit errors occurred in the last +minute.</dd> + +<dt><tt>0x2</tt> +<dd>Symbol alarm. The probability of correct decoding for a digit or +miscellaneous bit has fallen below the threshold.</dd> + +<dt><tt>0x1</tt> +<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. + +<dt><tt>yyyy ddd hh:mm:ss.fff</tt></tt> +<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> +<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> +<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> +<dd>The DUT sign and magnitude shows the current UT1 offset relative to +the displayed UTC time, in deciseconds.</dd> + +<dt><tt>lset</tt> +<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> +<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. + +<dt><tt>ident</tt> +<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> +<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> +<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> +<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> +<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><tt> 0 2000 006 22:36:00.000 S +3 1 115 C20 6 5 66.4 1024</tt> + +<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. + +<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. +<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></a><address><a +href=mailto:mills@udel.edu> David L. Mills <mills@udel.edu></a> +</address></a></body></html> |