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diff --git a/contrib/ntp/html/driver7.htm b/contrib/ntp/html/driver7.htm deleted file mode 100644 index 029ac04..0000000 --- a/contrib/ntp/html/driver7.htm +++ /dev/null @@ -1,657 +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 CHU Audio Demodulator/Decoder</title> -</head> -<body> -<h3>Radio CHU Audio Demodulator/Decoder</h3> - -<hr> -<h4>Synopsis</h4> - -Address: 127.127.7.<i>u</i> <br> -Reference ID: <tt>CHU</tt> <br> -Driver ID: <tt>CHU</tt> <br> -Modem Port: <tt>/dev/chu<i>u</i></tt>; 300 baud, 8-bits, no parity -<br> -Autotune Port: <tt>/dev/icom</tt>; 1200/9600 baud, 8-bits, no -parity <br> -Audio Device: <tt>/dev/chu_audio</tt> and <tt>/dev/audioctl</tt> - -<h4>Description</h4> - -<p>This driver synchronizes the computer time using data encoded in -radio transmissions from Canadian time/frequency station CHU in -Ottawa, Ontario. It replaces an earlier one, built by Dennis -Ferguson in 1988, which required a special line discipline to -preprocessed the signal. The new driver includes more powerful -algorithms implemented directly in the driver and requires no -preprocessing.</p> - -<p>CHU transmissions are made continuously on 3330 kHz, 7335 kHz -and 14670 kHz in upper sideband, compatible AM mode. 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 as propagation conditions change throughout the -day and night. The performance of this driver when tracking the -station is ordinarily better than 1 ms in time with frequency drift -less than 0.5 PPM when not tracking the station.</p> - -<p>While there are currently no known commercial CHU receivers, a -simple but effective receiver/demodulator can be constructed from -an ordinary shortwave receiver and Bell 103 compatible, 300-b/s -modem or modem chip, as described in the <a href="gadget.htm"> -Gadget Box PPS Level Converter and CHU Modem</a> page. The driver -can use the modem to receive the radio signal and demodulate the -data or, if available, the driver can use the audio codec of the -Sun workstation or another with compatible audio interface. In the -latter case, the driver implements the modem using DSP routines, so -the radio can be connected directly to either the microphone on -line input port.</p> - -<p>This driver incorporates several features in common with other -audio drivers such as described in the <a href="driver36.htm">Radio -WWV/H 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>Ordinarily, the driver poll interval is set to 14 (about 4.5 h), -although this can be changed with configuration commands. As long -as the clock is set or verified at least once during this interval, -the NTP algorithms will consider the source reachable and -selectable to discipline the system clock. However, if this does -not happen for eight poll intervals, the algorithms will consider -the source unreachable and some other source will be chosen (if -available) to discipline the system clock.</p> - -<p>The decoding algorithms process the data using -maximum-likelihood techniques which exploit the considerable degree -of redundancy available in each broadcast message or burst. As -described below, every character is sent twice and, in the case of -format A bursts, the burst is sent eight times every minute. In the -case of format B bursts, which are sent once each minute, the burst -is considered correct only if every character matches its -repetition in the burst. In the case of format A messages, a -majority decoder requires at least six repetitions for each digit -in the timecode and more than half of the repetitions decode to the -same digit. Every character in every burst provides an independent -timestamp upon arrival with a potential total of over 60 timestamps -for each minute.</p> - -<p>A timecode in the format described below is assembled when all -bursts have been received in the minute. The timecode is considered -valid and the clock set when at least one valid format B burst has -been decoded and the above requirements are met. The <tt>yyyy</tt> -year field in the timecode indicates whether a valid format B burst -has been received. Upon startup, this field is initialized at zero; -when a valid format B burst is received, it is set to the current -Gregorian year. The <tt>q</tt> quality character field in the -timecode indicates whether a valid timecode has been determined. If -any of the high order three bits of this character are set, the -timecode is invalid.</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 is lost. Since the signals are almost always -available during some period of the day and the NTP clock -discipline algorithms are designed to work well even in this case, -it is unlikely that the system clock could drift more than a few -tens of milliseconds during periods of signal loss. 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>The last three fields in the timecode are useful in assessing -the quality of the radio channel during the most recent minute -bursts were received. The <tt>bcnt</tt> field shows the number of -format A bursts in the range 1-8. The <tt>dist</tt> field shows the -majority decoder distance, or the minimum number of sample -repetitions for each digit of the timecode in the range 0-16. The -<tt>tsmp</tt> field shows the number of timestamps determined in -the range 0-60. For a valid timecode, <tt>bcnt</tt> must be at -least 3, <tt>dist</tt> must be greater than <tt>bcnt</tt> and <tt> -tsmp</tt> must be at least 20.</p> - -<h4>Program Operation</h4> - -<p>The program consists of four major parts: the DSP modem, maximum -likelihood UART, burst assembler and majority decoder. The DSP -modem demodulates Bell 103 modem answer-frequency signals; that is, -frequency-shift keyed (FSK) tones of 2225 Hz (mark) and 2025 Hz -(space). This is done using a 4th-order IIR filter and -limiter/discriminator with 500-Hz bandpass centered on 2125 Hz and -followed by a FIR raised-cosine lowpass filter optimized for the -300-b/s data rate. Alternately, the driver can be compiled to -delete the modem and input 300 b/s data directly from an external -modem via a serial port.</p> - -<p>The maximum likelihood UART is implemented using a set of eight -11-stage shift registers, one for each of eight phases of the -300-b/s bit clock. At each phase a new baseband signal value from -the DSP modem is shifted into the corresponding register and the -maximum and minimum over all 11 samples computed. This establishes -a slice level midway between the maximum and minimum over all -stages. For each stage, a signal level above this level is a mark -(1) and below is a space (0). A quality metric is calculated for -each register with respect to the slice level and the a-priori -signal consisting of a mark bit (previous stop bit), space (start) -bit, eight arbitrary information bits and the first of the two mark -(stop) bits.</p> - -<p>The shift registers are processed in round-robin order as each -modem value arrives until one of them shows a valid framing pattern -consisting of a mark bit, space bit, eight arbitrary data bits and -a mark bit. When found, the data bits from the register with the -best metric is chosen as the maximum likelihood character and the -UART begins to process the next character.</p> - -<p>The burst assembler processes characters either from the maximum -likelihood UART or directly from the serial port as configured. A -burst begins when a character is received and is processed after a -timeout interval when no characters are received. If the interval -between characters is greater than two characters, but less than -the timeout interval, the burst is rejected as a runt and a new -burst begun. As each character is received, a timestamp is captured -and saved for later processing.</p> - -<p>A valid burst consists of ten characters in two replicated -five-character blocks. A format B block contains the year and other -information in ten hexadecimal digits. A format A block contains -the timecode in ten decimal digits, the first of which is a framing -code (6). The burst assembler must deal with cases where the first -character of a format A burst is lost or is noise. This is done -using the framing code to correct the phase, either one character -early or one character late.</p> - -<p>The burst distance is incremented by one for each bit in the -first block that matches the corresponding bit in the second block -and decremented by one otherwise. In a format B burst the second -block is bit-inverted relative to the first, so a perfect burst of -five 8-bit characters has distance -40. In a format A block the two -blocks are identical, so a perfect burst has distance +40. Format B -bursts must be perfect to be acceptable; however, format A bursts, -which are further processed by the majority decoder, are acceptable -if the distance is at least 28.</p> - -<p>Each minute of transmission includes eight format A bursts -containing two timecodes for each second from 31 through 39. The -majority decoder uses a decoding matrix of ten rows, one for each -digit position in the timecode, and 16 columns, one for each 4-bit -code combination that might be decoded at that position. In order -to use the character timestamps, it is necessary to reliably -determine the second number of each burst. In a valid burst, the -last digit of the two timecodes in the block must match and the -value must be in the range 2-9 and greater than in the previous -burst.</p> - -<p>As each hex digit of a valid burst is processed, the value at -the row corresponding to the digit position in the timecode and -column corresponding to the code found at that position is -incremented. At the end of each minute of transmission, each row of -the decoding matrix encodes the number of occurrences of each code -found at the corresponding position of the timecode. However, the -first digit (framing code) is always 6, the ninth (second tens) is -always 3 and the last (second units) changes for each burst, so are -not used.</p> - -<p>The maximum over all occurrences at each timecode digit position -is the distance for that position and the corresponding code is the -maximum likelihood candidate. If the distance is zero, the decoder -assumes a miss; if the distance is not more than half the total -number of occurrences, the decoder assumes a soft error; if two -different codes with the same distance are found, the decoder -assumes a hard error. In all these cases the decoder encodes a -non-decimal character which will later cause a format error when -the timecode is reformatted. The decoding distance is defined as -the minimum distance over the first nine digits; the tenth digit -varies over the seconds and is uncounted.</p> - -<p>The result of the majority decoder is a nine-digit timecode -representing the maximum likelihood candidate for the transmitted -timecode in that minute. Note that the second and fraction within -the minute are always zero and that the actual reference point to -calculate timestamp offsets is backdated to the first second of the -minute. At this point the timecode block is reformatted and the -year, days, hours and minutes extracted along with other -information from the format B burst, including DST state, DUT1 -correction and leap warning. The reformatting operation checks the -timecode for invalid code combinations that might have been left by -the majority decoder and rejects the entire timecode if found.</p> - -<p>If the timecode is valid, it is passed to the reference clock -interface along with the backdated timestamp offsets accumulated -over the minute. A perfect set of nine bursts could generate as -many as 90 timestamps, but the maximum the interface can handle is -60. These are processed by the interface using a median filter and -trimmed-mean average, so the resulting system clock correction is -usually much better than would otherwise be the case with radio -noise, UART jitter and occasional burst errors.</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 standard 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 <tt> -icom.h</tt> header 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 tune the radio to 3.330 MHz. -If after five minutes at this frequency not more than two format A -bursts have been received for any minute, the driver will tune to -7.335 MHz, then to 14.670 MHz, then return to 3.330 MHz and -continue in this cycle. 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> - -<h4>Radio Broadcast Format</h4> - -<p>The CHU time broadcast includes an audio signal compatible with -the Bell 103 modem standard (mark = 2225 Hz, space = 2025 Hz). It -consist of nine, ten-character bursts transmitted at 300 b/s and -beginning each second from second 31 to second 39 of the minute. -Each character consists of eight data bits plus one start bit and -two stop bits to encode two hex digits. The burst data consist of -five characters (ten hex digits) followed by a repeat of these -characters. In format A, the characters are repeated in the same -polarity; in format B, the characters are repeated in the opposite -polarity.</p> - -<p>Format A bursts are sent at seconds 32 through 39 of the minute -in hex digits</p> - -<p><tt>6dddhhmmss6dddhhmmss</tt></p> - -<p>The first ten digits encode a frame marker (<tt>6</tt>) followed -by the day (<tt>ddd</tt>), hour (<tt>hh</tt>), minute (<tt>mm</tt>) -and second (<tt>ss</tt>). Since format A bursts are sent during the -third decade of seconds the tens digit of <tt>ss</tt> is always 3. -The driver uses this to determine correct burst synchronization. -These digits are then repeated with the same polarity.</p> - -<p>Format B bursts are sent at second 31 of the minute in hex -digits</p> - -<p><tt>xdyyyyttaaxdyyyyttaa</tt></p> - -<p>The first ten digits encode a code (<tt>x</tt> described below) -followed by the DUT1 (<tt>d</tt> in deciseconds), Gregorian year -(<tt>yyyy</tt>), difference TAI - UTC (<tt>tt</tt>) and daylight -time indicator (<tt>aa</tt>) peculiar to Canada. These digits are -then repeated with inverted polarity.</p> - -<p>The <tt>x</tt> is coded</p> - -<dl> -<dt><tt>1</tt></dt> - -<dd>Sign of DUT (0 = +)/dd></dd> - -<dt><tt>2</tt></dt> - -<dd>Leap second warning. One second will be added.</dd> - -<dt><tt>4</tt></dt> - -<dd>Leap second warning. One second will be subtracted. This is not -likely to happen in our universe.</dd> - -<dt><tt>8</tt></dt> - -<dd>Even parity bit for this nibble.</dd> -</dl> - -<p>By design, the last stop bit of the last character in the burst -coincides with 0.5 second. Since characters have 11 bits and are -transmitted at 300 b/s, the last stop bit of the first character -coincides with 0.5 - 10 * 11/300 = 0.133 second. Depending on the -UART, character interrupts can vary somewhere between the beginning -of bit 9 and end of bit 11. These eccentricities can be corrected -along with the radio propagation delay using the <tt>fudge -time1</tt> variable.</p> - -<h4>Debugging Aids</h4> - -<p>The most convenient way to track the program 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 program is not discipline the system clock. -If the debugging trace feature (<tt>-d</tt> on the <tt>ntpd</tt> -command line)is enabled, the program 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>With debugging enabled the driver produces messages in the -following formats:</p> - -<p>A format <tt>chuA</tt> message is produced for each format A -burst received in seconds 32 through 39 of the minute:</p> - -<p><tt>chuA n b s code</tt></p> - -<p>where <tt>n</tt> is the number of characters in the burst -(0-11), <tt>b</tt> the burst distance (0-40), <tt>s</tt> the -synchronization distance (0-40) and <tt>code</tt> the burst -characters as received. Note that the hex digits in each character -are reversed and the last ten digits inverted, so the burst</p> - -<p><tt>11 40 1091891300ef6e76ecff</tt></p> - -<p>is interpreted as containing 11 characters with burst distance -40. The nibble-swapped timecode shows DUT1 +0.1 second, year 1998 -and TAI -UTC 31 seconds.</p> - -<p>A format <tt>chuB</tt> message is produced for each format B -burst received in second 31 of the minute:</p> - -<p><tt>chuB n b f s m code</tt></p> - -<p>where <tt>n</tt> is the number of characters in the burst -(0-11), <tt>b</tt> the burst distance (0-40), <tt>f</tt> the field -alignment (-1, 0, 1), <tt>s</tt>the synchronization distance -(0-16), <tt>m</tt>the burst number (2-9) and <tt>code</tt> the -burst characters as received. Note that the hex digits in each -character are reversed, so the burst</p> - -<p><tt>10 38 0 16 9 06851292930685129293</tt></p> - -<p>is interpreted as containing 11 characters with burst distance -38, field alignment 0, synchronization distance 16 and burst number -9. The nibble-swapped timecode shows day 58, hour 21, minute 29 and -second 39.</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 dut lset agc rfrq bcnt dist tsmp - - s sync indicator - q quality character - yyyy Gregorian year - ddd day of year - hh hour of day - mm minute of hour - ss second of minute - fff millisecond of second - l leap second warning - d DST state - dut DUT sign and magnitude in deciseconds - lset minutes since last set - agc audio gain (0-255) - rfrq radio frequency - bcnt burst count - dist decoding distance - tsmp timestamps captured -</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 the clock is correctly set.</dd> - -<dt><tt>q</tt></dt> - -<dd>The quality character is a four-bit hexadecimal code showing -which alarms have been raised during the most recent minute. Each -bit is associated with a specific alarm condition according to the -following: - -<dl> -<dt><tt>8</tt></dt> - -<dd>Decoder alarm. A majority of repetitions for at least one digit -of the timecode fails to agree.</dd> - -<dt><tt>4</tt></dt> - -<dd>Timestamp alarm. Fewer than 20 timestamps have been -determined.</dd> - -<dt><tt>2</tt></dt> - -<dd>Format alarm. The majority timecode contains invalid bit -combinations.</dd> - -<dt><tt>1</tt></dt> - -<dd>Frame alarm. A framing or format error occurred on at least one -burst during the minute.</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. Note that the -Gregorian year is decoded directly from the transmitted -timecode.</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 code for Canada encodes the state for all -provinces.</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 program 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>rfrq</tt></dt> - -<dd>The current radio frequency, if the CI-V interface is active, -or 'X' if not.</dd> - -<dt><tt>bcnt</tt></dt> - -<dd>The number of format A bursts received during the most recent -minute bursts were received.</dd> - -<dt><tt>dist</tt></dt> - -<dd>The minimum decoding distance determined during the most recent -minute bursts were received.</dd> - -<dt><tt>tsmp</tt></dt> - -<dd>The number of timestamps determined during the most recent -minute bursts were received.</dd> -</dl> - -<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 CHU (45:18N 75:45N), in -seconds and fraction, with default 0.0.</dd> - -<dt><tt>time2 <i>time</i></tt></dt> - -<dd>Not used by this driver.</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>Specifies the driver reference identifier, an ASCII string from -one to four characters, with default <tt>CHU</tt>.</dd> - -<dt><tt>flag1 0 | 1</tt></dt> - -<dd>Not used by this driver.</dd> - -<dt><tt>flag2 0 | 1</tt></dt> - -<dd>When the audio driver is compiled, this flag selects the audio -input port, where 0 is the mike port (default) and 1 is the line-in -port. It does not seem useful to select the compact disc player -port.</dd> - -<dt><tt>flag3 0 | 1</tt></dt> - -<dd>When the audio driver is compiled, this flag 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> - |
