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diff --git a/html/drivers/driver6.html b/html/drivers/driver6.html index 8a51f16f035f..eb12bdd39f99 100644 --- a/html/drivers/driver6.html +++ b/html/drivers/driver6.html @@ -1,54 +1,37 @@ <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> - <html> - <head> <meta http-equiv="content-type" content="text/html;charset=iso-8859-1"> <meta name="generator" content="HTML Tidy, see www.w3.org"> <title>IRIG Audio Decoder</title> <link href="scripts/style.css" type="text/css" rel="stylesheet"> </head> - <body> <h3>IRIG Audio Decoder</h3> <hr> <h4>Synopsis</h4> - Address: 127.127.6.<i>u</i><br> - Reference ID: <tt>IRIG</tt><br> - Driver ID: <tt>IRIG_AUDIO</tt><br> - Audio Device: <tt>/dev/audio</tt> and <tt>/dev/audioctl</tt> - <p>Note: This driver supersedes an older one of the same name, address and ID which required replacing the original kernel audio driver with another which worked only on older Sun SPARC architectures and SunOS operating systems. The new driver requires no modification of the operating system and works on FreeBSD, SunOS and Solaris. While it is generic and likely portable to other systems, it is somewhat slower than the original, since the extensive signal conditioning, filtering and decoding is done in user space, not kernel space.</p> + Address: 127.127.6.<i>u</i><br> + Reference ID: <tt>IRIG</tt><br> + Driver ID: <tt>IRIG_AUDIO</tt><br> + Audio Device: <tt>/dev/audio</tt> and <tt>/dev/audioctl</tt> <h4>Description</h4> - <p>This driver supports the Inter-Range Instrumentation Group (IRIG) standard time distribution signal using the audio codec native to some workstations. This signal is generated by several radio clocks, including those made by Arbiter, Austron, Bancomm, Odetics, Spectracom and TrueTime, among others, although it is often an add-on option. The signal is connected via an optional attenuator box and cable to either the microphone or line-in port. The driver receives, demodulates and decodes the IRIG-B and IRIG-E signal formats using internal filters designed to reduce the effects of noise and interference.</p> + <p>This driver synchronizes the computer time using the Inter-Range Instrumentation Group (IRIG) standard time distribution signal. This signal is generated by several radio clocks, including those made by Arbiter, Austron, Bancomm, Odetics, Spectracom, Symmetricom and TrueTime, among others, although it is often an add-on option. The signal is connected via an optional attenuator and cable to either the microphone or line-in port of a workstation or PC.</p> + <p>The driver requires an audio codec or sound card with sampling rate 8 kHz and <font face="symbol">m</font>-law companding to demodulate the data. This is the same standard as used by the telephone industry and is supported by most hardware and operating systems, including Solaris, FreeBSD and Linux, among others. In this implementation, only one audio driver and codec can be supported on a single machine. In order to assure reliable signal capture, the codec frequency error must be less than 250 PPM (.025 percent). If necessary, the <tt>tinker codec</tt> configuration command can be used to bracket the codec frequency to this range.</p> + <p>For proper operation the IRIG signal source should be configured for analog signal levels, not digital TTL levels. In most radios the IRIG signal is driven ±10 V behind 50 Ohms. In such cases the cable should be terminated at the line-in port with a 50-Ohm resistor to avoid overloading the codec. Where feasible, the IRIG signal source should be operated with signature control so that, if the signal is lost or mutilated, the source produces an unmodulated signal, rather than possibly random digits. The driver automatically rejects the data and declares itself unsynchronized in this case. Some devices, in particular Spectracom radio/satellite clocks, provide additional year and status indication; other devices may not.</p> + <p>In general and without calibration, the driver is accurate within 500 <font face="symbol">m</font>s relative to the IRIG time. After calibrating relative to the PPS signal from a GPS receiver, the mean offset with a 2.4-GHz P4 running FreeBSD 6.1 is less than 20 <font face="symbol">m</font>s with standard deviation 10 <font face="symbol">m</font>s. Most of this is due to residuals after filtering and averaging the raw codec samples, which have an inherent jitter of 125 <font face="symbol">m</font>s. The processor load due to the driver is 0.6 percent on the P4.</p> + <p>However, be acutely aware that the accuracy with Solaris 2.8 and beyond has been seriously degraded to the order of several milliseconds. The Sun kernel driver has a sawtooth modulation with amplitude over 5 ms P-P and period 5.5 s. This distortion is especially prevalent with Sun Blade 1000 and possibly other systems.</p> + <p>The driver performs a number of error checks to protect against overdriven or underdriven input signal levels, incorrect signal format or improper hardware configuration. The specific checks are detailed later in this page. Note that additional checks are done elsewhere in the reference clock interface routines.</p> <p>This driver incorporates several features in common with other audio drivers such as described in the <a href="driver7.html">Radio CHU Audio Demodulator/Decoder</a> and the <a href="driver36.html">Radio WWV/H Audio Demodulator/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.html">Reference Clock Audio Drivers</a> page.</p> - <p>The IRIG signal format uses an amplitude-modulated carrier with pulse-width modulated data bits. For IRIG-B, the carrier frequency is 1000 Hz and bit rate 100 b/s; for IRIG-E, the carrier frequenchy is 100 Hz and bit rate 10 b/s. While IRIG-B provides the best accuracy, generally within a few tens of microseconds relative to IRIG time, it can also generate a significant load on the processor with older workstations. Generally, the accuracy with IRIG-E is about ten times worse than IRIG-B, but the processor load is ten times less.</p> - <p>The program processes 8000-Hz <font face="symbol">m</font>-law companded samples using separate signal filters for IRIG-B and IRIG-E, a comb filter, envelope detector and automatic threshold corrector. Cycle crossings relative to the corrected slice level determine the width of each pulse and its value - zero, one or position identifier. The data encode 20 BCD digits which determine the second, minute, hour and day of the year and sometimes the year and synchronization condition. The comb filter exponentially averages the corresponding samples of successive baud intervals in order to reliably identify the reference carrier cycle. A type-II phase-lock loop (PLL) performs additional integration and interpolation to accurately determine the zero crossing of that cycle, which determines the reference timestamp. A pulse-width discriminator demodulates the data pulses, which are then encoded as the BCD digits of the timecode. The timecode and reference timestamp are updated once each second with IRIG-B (ten seconds with IRIG-E) and local clock offset samples saved for later processing. At poll intervals of 64 s, the saved samples are processed by a trimmed-mean filter and used to update the system clock.</p> - <p>Infinite impulse response (IIR) filters are used with both IRIG-B and IRIG-E formats. An 800-Hz highpass filter is used for IRIG-B and a 130-Hz lowpass filter for IRIG-E. These are intended for use with noisy signals, such as might be received over a telephone line or radio circuit, or when interfering signals may be present in the audio passband. The driver determines which IRIG format is in use by sampling the amplitude of each filter output and selecting the one with maximum signal. An automatic gain control feature provides protection against overdriven or underdriven input signal amplitudes. It is designed to maintain adequate demodulator signal amplitude while avoiding occasional noise spikes. In order to assure reliable capture, the decompanded input signal amplitude must be greater than 100 units and the codec sample frequency error less than 250 PPM (.025 percent).</p> - <p>The program performs a number of error checks to protect against overdriven or underdriven input signal levels, incorrect signal format or improper hardware configuration. The specific checks are detailed later in this page. Note that additional checks are done elsewhere in the reference clock interface routines.</p> - <p>Unlike other drivers, which can have multiple instantiations, this one supports only one. It does not seem likely that more than one audio codec would be useful in a single machine. More than one would probably chew up too much CPU time anyway.</p> - <h4>IRIG-B Timecode Format</h4> - <p>The 100 elements of the IRIG timecode are numbered from 0 through 99. Position identifiers occur at elements 0, 9, 19 and every ten thereafter to 99. The control function (CF) elements begin at element 50 (CF 1) and extend to element 78 (CF 27). The straight-binary-seconds (SBS) field, which encodes the seconds of the UTC day, begins at element 80 (CF 28) and extends to element 97 (CF 44). The encoding of elements 50 (CF 1) through 78 (CF 27) is device dependent. This driver presently decodes the CF elements, but does nothing with them.</p> - <p>Where feasible, the IRIG signal source should be operated with signature control so that, if the signal is lost or mutilated, the source produces an unmodulated signal, rather than possibly random digits. The driver will automatically reject the data and declare itself unsynchronized in this case. Some devices, in particular Spectracom radio/satellite clocks, provide additional year and status indication in the format:</p> - <pre> - Element CF Function - ------------------------------------- - 55 6 time sync status - 60-63 10-13 BCD year units - 65-68 15-18 BCD year tens -</pre> - Other devices set these elements to zero. - <h4>Performance and Horror Stories</h4> - <p>The <font face="symbol">m</font>-law companded data format allows considerable latitude in signal levels; however, an automatic gain control (AGC) function is implemented to further compensate for varying input signal levels and to avoid signal distortion. For proper operation, the IRIG signal source should be configured for analog signal levels, NOT digital TTL levels.</p> - <p>The accuracy of the system clock synchronized to the IRIG-B source with this driver and the <tt>ntpd</tt> daemon is 10-20 <font face="symbol">m</font>s with a Sun UltraSPARC II running Solaris 2.6 and maybe twice that with a Sun SPARC IPC running SunOS 4.1.3. Be however acutely aware that the accuracy with Solaris 2.8 and presumably beyond has seriously degraded to the order of several milliseconds. The Sun kernel driver has a sawtooth modulation with amplitude over 5 ms peak-peak and period 5.5 s. The crafty IRIG driver uses a transverse filter to remove the modulation and something called a botttom-fisher to remove incidental positive spikes especially prevalent with Sun Blade 1000 and possibly other systems. The result is nominal accuracy and jitter something less than 0.5 ms, but the this is still far inferior to the performance with older systems.</p> - <p>The processor resources consumed by the daemon can be significant, ranging from about 1.2 percent on the faster UltraSPARC II to 38 percent on the slower SPARC IPC. However, the overall timing accuracy is limited by the resolution and stability of the CPU clock oscillator and the interval between clock corrections, which is 64 s with this driver. This performance, while probably the best that can be achieved by the daemon itself, can be improved with assist from the PPS discipline as described elsewhere in this documentation.</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.</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 on the <a href="../audio.html">Reference Clock Audio Drivers</a> page. 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 CHU. 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>Technical Overview</h4> + <p>The IRIG signal format uses an amplitude-modulated carrier with pulse-width modulated data bits. For IRIG-B, the carrier frequency is 1000 Hz and bit rate 100 b/s; for IRIG-E, the carrier frequenchy is 100 Hz and bit rate 10 b/s. While IRIG-B provides the best accuracy, generally within a few tens of microseconds relative to IRIG time, it can also generate a significant processor load with older workstations. Generally, the accuracy with IRIG-E is about ten times worse than IRIG-B, but the processor load is somewhat less. Technical details about the IRIG formats can be found in <a href="http://handle.dtic.mil/100.2/ADA346250">IRIG Standard 200-98</a>.</p> + <p>The driver processes 8000-Hz <font face="symbol">m</font>-law companded samples using separate signal filters for IRIG-B and IRIG-E, a comb filter, envelope detector and automatic threshold corrector. An infinite impulse response (IIR) 1000-Hz bandpass filter is used for IRIG-B and an IIR 130-Hz lowpass filter for IRIG-E. These are intended for use with noisy signals, such as might be received over a telephone line or radio circuit, or when interfering signals may be present in the audio passband. The driver determines which IRIG format is in use by sampling the amplitude of each filter output and selecting the one with maximum signal.</p> + <p>Cycle crossings relative to the corrected slice level determine the width of each pulse and its value - zero, one or position identifier (PI). The data encode ten characters (20 BCD digits) which determine the second, minute, hour and day of the year and with some IRIG generators the year and synchronization condition. The comb filter exponentially averages the corresponding samples of successive baud intervals in order to reliably identify the reference carrier cycle.</p> + <p>A type-II phase-lock loop (PLL) performs additional integration and interpolation to accurately determine the zero crossing of that cycle, which determines the reference timestamp. A pulse-width discriminator demodulates the data pulses, which are then encoded as the BCD digits of the timecode. The timecode and reference timestamp are updated once each second with IRIG-B (ten seconds with IRIG-E) and local clock offset samples saved for later processing. At poll intervals of 64 s, the saved samples are processed by a median filter and used to update the system clock.</p> <h4>Monitor Data</h4> - The timecode format used for debugging and data recording includes data helpful in diagnosing problems with the IRIG signal and codec connections. With debugging enabled (-d on the ntpd command line), the driver produces one line for each timecode in the following format: - <p><tt>00 1 98 23 19:26:52 721 143 0.694 47 20 0.083 66.5 3094572411.00027</tt></p> - <p>The first field containes the error flags in hex, where the hex bits are interpreted as below. This is followed by the IRIG status indicator, year of century, day of year and time of day. The status indicator and year are not produced by some IRIG devices. Following these fields are the carrier amplitude (0-8100), codec gain (0-255), field phase (0-79), time constant (2-20), modulation index (0-1), carrier phase error (0±0.5) and carrier frequency error (PPM). The last field is the on-time timestamp in NTP format. The fraction part is a good indicator of how well the driver is doing. With an UltrSPARC 30, this is normally within a few tens of microseconds relative to the IRIG-B signal and within a few hundred microseconds with IRIG-E.</p> + The timecode format used for debugging and data recording includes data helpful in diagnosing problems with the IRIG signal and codec connections. The driver produces one line for each timecode in the following format: + <p><tt>00 00 98 23 19:26:52 2782 143 0.694 10 0.3 66.5 3094572411.00027</tt></p> + <p>If clockstats is enabled, the most recent line is written to the clockstats file every 64 s. If verbose recording is enabled (fudge flag 4) each line is written as generated.</p> + <p>The first field containes the error flags in hex, where the hex bits are interpreted as below. This is followed by the year of century, day of year and time of day. Note that the time of day is for the previous minute, not the current time. The status indicator and year are not produced by some IRIG devices and appear as zeros. Following these fields are the carrier amplitude (0-3000), codec gain (0-255), modulation index (0-1), time constant (4-10), carrier phase error (0±0.5) and carrier frequency error (PPM). The last field is the on-time timestamp in NTP format.</p> <p>The error flags are defined as follows in hex:</p> <dl> <dt><tt>x01</tt> @@ -65,6 +48,8 @@ <dd>Seconds numbering discrepancy. The decoder second does not match the IRIG second. This is usually the result of an overdriven codec, wrong signal format or noisy IRIG signal. <dt><tt>x40</tt> <dd>Codec error (overrun). The machine is not fast enough to keep up with the codec. + <dt><tt>x80</tt> + <dd>Device status error (Spectracom). </dl> <h4>Fudge Factors</h4> <dl> @@ -88,5 +73,4 @@ <hr> <script type="text/javascript" language="javascript" src="scripts/footer.txt"></script> </body> - </html>
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