2001-08-29 14:35:15 +00:00
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<title>IRIG Audio Decoder</title>
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</head>
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<body>
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<h3>IRIG Audio Decoder</h3>
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<hr>
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<h4>Synopsis</h4>
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Address: 127.127.6.<i>u</i> <br>
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Reference ID: <tt>IRIG</tt> <br>
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Driver ID: <tt>IRIG_AUDIO</tt> <br>
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Audio Device: <tt>/dev/audio</tt> and <tt>/dev/audioctl</tt>
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<p>Note: This driver supersedes an older one of the same name,
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address and ID which required replacing the original kernel audio
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driver with another which works only on older Sun SPARCstation
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systems. The new driver described here uses the stock kernel audio
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driver and works in SunOS 4.1.3 and Solaris 2.6 versions and
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probably all versions in between. The new driver requires no
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modification of the operating system. While it is generic and
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likely portable to other systems, it is somewhat slower than the
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original, since the extensive signal conditioning, filtering and
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decoding is done in user space, not kernel space.</p>
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<h4>Description</h4>
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This driver supports the Inter-Range Instrumentation Group (IRIG)
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standard time distribution signal using the audio codec native to
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some workstations. This signal is generated by several radio
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clocks, including those made by Arbiter, Austron, Bancomm, Odetics,
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Spectracom and TrueTime, among others, although it is often an
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add-on option. The signal is connected via an optional attenuator
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box and cable to either the microphone or line-in port. The driver
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receives, demodulates and decodes the IRIG-B and IRIG-E signal
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formats using internal filters designed to reduce the effects of
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noise and interference.
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<p>This driver incorporates several features in common with other
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audio drivers such as described in the <a href="driver7.htm">Radio
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CHU Audio Demodulator/Decoder</a> and the <a href="driver36.htm">
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Radio WWV/H Audio Demodulator/Decoder</a> pages. They include
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automatic gain control (AGC), selectable audio codec port and
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signal monitoring capabilities. For a discussion of these common
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features, as well as a guide to hookup, debugging and monitoring,
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see the <a href="audio.htm">Reference Clock Audio Drivers</a>
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page.</p>
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<p>The IRIG signal format uses an amplitude-modulated carrier with
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pulse-width modulated data bits. For IRIG-B, the carrier frequency
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is 1000 Hz and bit rate 100 b/s; for IRIG-E, the carrier frequenchy
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is 100 Hz and bit rate 10 b/s. While IRIG-B provides the best
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accuracy, generally within a few tens of microseconds relative to
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IRIG time, it can also generate a significant load on the processor
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with older workstations. Generally, the accuracy with IRIG-E is
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about ten times worse than IRIG-B, but the processor load is ten
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times less.</p>
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<p>The program processes 8000-Hz mu-law companded samples using
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separate signal filters for IRIG-B and IRIG-E, a comb filter,
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envelope detector and automatic threshold corrector. Cycle
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crossings relative to the corrected slice level determine the width
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of each pulse and its value - zero, one or position identifier. The
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data encode 20 BCD digits which determine the second, minute, hour
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and day of the year and sometimes the year and synchronization
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condition. The comb filter exponentially averages the corresponding
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samples of successive baud intervals in order to reliably identify
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the reference carrier cycle. A type-II phase-lock loop (PLL)
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performs additional integration and interpolation to accurately
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determine the zero crossing of that cycle, which determines the
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reference timestamp. A pulse-width discriminator demodulates the
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data pulses, which are then encoded as the BCD digits of the
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timecode. The timecode and reference timestamp are updated once
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each second with IRIG-B (ten seconds with IRIG-E) and local clock
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offset samples saved for later processing. At poll intervals of 64
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s, the saved samples are processed by a trimmed-mean filter and
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used to update the system clock.</p>
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<p>Infinite impulse response (IIR) filters are used with both
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IRIG-B and IRIG-E formats. An 800-Hz highpass filter is used for
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IRIG-B and a 130-Hz lowpass filter for IRIG-E. These are intended
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for use with noisy signals, such as might be received over a
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telephone line or radio circuit, or when interfering signals may be
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present in the audio passband. The driver determines which IRIG
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format is in use by sampling the amplitude of each filter output
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and selecting the one with maximum signal. An automatic gain
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control feature provides protection against overdriven or
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underdriven input signal amplitudes. It is designed to maintain
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adequate demodulator signal amplitude while avoiding occasional
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noise spikes. In order to assure reliable capture, the decompanded
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input signal amplitude must be greater than 100 units and the codec
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sample frequency error less than 250 PPM (.025 percent).</p>
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<p>The program performs a number of error checks to protect against
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overdriven or underdriven input signal levels, incorrect signal
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format or improper hardware configuration. Specifically, if any of
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the following errors occur for a timecode, the data are rejected.
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Secifically, if any of the following errors occur for a time
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measurement, the data are rejected.</p>
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<ol>
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<li>The peak carrier amplitude is less than 100 units. This usually
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means dead IRIG signal source, broken cable or wrong input
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port.</li>
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<li>The frequency error is greater than ±250 PPM (.025
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percent). This usually means broken codec hardware or wrong codec
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configuration.</li>
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<li>The modulation index is less than 0.5. This usually means
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overdriven IRIG signal or wrong IRIG format.</li>
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<li>A frame synchronization error has occured. This usually means
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wrong IRIG signal format or the IRIG signal source has lost
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synchronization (signature control).</li>
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<li>A data decoding error has occured. This usually means wrong
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IRIG signal format.</li>
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<li>The current second of the day is not exactly one greater than
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the previous one. This usually means a very noisy IRIG signal or
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insufficient CPU resources.</li>
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<li>An audio codec error (overrun) occured. This usually means
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insufficient CPU resources, as sometimes happens with Sun SPARC
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IPCs when doing something useful.</li>
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</ol>
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Note that additional checks are done elsewhere in the reference
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clock interface routines.
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<p>Unlike other drivers, which can have multiple instantiations,
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this one supports only one. It does not seem likely that more than
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one audio codec would be useful in a single machine. More than one
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would probably chew up too much CPU time anyway.</p>
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<h4>IRIG-B Timecode Format</h4>
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The 100 elements of the IRIG timecode are numbered from 0 through
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99. Position identifiers occur at elements 0, 9, 19 and every ten
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thereafter to 99. The control function (CF) elements begin at
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element 50 (CF 1) and extend to element 78 (CF 27). The
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straight-binary-seconds (SBS) field, which encodes the seconds of
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the UTC day, begins at element 80 (CF 28) and extends to element 97
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(CF 44). The encoding of elements 50 (CF 1) through 78 (CF 27) is
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device dependent. This driver presently decodes the CF elements,
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but does nothing with them.
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<p>Where feasible, the IRIG signal source should be operated with
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signature control so that, if the signal is lost or mutilated, the
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source produces an unmodulated signal, rather than possibly random
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digits. The driver will automatically reject the data and declare
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itself unsynchronized in this case. Some devices, in particular
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Spectracom radio/satellite clocks, provide additional year and
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status indication in the format:</p>
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<pre>
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Element CF Function
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-------------------------------------
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55 6 time sync status
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60-63 10-13 BCD year units
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65-68 15-18 BCD year tens
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</pre>
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Other devices set these elements to zero.
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<h4>Performance</h4>
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The mu-law companded data format allows considerable latitude in
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signal levels; however, an automatic gain control (AGC) function is
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implemented to further compensate for varying input signal levels
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and to avoid signal distortion. For proper operation, the IRIG
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signal source should be configured for analog signal levels, NOT
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digital TTL levels.
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<p>The accuracy of the system clock synchronized to the IRIG-B
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source with this driver and the <tt>ntpd</tt> daemon is 10-20 <font
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face="symbol">m</font>s with a Sun UltraSPARC II and maybe twice
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that with a Sun SPARC IPC. The processor resources consumed by the
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daemon can be significant, ranging from about 1.2 percent on the
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faster UltraSPARC II to 38 percent on the slower SPARC IPC.
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However, the overall timing accuracy is limited by the resolution
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and stability of the CPU clock oscillator and the interval between
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clock corrections, which is 64 s with this driver. This
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performance, while probably the best that can be achieved by the
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daemon itself, can be improved with assist from the PPS discipline
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as described elsewhere in the documentation.</p>
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<h4>Monitor Data</h4>
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The timecode format used for debugging and data recording includes
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data helpful in diagnosing problems with the IRIG signal and codec
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connections. With debugging enabled (-d on the ntpd command line),
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the driver produces one line for each timecode in the following
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format:
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<p><tt>00 1 98 23 19:26:52 721 143 0.694 47 20 0.083 66.5
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3094572411.00027</tt></p>
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<p>The first field containes the error flags in hex, where the hex
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bits are interpreted as below. This is followed by the IRIG status
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indicator, year of century, day of year and time of day. The status
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indicator and year are not produced by some IRIG devices. Following
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these fields are the signal amplitude (0-8100), codec gain (0-255),
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field phase (0-79), time constant (2-20), modulation index (0-1),
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carrier phase error (0±0.5) and carrier frequency error
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(PPM). The last field is the on-time timestamp in NTP format. The
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fraction part is a good indicator of how well the driver is doing.
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With an UltrSPARC 30, this is normally within a few tens of
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microseconds relative to the IRIG-B signal and within a few hundred
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microseconds with IRIG-E.</p>
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<h4>Fudge Factors</h4>
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<dl>
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<dt><tt>time1 <i>time</i></tt></dt>
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<dd>Specifies the time offset calibration factor, in seconds and
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fraction, with default 0.0.</dd>
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<dt><tt>time2 <i>time</i></tt></dt>
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<dd>Not used by this driver.</dd>
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<dt><tt>stratum <i>number</i></tt></dt>
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<dd>Specifies the driver stratum, in decimal from 0 to 15, with
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default 0.</dd>
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<dt><tt>refid <i>string</i></tt></dt>
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<dd>Specifies the driver reference identifier, an ASCII string from
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one to four characters, with default <tt>IRIG</tt>.</dd>
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<dt><tt>flag1 0 | 1</tt></dt>
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<dd>Not used by this driver.</dd>
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<dt><tt>flag2 0 | 1</tt></dt>
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<dd>Specifies the microphone port if set to zero or the line-in
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port if set to one. It does not seem useful to specify the compact
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disc player port.</dd>
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<dt><tt>flag3 0 | 1</tt></dt>
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<dd>Enables audio monitoring of the input signal. For this purpose,
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the speaker volume must be set before the driver is started.</dd>
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<dt><tt>flag4 0 | 1</tt></dt>
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<dd>Enable verbose <tt>clockstats</tt> recording if set.</dd>
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</dl>
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<h4>Additional Information</h4>
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<a href="refclock.htm">Reference Clock Drivers</a> <br>
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<a href="audio.htm">Reference Clock Audio Drivers</a>
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<hr>
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<a href="index.htm"><img align="left" src="pic/home.gif" alt=
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"gif"></a>
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2001-08-29 14:35:15 +00:00
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<address><a href="mailto:mills@udel.edu">David L. Mills
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<mills@udel.edu></a></address>
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</body>
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</html>
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1999-12-09 13:01:21 +00:00
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