243 lines
11 KiB
HTML
243 lines
11 KiB
HTML
<html><head><title>
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IRIG Audio Decoder
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</title></head><body><h3>
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IRIG Audio Decoder
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</h3><hr>
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<H4>Synopsis</H4>
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Address: 127.127.6.<I>u</I>
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<BR>Reference ID: <TT>IRIG</TT>
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<BR>Driver ID: <TT>IRIG_AUDIO</TT>
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<BR>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, address
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and ID which required replacing the original kernel audio driver with
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another which works only on older Sun SPARCstation systems. The new
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driver described here uses the stock kernel audio driver and works in
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SunOS 4.1.3 and Solaris 2.6 versions and probably all versions in
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between. The new driver requires no modification of the operating
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system. While it is generic and likely portable to other systems, it is
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somewhat slower than the original, since the extensive signal
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conditioning, filtering and decoding is done in user space, not kernel
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space.
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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 some
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workstations. This signal is generated by several radio clocks,
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including those made by Arbiter, Austron, Bancomm, Odetics, Spectracom
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and TrueTime, among others, although it is often an add-on option. The
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signal is connected via an optional attenuator box and cable to either
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the microphone or line-in port. The driver receives, demodulates and
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decodes the IRIG-B and IRIG-E signal formats using internal filters
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designed to reduce the effects of noise and interference.
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<p>This driver incorporates several features in common with other audio
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drivers such as described in the <a href=driver7.htm>Radio CHU Audio
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Demodulator/Decoder</a> and the <a href=driver36.htm>Radio WWV/H Audio
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Demodulator/Decoder</a> pages. They include automatic gain control
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(AGC), selectable audio codec port and signal monitoring capabilities.
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For a discussion of these common features, as well as a guide to hookup,
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debugging and monitoring, see the <a href=audio.htm>Reference Clock
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Audio Drivers</a> page.
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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 is
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1000 Hz and bit rate 100 b/s; for IRIG-E, the carrier frequenchy is 100
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Hz and bit rate 10 b/s. While IRIG-B provides the best accuracy,
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generally within a few tens of microseconds relative to IRIG time, it
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can also generate a significant load on the processor with older
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workstations. Generally, the accuracy with IRIG-E is about ten times
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worse than IRIG-B, but the processor load is ten times less.
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<P>The program processes 8000-Hz mu-law companded samples using separate
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signal filters for IRIG-B and IRIG-E, a comb filter, envelope detector
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and automatic threshold corrector. Cycle crossings relative to the
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corrected slice level determine the width of each pulse and its value -
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zero, one or position identifier. The data encode 20 BCD digits which
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determine the second, minute, hour and day of the year and sometimes the
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year and synchronization condition. The comb filter exponentially
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averages the corresponding samples of successive baud intervals in order
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to reliably identify the reference carrier cycle. A type-II phase-lock
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loop (PLL) performs additional integration and interpolation to
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accurately determine the zero crossing of that cycle, which determines
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the reference timestamp. A pulse-width discriminator demodulates the
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data pulses, which are then encoded as the BCD digits of the timecode.
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The timecode and reference timestamp are updated once each second with
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IRIG-B (ten seconds with IRIG-E) and local clock offset samples saved
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for later processing. At poll intervals of 64 s, the saved samples are
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processed by a trimmed-mean filter and used to update the system clock.
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<P>Infinite impulse response (IIR) filters are used with both IRIG-B and
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IRIG-E formats. An 800-Hz highpass filter is used for IRIG-B and a
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130-Hz lowpass filter for IRIG-E. These are intended for use with noisy
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signals, such as might be received over a telephone line or radio
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circuit, or when interfering signals may be present in the audio
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passband. The driver determines which IRIG format is in use by sampling
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the amplitude of each filter output and selecting the one with maximum
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signal. An automatic gain control feature provides protection against
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overdriven or underdriven input signal amplitudes. It is designed to
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maintain adequate demodulator signal amplitude while avoiding occasional
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noise spikes. In order to assure reliable capture, the decompanded input
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signal amplitude must be greater than 100 units and the codec sample
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frequency error less than 250 PPM (.025 percent).
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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 format
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or improper hardware configuration. Specifically, if any of the
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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.
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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 port.</LI>
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<LI>The frequency error is greater than ±250 PPM (.025 percent).
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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 overdriven
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IRIG signal or wrong IRIG format.</LI>
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<LI>A frame synchronization error has occured. This usually means wrong
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IRIG signal format or the IRIG signal source has lost synchronization
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(signature control).</LI>
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<LI>A data decoding error has occured. This usually means wrong IRIG
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signal format.</LI>
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<LI>The current second of the day is not exactly one greater than the
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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 IPCs
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when doing something useful.</LI>
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</OL>
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Note that additional checks are done elsewhere in the reference clock
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interface routines.
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<P>Unlike other drivers, which can have multiple instantiations, this
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one supports only one. It does not seem likely that more than one audio
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codec would be useful in a single machine. More than one would probably
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chew up too much CPU time anyway.
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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 99.
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Position identifiers occur at elements 0, 9, 19 and every ten thereafter
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to 99. The control function (CF) elements begin at element 50 (CF 1) and
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extend to element 78 (CF 27). The straight-binary-seconds (SBS) field,
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which encodes the seconds of the UTC day, begins at element 80 (CF 28)
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and extends to element 97 (CF 44). The encoding of elements 50 (CF 1)
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through 78 (CF 27) is device dependent. This driver presently decodes
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the CF elements, 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 itself
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unsynchronized in this case. Some devices, in particular Spectracom
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radio/satellite clocks, provide additional year and status indication in
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the format:
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<PRE> 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 signal
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levels; however, an automatic gain control (AGC) function is implemented
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to further compensate for varying input signal levels and to avoid
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signal distortion. For proper operation, the IRIG signal source should
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be configured for analog signal levels, NOT digital TTL levels.
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<P>The accuracy of the system clock synchronized to the IRIG-B source
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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 that with
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a Sun SPARC IPC. The processor resources consumed by the daemon can be
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significant, ranging from about 1.2 percent on the faster UltraSPARC II
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to 38 percent on the slower SPARC IPC. However, the overall timing
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accuracy is limited by the resolution and stability of the CPU clock
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oscillator and the interval between clock corrections, which is 64 s
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with this driver. This performance, while probably the best that can be
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achieved by the daemon itself, can be improved with assist from the PPS
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discipline as described elsewhere in the documentation.
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<H4>Monitor Data</H4>
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The timecode format used for debugging and data recording includes data
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helpful in diagnosing problems with the IRIG signal and codec
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connections. With debugging enabled (-d on the ntpd command line), the
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driver produces one line for each timecode in the following 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>
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<p>The first field containes the error flags in hex, where the hex bits
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are interpreted as below. This is followed by the IRIG status indicator,
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year of century, day of year and time of day. The status indicator and
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year are not produced by some IRIG devices. Following these fields are
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the signal amplitude (0-8100), codec gain (0-255), field phase (0-79),
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time constant (2-20), modulation index (0-1), carrier phase error
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(0±0.5) and carrier frequency error (PPM). The last field is the
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on-time timestamp in NTP format. The fraction part is a good indicator
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of how well the driver is doing. With an UltrSPARC 30, this is normally
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within a few tens of microseconds relative to the IRIG-B signal and
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within a few hundred microseconds with IRIG-E.
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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 default
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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 one
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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 port if
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set to one. It does not seem useful to specify the compact disc player
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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, the
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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>
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<br><A HREF="audio.htm">Reference Clock Audio Drivers</A>
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<hr><a href=index.htm><img align=left src=pic/home.gif></a><address><a
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href=mailto:mills@udel.edu> David L. Mills <mills@udel.edu></a>
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</address></a></body></html>
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