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<html><head><title>
IRIG Audio Decoder
</title></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 works only on older Sun SPARCstation systems. The new
driver described here uses the stock kernel audio driver and works in
SunOS 4.1.3 and Solaris 2.6 versions and probably all versions in
between. The new driver requires no modification of the operating
system. 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.
<H4>Description</H4>
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>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=driver36.htm>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.htm>Reference Clock
Audio Drivers</a> page.
<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>The program processes 8000-Hz mu-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>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>The program performs a number of error checks to protect against
overdriven or underdriven input signal levels, incorrect signal format
or improper hardware configuration. Specifically, if any of the
following errors occur for a timecode, the data are rejected.
Secifically, if any of the following errors occur for a time
measurement, the data are rejected.
<OL>
<LI>The peak carrier amplitude is less than 100 units. This usually
means dead IRIG signal source, broken cable or wrong input port.</LI>
<LI>The frequency error is greater than ±250 PPM (.025 percent).
This usually means broken codec hardware or wrong codec
configuration.</LI>
<LI>The modulation index is less than 0.5. This usually means overdriven
IRIG signal or wrong IRIG format.</LI>
<LI>A frame synchronization error has occured. This usually means wrong
IRIG signal format or the IRIG signal source has lost synchronization
(signature control).</LI>
<LI>A data decoding error has occured. This usually means wrong IRIG
signal format.</LI>
<LI>The current second of the day is not exactly one greater than the
previous one. This usually means a very noisy IRIG signal or
insufficient CPU resources.</LI>
<LI>An audio codec error (overrun) occured. This usually means
insufficient CPU resources, as sometimes happens with Sun SPARC IPCs
when doing something useful.</LI>
</OL>
Note that additional checks are done elsewhere in the reference clock
interface routines.
<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.
<H4>IRIG-B Timecode Format</H4>
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>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:
<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</H4>
The mu-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>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 and maybe twice that with
a Sun SPARC IPC. 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 the documentation.
<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>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 signal 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.
<H4>Fudge Factors</H4>
<DL>
<DT><TT>time1 <I>time</I></TT></DT>
<DD>Specifies the time offset calibration factor, 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>IRIG</TT>.</DD>
<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>
|
