freebsd-dev/contrib/ntp/html/driver6.htm
2001-08-29 14:35:15 +00:00

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<title>IRIG Audio Decoder</title>
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<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.</p>
<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>
<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 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>
<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. 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.</p>
<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 &plusmn;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.</p>
<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:</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</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.</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 signal amplitude (0-8100), codec gain (0-255),
field phase (0-79), time constant (2-20), modulation index (0-1),
carrier phase error (0&plusmn;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>
<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" alt=
"gif"></a>
<address><a href="mailto:mills@udel.edu">David L. Mills
&lt;mills@udel.edu&gt;</a></address>
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