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diff --git a/html/driver6.htm b/html/driver6.htm new file mode 100644 index 0000000..501f697 --- /dev/null +++ b/html/driver6.htm @@ -0,0 +1,271 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<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.</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 ±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±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 +<mills@udel.edu></a></address> +</body> +</html> + |
