Update of /usr/cvsroot/asterisk/channels
In directory mongoose.digium.com:/tmp/cvs-serv17874/channels
Modified Files:
chan_iax2.c
Log Message:
add explanatory comment about timestamp compensation (bug #4763)
Index: chan_iax2.c
===================================================================
RCS file: /usr/cvsroot/asterisk/channels/chan_iax2.c,v
retrieving revision 1.322
retrieving revision 1.323
diff -u -d -r1.322 -r1.323
--- chan_iax2.c 20 Jul 2005 22:46:58 -0000 1.322
+++ chan_iax2.c 25 Jul 2005 23:13:20 -0000 1.323
@@ -3420,11 +3420,23 @@
if (voice) {
/* On a voice frame, use predicted values if
appropriate */
if (p->notsilenttx && abs(ms - p->nextpred) <=
MAX_TIMESTAMP_SKEW) {
- /* Adjust our txcore, keeping voice and
- non-voice synchronized */
- /* We need someone who understands this code to
comment here on
- why the 'adjust' value is handled as if it
was in units
- of 10,000 microseconds, instead of
milliseconds
+ /* Adjust our txcore, keeping voice and
non-voice synchronized */
+ /* AN EXPLANATION:
+ When we send voice, we usually send
"calculated" timestamps worked out
+ on the basis of the number of samples sent.
When we send other frames,
+ we usually send timestamps worked out from
the real clock.
+ The problem is that they can tend to drift
out of step because the
+ source channel's clock and our clock may not
be exactly at the same rate.
+ We fix this by continuously "tweaking"
p->offset. p->offset is "time zero"
+ for this call. Moving it adjusts timestamps
for non-voice frames.
+ We make the adjustment in the style of a
moving average. Each time we
+ adjust p->offset by 10% of the difference
between our clock-derived
+ timestamp and the predicted timestamp.
That's why you see "10000"
+ below even though IAX2 timestamps are in
milliseconds.
+ The use of a moving average avoids offset
moving too radically.
+ Generally, "adjust" roams back and forth
around 0, with offset hardly
+ changing at all. But if a consistent
different starts to develop it
+ will be eliminated over the course of 10
frames (200-300msecs)
*/
adjust = (ms - p->nextpred);
if (adjust < 0)
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