[mythtv] Tinny audio in some Hauppauge PVR recordings

John Biundo johnbiundo at sbcglobal.net
Thu Apr 6 05:58:03 UTC 2006


A number of people have experienced "tinny audio" (low volume, clipped 
sound) on some recordings (approximately 15-20% are "tinny", the rest 
are normal) made with mythtv and the Hauppauge PVR cards.  This problem 
has been discussed at some length on the ivtv list.  Recently, several 
of us have found that the newest cx25840 firmware prerelease seems to 
have a positive impact on the problem.

Previously, the problem could be easily reproduced by running a bash 
script with an infinite loop containing the command

ivtvctl -q0

While viewing a livetv stream in mythtv.

This command will *always* correct the tinny audio problem if it's 
occuring, but will also, *sometimes*, cause a stream with good audio to 
become tinny.  In other words, it reliably toggles a bad stream to good, 
but also sometimes toggles a good stream to bad.  (So, it's 
unfortunately not suitable as a "brute force" method to correct the 
problem; blindly running this command will sometimes corrupt a good stream).

As of the latest firmware pre-release, it seems that one can no longer 
induce the tinny audio problem directly through ivtvctl.  This was a 
promising development.

Unfortunately, the problem still does occur on some percentage of mythtv 
recordings.  Early findings suggest that the problem occurs *less 
frequently* with this new firmware release (perhaps now only 10% of the 
time), but it still does occur.

The problem appears to be somewhat stalled in ivtv-land (due to lack of 
resources).  I'm wondering if there might be something that could be 
done from the mythtv side of things.  I wonder whether we can infer 
anything useful about the way mythtv interfaces to ivtv that might 
suggest a fix or a workaround for the problem from these experimental 
results?  While it's unclear exactly what triggers the problem in 
mythtv, one fairly reliable mechanism of triggering it directly in ivtv 
no longer does so.

The obvious "brute force" solution of running a periodic script (say 
through a cron job running every n minutes) with ivtvctl -q0, while now 
possible, has the undesireable side effect of interrupting the audio 
stream momentarily -- resulting in an audible "skip".  If there were a 
way for the user to trigger invocation of a shell script with this 
command at the beginning of every recording, that might work.  I'm not 
aware of such a capability without modifying source code.  While I don't 
have a lot of experience, I might be tempted to try to modify the source 
code to do just this if I get some feedback from this list suggesting 
that's a viable solution.

Looking forward to any comments or suggestions.


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