[mythtv-users] HD-PVR SPDIF audio trashed at beginning of recordings

[John P Poet]

On Mon, Apr 6, 2009 at 2:55 PM, JS Boyd <mythtv at futures.com> wrote:
> Hello All,
>
> I was wondering if anyone else with an HD-PVR is getting from 1 second to 20
> minute burst of static in the audio at the beginning of their recordings?
>
> It's almost always only about 1 second, but sometimes much longer. Once the
> static disappears it never reoccurs within that recording. It's definitely
> in the stream being passed to Myth. Playing the files with mplayer produces
> the identical audio that the Myth Internal player plays. And given that Myth
> does not process HD-PVR files while recording, I suspect it is the HDPVR or
> maybe the cable box. Though directly connecting to the AV receiver via SPDIF
> from the cablebox produces no static.


Happens to me occasionally.  I have never had the static last more
than 2 seconds, though.

I was able to reduce the problem by waiting for the second "keyframe"
in Myth's recorder before starting to "record" the stream from the
HD-PVR.  However, that is the way it is in current trunk, so I am
surprised that you are having much of a problem there.

You could try H264Parser-fixes-v1.1.patch from
http://svn.mythtv.org/trac/ticket/6243 .  With that patch, I actually
get 1 second of static at the beginning *more* often then I do with
the committed code, but it might work better for you.  That patch is
actually more "correct" than what is currently committed to trunk.


>
> The HD-PVR is connected to a Motorola DCH3416 cablebox via SPDIF. The analog
> audio is disconnected. I am using the latest driver from the Haupauge
> website: 1.0.5.301


I have mine hooked up to a Directv HD receiver.


> The static seems to always occur after a channel change, but even then it
> does not always happen. I am using:


I am guessing that the static is caused by the AC3 stream being in the
middle of a frame when the recording starts.  So, it won't happen if
the recording starts at the beginning of a frame.


> mythchanger -f 6 -c
>
> over firewire to change the cablebox channels.


You could try adding a 1 or 2 second sleep at the end of the channel
changing script.  I have a 1 second sleep at the end of mine, and that
may be why this is not as much of a problem for me.

It takes the HD-PVR a relatively long time to "sync" up with the
incoming audio & video after a channel change (especially if the video
resolution changes).  Myth wants to start processing the data from the
HD-PVR *immediately* after the channel change script completes.
Sometimes myth tries to process the data before the HD-PVR has settled
down, and I think that may be part of the problem.


John
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