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

John P Poet jppoet at gmail.com
Thu Apr 23 17:54:38 UTC 2009


On Sat, Apr 11, 2009 at 8:38 AM, JS Boyd <mythtv at futures.com> wrote:

> John P Poet wrote:
>>
>> 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
>>
>
>
> I found that if I used the slower channel change over firewire command:
>
> mythchanger -f 7 -c
>
> and added a 1 second sleep after that channel change that the noise
> disappeared.
>
> But.... I get a new but more pleasant side effect: The audio will appear
> immediately in the recording and then disappear about a second or two into
> the recording and then begin again at the next scene change always within 3
> to 10 seconds, which appears to be the next keyframe.
>
> Thank you John for the info and insight.
>
> JS.


I have been running a new patch for the last couple of weeks which has
completely eliminated any AC3 audio static at the beginning of HD-PVR
recordings.  Unfortunately, it makes LiveTV channel changes more
likely to fail (due to timing out, I think).

If you want the patch, just let me know.


John
-- 
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