Mailing List Archive: MythTV: Users

What would cause "crackling" audio in some video playback

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mythtv at derdev

Dec 7, 2009, 6:17 AM

Post #1 of 4 (724 views)
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What would cause "crackling" audio in some video playback

Was showing off my mythtv system over the weekend and some videos
unexpectedly had some crackling in the audio playback. I didn't take
inventory of which videos by file type or codec detail had the issue but I
know that they were not from TV shows that myth had recorded. Others
played fine. The ones that crackled weren't particularly intense audio
passages nor was the video hi-end HD.

Since my kit is SPDIF/Alsa audio, I could only think of a few reasons and
would appreciate some tips...

1) Heat in the FE messing with the works
2) The files themselves actually have the crackling embedded (will be
checking with my Windows laptop tonight)
3) The codecs for selected audio/video are interpreting a channel in a way
that leads to crackling - or the myth audio upsampling code was
introducing this crackling (how would I figure this out?)

I wasn't playing at particularly loud levels so I'm not thinking I taxed
the receiver or speakers to any measure.

Is is possible that an SDPIF cable could have a flaw that would lead to
such a thing? Doesn't seem possible.

THANKS!

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aaron at rb303

Dec 7, 2009, 8:10 AM

Post #2 of 4 (698 views)
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Re: What would cause "crackling" audio in some video playback [In reply to]

I have seen a couple threads on this recently, and have witnessed
first hand some of it. My issue was playing DTS audio via SPDIF;
Dolby Digital plays fine.

One note, I switched to XBMC for my movies and have no audio issues
(everything plays fine) So it really leads me to believe the problem
is in Myth. I also do not have issues playing DTS tracks via Mplayer,
Xine, or VLC.

I personally can't say it's a ALSA issue because the other players play fine.

Some threads to look at regarding this issue:
[mythtv-users] Stuttering audio with DTS passthrough in 0.22-fixes
[mythtv-users] some audio channels cause, clicky static sounds
[mythtv-users] audio issue using Internal player
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mtdean at thirdcontact

Dec 7, 2009, 2:37 PM

Post #3 of 4 (690 views)
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Re: What would cause "crackling" audio in some video playback [In reply to]

On 12/07/2009 09:17 AM, Dave Richardson wrote:
> Was showing off my mythtv system over the weekend and some videos
> unexpectedly had some crackling in the audio playback. I didn't take
> inventory of which videos by file type or codec detail had the issue but I
> know that they were not from TV shows that myth had recorded. Others
> played fine. The ones that crackled weren't particularly intense audio
> passages nor was the video hi-end HD.
>
> Since my kit is SPDIF/Alsa audio, I could only think of a few reasons and
> would appreciate some tips...
>
> 1) Heat in the FE messing with the works
> 2) The files themselves actually have the crackling embedded (will be
> checking with my Windows laptop tonight)
> 3) The codecs for selected audio/video are interpreting a channel in a way
> that leads to crackling - or the myth audio upsampling code was
> introducing this crackling (how would I figure this out?)
>
> I wasn't playing at particularly loud levels so I'm not thinking I taxed
> the receiver or speakers to any measure.
>
> Is is possible that an SDPIF cable could have a flaw that would lead to
> such a thing? Doesn't seem possible.

Extra audio buffering
Enable this setting if MythTV is playing "crackly" audio. This setting
affects digital tuners (QAM/DVB/ATSC) and hardware encoders. It will
have no effect on framegrabbers (MPEG-4/RTJPEG). MythTV will keep extra
audio data in its internal buffers to workaround this bug.

(which, BTW, is enabled by default).

Mike

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william_munson at comcast

Dec 7, 2009, 6:58 PM

Post #4 of 4 (683 views)
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Re: What would cause "crackling" audio in some video playback [In reply to]

Michael T. Dean wrote:
> On 12/07/2009 09:17 AM, Dave Richardson wrote:
>> Was showing off my mythtv system over the weekend and some videos
>> unexpectedly had some crackling in the audio playback. I didn't take
>> inventory of which videos by file type or codec detail had the issue
>> but I
>> know that they were not from TV shows that myth had recorded. Others
>> played fine. The ones that crackled weren't particularly intense audio
>> passages nor was the video hi-end HD.
>>
>> Since my kit is SPDIF/Alsa audio, I could only think of a few reasons
>> and
>> would appreciate some tips...
>>
>> 1) Heat in the FE messing with the works
>> 2) The files themselves actually have the crackling embedded (will be
>> checking with my Windows laptop tonight)
>> 3) The codecs for selected audio/video are interpreting a channel in
>> a way
>> that leads to crackling - or the myth audio upsampling code was
>> introducing this crackling (how would I figure this out?)
>>
>> I wasn't playing at particularly loud levels so I'm not thinking I taxed
>> the receiver or speakers to any measure.
>>
>> Is is possible that an SDPIF cable could have a flaw that would lead to
>> such a thing? Doesn't seem possible.
>
> Extra audio buffering
> Enable this setting if MythTV is playing "crackly" audio. This
> setting affects digital tuners (QAM/DVB/ATSC) and hardware encoders.
> It will have no effect on framegrabbers (MPEG-4/RTJPEG). MythTV will
> keep extra audio data in its internal buffers to workaround this bug.
>
> (which, BTW, is enabled by default).
>
> Mike
>
> _______________________________________________
> mythtv-users mailing list
> mythtv-users [at] mythtv
> http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users
>
I have that same problem with some videos and have the same audio setup.
My buffering is enabled and it still happens. I have noticed that it
only occurs on the initial sound at the start of the first thing spoken
after a short period of silence. It sounds very much like there is an
AGC circuit in effect pumping up the audio gain which then overloads the
decoder until the gain cuts back again.
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