[mythtv-users] What would cause "crackling" audio in some video playback

William william_munson at comcast.net
Tue Dec 8 02:58:41 UTC 2009


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
>
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>
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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