[mythtv-users] Digital audio: centre channel drops out intermittently

Marc Sherman msherman at projectile.ca
Mon Jan 16 02:16:31 UTC 2012


On 1/15/2012 6:26 PM, Jean-Yves Avenard wrote:
>
> You can't do upmixing 2->5 outside of myth (say using ALSA custom config).
> If that's what you are hoping to achieve it will never work as spdif
> is stereo maximum, everything more must be compressed into a digital
> stream
>
> you wrote in the previous post that upmixing was disabled in your myth
> config. I can only go by the information you provide.

It is disabled in my myth config. The only upmixing that is enabled (and 
it is 2->5 upmixing) is in my JVC amp. The only reason I mentioned that 
was to note that when MythTV is playing a recording with a stereo audio 
track, the amp upmixes it to 5 channels and drives the centre channel 
with no audible dropouts. So that establishes that the amp is capable of 
sending output to the centre channel without dropouts.

The dropouts only occur when MythTV is playing a recording with a 5.1 
audio track. In that situation, the centre channel drops out intermittently.

Sorry if I confused matters by describing too many different scenarios 
all in one email initially.

>> Well, if myth is decoding/processing/reencoding the AC3 stream as I
>> suggested above, then it could be dropping one of the channels in this code
>> path.
>>
>
> You obviously have a clear idea of what is happening, so I'll leave you to it

No, I really don't. All I think I've done is establish that it could 
possibly be a mythtv bug (possibly where it re-encodes AC3 5.1 audio), 
or it could possibly be a hardware/firmware failure in my jvc amp when 
it decodes the AC3 audio. I really have no idea how to proceed to 
isolate this further, so I can report a proper bug or replace the broken 
amp, as needed. Any suggestions would be greatly appreciated.

One thing I did try this morning is configure the frontend with 
"--verbose playback,audio,libav". I then tried watching a recording that 
had previously had dropouts (last night's SNL broadcast) twice back to 
back, and no dropouts occurred. So either I've just got terrible luck, 
or perhaps the bug was a race of some kind and the extra verbose logging 
has tweaked the timings enough to avoid triggering it. :/

I'll keep an eye on it, and if it recurs, I'll check the logs to see if 
anything strange shows up.

- Marc


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