[mythtv-users] "Woodchuck" sound on TV playback

Allen Edwards allen.edwards at oldpaloalto.com
Sun Aug 17 21:34:12 UTC 2008


On Sun, Aug 17, 2008 at 1:17 PM, John Welch <jrw3319 at gmail.com> wrote:
> On Sun, Aug 17, 2008 at 3:50 PM, Michael T. Dean
> <mtdean at thirdcontact.com> wrote:
>> On 08/17/2008 03:01 PM, John Welch wrote:
>>> Yes!  Thank you, thank you, thank you!  My problem is solved.  The
>>> keys were the changes in steps 3 & 4.
>>>
>>> Maybe this is explained elsewhere, but I couldn't find it, so can
>>> anyone tell me what "ALSA:iec958:(AESO 0x02)" means, particularly the
>>> "AES 0x02" part?
>>
>> Says to set the IEC958 status bit for data type to non-audio (i.e.
>> encoded audio formats, like AC-3 or DTS).
>>
>>>   Also, can anyone tell me how to pass this along to
>>> mplayer?
>>
>> Well, you can do it like you're doing with Myth, by specifying a
>> partially correct***, but completely unintelligible, /ALSA/
>> configuration string within the settings for that one application
>> (something like:  mplayer -ao "alsa:device="iec958={CARD 0 AES0 0x02}"
>> -ac hwac3, /path/to/file), or you can set up your ALSA configuration
>> correctly and give the configuration a useful and easy to remember name,
>> like "passthrough" that can be used for any and all applications (i.e.
>> Myth "Passthrough output device" would be "passthrough", mplayer could
>> be started with mplayer -ao alsa:device=passthrough -ac hwac3,
>> /path/to/file).
>>
>> Oh, but wait, I must not know what I'm talking about because the wiki
>> says that you don't need an ALSA configuration file...
>>
>> Mike "doing it wrong because he puts ALSA configuration in the, er, ALSA
>> configuration file" Dean
>>
>> *** "partially correct" meaning that you really should specify all the
>> IEC958 status bits because someone may have left them set for some other
>> type of audio...  For Myth's HDTV recordings, AES0 0x06, AES1 0x82, AES2
>> 0x00, AES3=0x02 is probably what you'd want.
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>>
> Thanks for the explanation Mike.  BTW, I have had success using an
> ALSA configuration file in the past, so I'm certainly not calling you
> "wrong".  It just didn't seem to be working for me in this case; hence
> my calls for help.
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The issue for me with setting up an ALSA configuration file is that
ALSA will take the bitstream, decode it, manipulate it per the
configuration file, then encode it (poorly) before sending it to the
output.

I really wanted ALSA to do nothing, I just wanted the spdif
information sent unaltered to the output so that my external decoder
could deal with the pure bitstream.

The other problem I read is that people end up with bit rate specific
files.  You really don't want ALSA changing the bit rate for any
device.

The goal is bit true audio...

Allen


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