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

Allen Edwards allen.edwards at oldpaloalto.com
Sun Aug 17 21:37:14 UTC 2008


On Sun, Aug 17, 2008 at 2:34 PM, Allen Edwards
<allen.edwards at oldpaloalto.com> wrote:
> 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
>

I should have added, my understanding was that newer versions of ALSA
have built in default configurations that work well so it is not that
you don't need a config file, but that the built in one is a good one
to use.  Am I wrong?

Allen


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