[mythtv-users] stereo and 5.1?

[Jean-Yves Avenard]

Hi

On Saturday, 8 January 2011, David Knight <dlknight at sdf.lonestar.org> wrote:

>
> However if a digital passthrough device is configured then the selected
> digital audio types (e.g. DD/DTS) will be passed through to the device
> specified in the advanced configuration?

That's right.
>
> Otherwise what is the point in specifying the passthrough device?

The passthru device is really an option that should have been
obsoleted a long time ago; it is unnecessary for 99% of the user.
Everything should be working without any options in the advanced audio
setup modified. The main audio device will be configured automatically
for passthru should passthru audio had been enabled (ac3, dts etc)

I only left that option for the maybe dozen users that do want to use
different output.

>
> As I previously stated mythtv doesn't send the DD/DTS in its raw form to
> the passthrough device. Shouldn't it be doing so?

If you have configured the setup for 5.1 or 7.1 audio, checked that
AC3 and DTS were available then yes passthrough will be used for multi
channels audio under some circumstances.

Here are some examples were passthrough will not be used:
Your main audio device supports multi channels audio (analog or
digital such as hdmi) *and* some audio processing will be applied.

Audio processing includes: resampling, reformatting, volume control etc.

Myth has to decode the audio to perform those. As it detects that the
main audio device supports natively multichannel audio, once decoded,
it will NOT re-encore to AC3 or DTS. There's no point, it only slows
things down and introduce even more audio quality loss.

Now let's look at your particular case
You have set your main audio device as hdmi which would report up to 7.1 audio.
You have set a passthrough device as ALSA:iec958:{ AES0=6 } (the 6
here got nothing to do wil the number of channels)

You are now playing a 5.1 AC3. Somehow, due to your setup, audio
processing is required eg:
* input is 44.1kHz but you have checked to force 48kHz output -> it
must resampled
* you have enabled software volume control

So the audio is decoded, processing applied. As your main audio device
is multi channels capable and you have configured myth for 5.1 or 7.1
speakers

Now in your case, checking the "stereoPCM only" option would fix it
because after decoding the audio, myth will assume now that discrete
PCM isn't possible, and will re-encode to AC3.

Note that if you didn't check the force 48k resampling, audio would
probably not had to be decoded to start with, and disable software
volume control too.

At the end of the day, myth is doing exactly what you asked it to do
through your configuration; if you screwed it up like checking option
that shouldn't have been, the it will have a screwed up behaviour too.

>
> If I am understanding correctly then maybe two speaker configuration
> options are required? one for the main output device and one for the pass
> through device?

I will not add those. Myth supports one audio configuration and only
one.. I will not add multi-audio output at once ; it's too
complicated. There are ways to achieve this with either configuring
pulse audio or setting an alsa mixer.

JY