[mythtv-users] Simultaneous analogue (analog) and S/PDIF output

[Jean-Yves Avenard]

On 14 January 2011 08:53, Alex Butcher <mythlist at assursys.co.uk> wrote:

Because you've set:

>
> | AC3PassThru            | 0
> | DTSPassThru            | 0

then

> | PassThruOutputDevice   | ALSA:iec958:AES0=6
> | PassThruDeviceOverride | 1

will never be used.

And if:

> | MaxChannels            | 6

and

> | StereoPCM              | 1

then you never get more than stereo. So just as well set MaxChannels to stereo.


> | AudioOutputDevice      | ALSA:ajb
> | MythControlsVolume     | 1
> | MixerDevice            | ALSA:default
> | MixerControl           | PCM
>
> Together, this provides simultaneous stereo output on both the S/PDIF and
> the analogue front channels from my (stereo) UK DVB-T recordings and from
> DD2.0/DD5.1/DTS DVD sources. :-)

yes, because you've configured the audio in such a way that
multichannels audio is just not possible

>
> Because StereoPCM is set to 1 (enabled), multi-channel audio (e.g.  from
> Dolby Digital 5.1/DTS DVDs) is re-encoded to stereo.  Ah well, I've got a
> real DVD player for when I want surround sound from DVDs.  Setting
> AC3PassThru and/or DTSPassThru to 1 (enabled) silences the analogue output.
> :-(

this is to be expected. I already mentioned that:
-Stereo will go to both analog+spdif
-Multichannels will only go out via spdif

> If I unset StereoPCM and set AC3PassThru and DTSPassThru, I merely get
> distorted versions of the DD5.1 and DTS soundtracks on both S/PDIF and the

you can not unset stereoPCM, as iec958 output only supports stereo.

> If I set StereoPCM, AC3PassThru and DTSPassThru to 1 (enabled) and unset
> MythControlsVolume, thereby disabling MythTV's internal volume controls,
> i.e.  completely to Jean-Yves' recommendations, then the analogue outputs
> remain silent, but the AC3 and DTS passthru seems to work correctly apart
> from no LFE output.  Maybe that's the source DVD I was using, but I don't
> think so. I'd rather have analogue output and only 2.0 digital output from
> DD5.1/DTS sources.

if StereoPCM is set, and you have put a digital passthrough override,
then only the iec958 output will output something when playing
multichannels audio.

If you want to try digital passthrough through both analog and digital
(e.g. hear static on the analog), then disable the digital passthrough
override ; but to be honest, I have doubt digital audio will survive
being mixed by alsa like that.
I don't know why would you want to do that, it would sound awful anyway.

>
> Maybe there's a tweak someone can suggest to my .asoundrc that will allow
> AC3 and DTS passthru to work with >2 channel output, without silencing the
> output to the analogue channels, but unless that happens, I think I've had

again, you can't. I thought this was pretty clear why.

if you have set digital passthrough override *and* configure AC3/DTS
passthrough, then multi-channels audio will use the device configured
for digital and not the main audio device.

> enough of fiddling with ALSA for the forseeable future.  Or maybe it's just
> a bug in 0.24-fixes and the StereoPCM stuff. I'll let others pick through
> that.

certainly isn't.

Why would you expect audio to go out your analog device, when you've
configured it to go through another one (ALSA:iec958:AES0-6)

If stereo then audio goes out through ALSA:ajb
if 5.1 and audio is AC3 or DTS audio goes out through ALSA:iec958:AES0=6
If 5.1 and audio is not AC3 or DTS or if any audio processing is to be
performed, then audio gets re-encoded to AC3 and go out through
ALSA:iec958:AES0=6 (as StereoPCM is set)

You can NOT have multichannels digital audio working as the same time
as analog audio other than getting static (well, not exactly static,
just an AC3/DTS encoded stream).
Myth only output one stream of data at any given time.