[mythtv-users] spdif working but not passthrough - AV-710

Steve Hodge stevehodge at gmail.com
Sun Oct 30 17:45:14 EST 2005

On 10/31/05, Byron Poland <wpoland at gmail.com> wrote:
> So thats crazy.  I got it to work with mplayer with just the the -ac
> hwac3 option.  It didn't work with the -ao alsa:device=hw=0.2 in the
> command.

"-ac hwac3" tells mplayer to pass AC3 signals though without decoding.
If you don't have that mplayer will decode them to separate channels
so you definitely needed that. The device setting may or may not be
necessary depending on what the hardware does and what the default
device is. On my card I need to specify the device but I believe some
cards will send everything from the default analogue output is also
output to the digital outputs, provided the format is correct. Sounds
like yours might be like that, or maybe you've got spdif setup as the
default alsa device?

> So I think I don't have to specify a device.

It's safest to specify a device anyway so that it is obvious what is
going on. Otherwise if you change sound cards you might find it stops
working and it will be less clear where the problem lies.

> I have no idea why it never worked earlier, as I know this was one of
> the options I had tried.  only thing I did differently was got mplayer
> to work inbetween Mythtv tries.

I haven't used AC3 passthrough with MythTV so I'm not sure what was
wrong there. The reason mplayer didn't work previously was the missing
"-ac hwac3" option. I don't see why getting that to work would affect
MythTV so that is a bit of a mystery.

Incidentally with mplayer you should have a comma on the end of the ac
option, e.g. "-ac hwac3,". This tells mplayer to try the default audio
output codec if the hardware ac3 passthrough doesn't work, as in the
case of a non AC3 signal. The option I use is "-ac hwdts,hwac3,". This
tries DTS passthrough first, then AC3 passthrough, then the default
audio codec. That way if a video has several audio streams it will
favour DTS over AC3.


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