[mythtv-users] 32khz AC3 Streams was: Sound problem with AC3 passthrough

[Steve Hodge]

On 6/7/06, Steven Adeff <adeffs.mythtv at gmail.com> wrote:
> On 6/6/06, Steve Hodge <stevehodge at gmail.com> wrote:
> > I know in my case that the soundcard can only output 44.1kHz and 48kHz
> > spdif. Maybe it's the same with the nforce2?
>
> I was under the impression that AC3 passthrough did not touch the AC3
> stream, thus the "passthrough" convention.
>
> In other words, the software should not care what the AC3 stream
> holds, onec it sees the AC3 header it should just pass the stream
> untouched through the SPDIF output.

Passthrough means that the bitstream is not altered between the source
and the spdif output. However you still need to send those bits at the
correct rate. The bitrate of the SPDIF connection (32kHz, 44.1kHz or
48kHz) is something that needs to be set.

That's why we're getting audio that is too fast in some cases - alsa
is outputting the ac3 stream at 48kHz instead of 32kHz. But the stream
is unmodified and still valid and so the receiver can play it back
just fine, if a bit fast.

> Is it possible your decoder is unable to handle the 32khz stream?

Which is more likely to not support a particular format: a $1200
receiver or a $25 sound card? Seriously though, I know for a fact that
the soundcard does not support spdif output at 32kHz. It specifically
states that it only supports 44.1 and 48kHz. I haven't been able to
find anything in the manual of the receiver one way or the other and I
can't test it since I don't have anything that outputs a 32kHz spdif
signal.

Steve