[mythtv-users] Quick survey regarding passthrough device settings

Jean-Yves Avenard jyavenard at gmail.com
Tue Apr 20 15:55:12 UTC 2010


On 21 April 2010 01:13, Allen Edwards <allen.p.edwards at gmail.com> wrote:
> So, while I say that I want my MythTV box to do nothing to the digital
> stream, I may not mean that.  What I mean is that I don't want it decoding
> the audio in floating point anything and then re-encoding it. I want the
> digital audio information sent to my pre-amp and let it figure out how to
> split that up to my 2.0 TV or 4.1 surround system (I have no center speaker
> because my Thiel 3.6 fronts are so much better than my Theil 2.0 center that
> the system sounds better without the center).

This is exactly what is happening now and in future update.
No conversion/processing is done unless the user specifically
requested it or is trying to play content that its hardware doesn't

That is:
-software volume
-resampling (like the audio card doesn't support the original sample rate)
-downconversion (24 bits depth audio -> 16 bits)

You are using SPDIF, as far as ALSA is concern, this audio card only
supports stereo ; so for any multi-channels stuff to occur, the data
must be sent as passthrough or (if processing was requested)
re-encoded as AC3.

In your case, you will keep you existing settings, that is a 5.1
channel configuration.

The only processing that will occur is if you are trying to play > 16
bits audio (E-AC3, TrueHD etc), in which case the audio will be
converted from 24 bits depth to 16 bits and if the audio is 7.1, it
will be downmixed to 5.1

Better applying some processing than hearing nothing is my opinion.

For people with a system that supports native 5.1 without requiring
AC3 or DTS), typically HDMI people: if no processing was required, an
AC3 or DTS track will be passthrough.
If processing is required (see above), the audio however will not be
re-encoded into AC3, but instead send as 6 channels LPCM because the
re-encoding isn't really necessary (it sounds exactly the same) but
save quite a fair bit of CPU cycles (Atom people with Ion chipset will
be happier).
This is slightly different to 0.23, where audio would always be
re-encoded as AC3.

The reason for this, is that it's the only way mythtv in 0.23 and
earlier can send digital multi-channels audio out.
0.24 will fully support various multi-channels audio ; the only
downside is that you won't always get the AC3 logo lighting up on your
amp and I know that some people are very found of that icon.


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