[mythtv-users] Quick survey regarding passthrough device settings

Allen Edwards allen.p.edwards at gmail.com
Tue Apr 20 15:13:47 UTC 2010

On Mon, Apr 19, 2010 at 6:47 AM, Allen Edwards <allen.p.edwards at gmail.com>wrote:

> On Mon, Apr 19, 2010 at 3:54 AM, Jean-Yves Avenard <jyavenard at gmail.com>
> wrote:
> >
> > Hi there...
> >
> > As I'm currently rewriting the audio code once again with
> > foobum at gmail.com (adds supports for > 16 bits audio (E-AC3, TrueHD,
> > LPCM etc) and various channel configurations (stereo, 3.0, 4.0, 5.1,
> > 7.1)
> >
> > I am wondering on wheither we actually need the passthrough device in
> > the audio settings.
> >
> > On all my machines, it's always set as default, or the same physical
> > device as what's used in the main audio device....
> >
> > So I'm interested to know from people who used it with a different
> > value, why they did so and if they tried any different ways ...
> >
> > Thank you
> > Jean-Yves
> > _______________________________________________
> > mythtv-users mailing list
> > mythtv-users at mythtv.org
> > http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users
> From the wiki I wrote (http://www.mythtv.org/wiki/Digital_Audio_Tutorial)
> Audio output device: ALSA:default
> Passthrough output device: Default
> Max Audio Channels: Stereo (MUST BE SET TO STEREO as of 6/2/2009 for
> passthrough to work)
> Upmix: Passive
> Enable AC3 to SPDIF passthrough checked
> Enable DTS to SPDIF passthrough checked
> Aggressive sound card buffering off
> Use internal volume controls off
> Allen

I see from other posts that some of this discussion has turned into a what
people want mythtv to do with the audio.  I may be missing something but I
am thinking what I want is for myth to do nothing except take the digital
audio string and put it out to my receiver.  That said, this may not
actually be true.  I have used passthrough to alsa default and had some
settings in alsa that did not work very well.  Let me define "not work very
well".  I have two decoders, running on two SPDIF outputs from my MB.  One
goes to a very nice pre-amp for my surround system and the other goes to an
older dolby/DTS decoder that feeds stereo to the TV.  I have had settings
for alsa that do not work for the pre-amp while at the same time working for
the Dolby/DTS decoder.  I discuss this in my wiki, which I assume you have
read.  I might point out that digital audio setups were a constant thread on
this board until I wrote that wiki and once it started showing up in search
engines, that discussion went down to a trickle.  I monitored all these
threads for people with digital audio problems and pointed them to the wiki
for many months and it seemed to solve their problems.

Back to what worked and what did not.

This did not work:

   pcm.!default {
       type hw
       card 0
       device 1

 This did work

  pcm.!default {
     type plug
     slave {
        pcm "hw:0,1"
        format S32_LE

This also worked (as did some other settings, see the wiki):

  pcm.!default {
     type plug
     slave.pcm "spdif"

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).

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