[mythtv-users] alsa upmixing to spdif

Michael T. Dean mtdean at thirdcontact.com
Thu Jul 12 02:46:24 UTC 2007


On 07/11/2007 10:24 PM, Nick Rout wrote:
> All of my sound goes to a device called ALSA:spdif2 which feeds a DTS/AC3
> THX certified amplifier/speaker setup. (yes I have 2 sound cards, the
> operative one is a Xitel HiFiLink Pro (USB soundcard).
>
> Anyway, 5.1 material plays fine, but 2 channel material gets upmixed by
> the amplifier in a fashion that I do not like. I would like to try getting
> alsa upmixing to work. I have had a look at thrwads like this
> http://www.gossamer-threads.com/lists/mythtv/users/134552 and
> http://www.gossamer-threads.com/lists/mythtv/users/163997
>
> I have a couple of questions:
>
> 1. The specific setups I have seen are for the situation where the
> soundcard is directly feeding the speakers, rather than my situation where
> the stream needs to be fed out the spdif port - will this work for me?
>   

Nope.  Since you can only send 2-channel PCM data through an IEC958
connection, you'd have to encode the resultant "mixed" audio to AC-3 or
DTS.  While there are ways to plug in external encoders, doing so would
likely cause serious A/V sync issues.

What you need is a sound card that supports HDMI 1.3+'s 8-channel PCM
data.  When you find one, please let me know as I want one too (enough
to hack some drivers for it ;).  In the meantime, look at Mark Spieth's
patches on #1104.

> 2. I can see what happens when such a setup is fed a 2.0 channel signal -
> the left front is fed to the left rear, ditto right front/rear, left and
> right mixed into centre and LFE. Sensible. However what happens when a 5.1
> signal is fed into it? Obviously you don't want this mixing going on to a
> stream that is apready 5.1. I want to have the same alsa device for all
> output (otherwise I would have to set different player commands for
> different video files).
>   

In the other thread, it was easy because the user was using xine (which
allows specifying a different device for 2-channel, 4-channel,
5.1-channel, etc.).  Myth doesn't really allow that.  However, since
you're using AC-3/DTS passthrough, you can specify a different ALSA
device for the Passthrough Audio Device, so non AC-3/DTS uses the ttable
device an AC-3/DTS uses the S/PDIF.  This is a moot point, though, if
you can't use (or choose not to use) the analog outputs on your sound
card.  Also, if the IEC958 device is properly set to non-audio mode
(meaning data, such as AC-3 or DTS--which Myth doesn't currently do),
the ALSA "filtering" should be bypassed regardless of audio device.

> 3. It doesn't seem right to just send L+R to the LFE - is there any
> possibility of a filter to just send below a certain frequency to this
> channel?

Yeah.  Takes a bit more ALSA conf magic, but your sub should have a
pretty good "hardware" filter in it, anyway, so...

Mike


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