[mythtv-users] alsa upmixing to spdif

[Nick Rout]

On Thu, July 12, 2007 2:46 pm, Michael T. Dean wrote:
> 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.


Ahh interesting, I didn't know that about spdif/iec958. Owing to this
answer, the rest is probably moot, and I'll stick to the upmixing provided
by the amplifier. But thanks for the info, the knowledge on this list is
fantastic.

>
> 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.).

good point, a bonus for xine users I guess.

> 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.

I was more concerned about mythvideo, in which i use mplayer.

> 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...

ahh yes I get you - it wouldn't matter in that case.

Thanks again :-)


-- 
Nick Rout