[mythtv-users] More Digital Audio Q's

[Michael T. Dean]

Steve Adeff wrote:

>My mistake on the AC3 bit-perfect thing, apparently its a Windows only problem 
>with how Windows handles this sort of thing.
>
>For things like DTS audio CD's where the card has to be able to send the 
>signal properly, which is a hardware issue not a software issue. The issue is 
>with forced sampling from 44.1 to 44.8 which ends up mangling things. So for 
>DTS audio CDs, and regular audio cd's you want bit-perfect (ie the output 
>must be able to do 44.1 output and not just 44.8). It seems that all the 
>newer chipsets (Live! as well) can output 44.1Khz through its SPDIF 
>connector, but for a while windows wouldn't let it. 
>
>If I understand what I've read correctly, if you play an audio CD with mplayer 
>-ac hwac3 it should send the PCM signal straight through the SPDIF bypassing 
>any asound mixing that might get done. So as long as you force passthrough 
>you should be ok.
>You can test this if you have a DTS audio cd, play it with -ac hwac3 and if 
>your stereo outputs garbage sound then your hardware is not capable of doing 
>the bit perfect passthrough. This will also affect CD playback in that it 
>will be converting 44.1 to something(dependent on the soundcard) which isn't 
>especially noticeable depending on the quality of your equipment.
>
>In linux the DD/DTS compressed movie streams are sent directly through to the 
>SPDIF output when using the whole passthrough thing, in Windows they can go 
>through its kmixer thingy which will mangle things, so not important for us 
>linux folks since we can easily force passthrough.
>  
>
So, basically, I'm reading this to mean anything that passes through 
Windows comes out garbage...  No arguments here.  ;)

>Apparently as well, there are cards that can do AC3 encoding (and some that 
>now do DTS encoding as well!) for output. The sound hardware on the Xbox can 
>do AC3 encoding which is how XBMC can output a stereo mp3 to all speakers!
>This is a feature I love, so I'm going to look into these boards and see what 
>the linux support is for this feature. hopefully just setting up another 
>asound output will let me do this with MythMusic while leaving TV and 
>MythVideo output alone =)
>
ttable ...  It won't be AC-3 or DTS, but PCM works (better, actually, 
because there's no lossy encoding) if you've got the bandwidth (and you 
do when piping to the receiver for output).  (Actually, I haven't tested 
this with digital out, but I can't find any reason it won't work.  
Please let me know if you find one.  :)

I was teaching Franco how to do something slightly different (5.1 to 
7.1), but the same ideas apply, so you should find everything you need 
here: http://www.gossamer-threads.com/lists/mythtv/users/134552#134552
Reading the entire thread would give you some additional info that might 
be useful, too.  I have to admit that I really thought he would be 
destroying the effect with 5.1 to 7.1 (and that this should only be 
attempted with 1- or 2-channel upmix), but he and his friends felt that 
the the 7.1 output was even better than the 7.1 upmix he got from 
switching the setting on his Creative GigaWorks amp to use its built-in 
upmix.

Oh, and thanks for the reply.  You have no idea how out-of-kilter my 
entire world would have been if passthrough didn't mean passthrough (on 
Linux, at least :).

Mike