[mythtv-users] More Digital Audio Q's

Steve Adeff adeffs at gmail.com
Sun Nov 27 19:06:23 EST 2005

On Sunday 27 November 2005 17:33, Robert Johnston wrote:
> On 11/27/05, Steve Adeff <adeffs at gmail.com> wrote:
> > On Sunday 27 November 2005 16:26, Michael T. Dean wrote:
> > > Steve Adeff wrote:
> > > What exactly does this mean?  If you're doing AC-3 passthrough from
> > > Myth, won't the S/PDIF output be as bit-perfect as was sent to you by
> > > the digital TV broadcaster/DVD production house?  Or, are you saying
> > > that some sound card's mangle the stream they're attempting to pass
> > > through?
> > >
> > > Mike
> >
> > Yes, 'mangle', because the chipset will do freq/bitrate conversion if
> > needed to match the abilities of the output circuitry. If the output
> > circuits can handle 24/196 then no change is needed and the chip just
> > sends out the signal. Creative's output circuits can't handle 24/196 so
> > the signal gets reprocessed to match its output (20/92 iirc). This is
> > whether the signal is DD/DTS or PCM.
> Here's the thing. The S/P-DIF specifications state that signals have
> to be transferred at either 44.1Khz/16-Bit or 48Khz/16-Bit for PCM,
> DD/DTS (Which is a raw mix from the source), or AC-3 (5.1 Channel,
> 48/44.1Khz, 16-Bit).
> However, Creative's SB Live! cards add to this, by being able to
> decode (On-card) DD5.1 to AC3 (And multiple outputs). But this is
> optional, and can be switched off (At least in the Windows Drivers, I
> don't know about the Linux ones). If it's switched off, and the
> S/P-DIF is addressed directly, then the format that's presented
> (DTS/DD/AC-3) will be piped directly to the S/P-DIF and out to the Amp
> via the S/P-DIF RCA or TOSLink connector.
> If the input stream is from a DVB/ATSC source, it should be in AC-3
> (Or DD, I'm not certain), And if it's from a DVD source, it'll be in
> 48Khz PCM/DD/DTS, which means that there will be no resampling. If the
> source is an PVR card, make sure it's set to capture at 48Khz, and
> there will be no resampling (As it'll get decoded into a PCM stream,
> that'll then hit the soundcard and S/P-DIF untouched). If the source
> is an Analog card (BTTV), then you may be stuck with 32Khz, and will
> have to upsample to 44.1/48Khz. This may introduce upsampling
> problems, BUT you will have this no matter what card you use, as the
> S/P-DIF forces to be upsampled.
> As another point, 99% of cards have an internal "Working Frequency",
> which means there can be resampling artefacts on virtually every
> soundcard out there. AC'97 (The general soundcard spec) states
> soundcards must accept and output 48Khz, and most cards use that. SB
> Audigy's use 92Khz (That's 48Khz x 2) so they can very simply convert
> into and back from S/P-DIF's 48Khz recommended frequency.
> However, as this is all real Audiophile details, the chances of any of
> this coming out in subjective listening (Without $100,000 sound
> equpiment) is very very slim indeed.
> --
> Robert "Anaerin" Johnston

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.

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 =)


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