[mythtv-users] mythmusic iec958 help

[Mark Greenwood]

On Sunday 06 July 2008 23:48:59 Joe Henley wrote:

Thanks Joe for that informative response.

> "I have the optical out connected to my surround amp (a Yamaha 
> DSP-E800). It doesn't mention anything about 44.1KHz spdif output. I 
> have the optical out connected to my surround amp (a Yamaha DSP-E800). 
> It doesn't mention anything about 44.1KHz spdif output. "
> 
> Does the Yamaha have any indicator lights to inform you of the type of 
> signal it's getting?  I have a Denon which does this; made my original 
> efforts at sorting all this junk out -- much easier.

Unfortunately not. It does have a CD input, but analogue only, which perhaps suggests it will only take 48KHz digital input signals.
 
> IIRC, MP3 is always at 48KHz, then compressed, then transmitted. 

I don't think that's correct. An MP3 file can be created from source material with any sampling rate from 8KHz up to 48KHz. The sampling rate is retained in the MP3 - it's one of the header fields.

> "The sampling rate of the source material should be irrelevant, 
> otherwise everything would have to be recorded at the same sampling rate 
> everywhere and you wouldn't be able to play DVDs and CDs on the same 
> machine. Which, basically, is the problem being discussed... I would not 
> expect any digital sound system to be incapable of doing that. Something 
> is very wrong somewhere if that isn't working. "
> 
> And yes, you're correct here, too.  There is something wrong with 
> pre-pending a header which says CD data sampled at 44.1KHz is really 
> sampled at 48KHz.  But most sound cards do this.

That's really crap. I've been doing digital audio for about 15 years and I've never come across a problem like that. That said, I've never used 'onboard' sound chips so that may be why. 

So it would seem that my sound card is doing the resampling correctly. However that's not the solution I thought everybody needed, because they all want bit-perfect audio which is not what I am getting. The only solution to this I can see, if you want pass-through audio to work correctly at any sample rate is to get a sound card that correctly sets the header *and* to have a processor that can accept multiple input sample rates.

My USB sound card, while it can output digital and analog at the same time, doesn't appear to support an output such as ALSA:spdif - although I have those entries in my setup, I get no sound when I select them which probably means they're referring to the onboard sound card. Which is odd, as that doesn't have a spdif connector.. ho hum.

Mark

> 
> Joe Henley
> _______________________________________________
> mythtv-users mailing list
> mythtv-users at mythtv.org
> http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users
>