[vox-tech] update on the audio CD problem

[Micah J. Cowan]

On Fri, Jan 27, 2006 at 04:54:14PM -0500, Peter Jay Salzman wrote:
> On Fri 27 Jan 06,  1:17 PM, Micah J. Cowan <micah at cowan.name> said:
> > On Fri, Jan 27, 2006 at 04:05:23PM -0500, Peter Jay Salzman wrote:
> > > On Fri 27 Jan 06, 12:50 PM, Bill Kendrick <nbs at sonic.net> said:
> > > > On Fri, Jan 27, 2006 at 05:06:09AM -0500, Peter Jay Salzman wrote:
> > > > > Digital extraction is where the cd is read, and the signal gets pumped
> > > > > through the ATA port to your speakers.  This is more efficient from the
> > > > > CPU's standpoint.
> > > > 
> > > > Woah, I would think this is way LESS efficient.
> > > 
> > > No.  The CPU is involved in many read-copy operations when the signal passes
> > > through the sound card.
> > > 
> > > AFAIK, the north and south bridge use DMA during digital extraction.
> > 
> > Pete, I'm pretty sure that the CPU isn't even /involved/ with analog
> > mode. It's analog. There is no read/copy. There's nothing to read/copy.
>  
> sure there is.
> 
> data needs to be copied from kernel space to user space.  that _definitely_
> requires the CPU.  that's why the whole "zero-copy user-space access" was
> such a big deal.

Kernel space? What the heck are you talking about?

There is a direct analog connection from the CD drive to the sound card.
It doesn't ever even touch the mother board. The signal sure as hell
isn't routed out through the soundcard to the motherboard, and then back
into the soundcard. No kernel, user, or any other sort of space is
involved.  It's an analog signal, which isn't even representable in
kernel or user space unless you convert it to digital, which kind of
defeats the purpose.

No PCM, in any space.

Odds are pretty good that you can rig an Analog playback with a /fried/
motherboard, so long as you have some way of getting power to the sound
card, and one of those CD drives that has a play button on the front of
it. Might take some hacking, and I'm not up to it, but... definitely no
digital magic needed. Heck, throw out the _sound card_, and just press
play on the front of your CD, with your headphones jacked into the
front: it's the same signal.

The only thing "digital extraction" mode is good for is that many times
that physical connection between between CD drive and soundcard has not
been made: or, you might have more than one CD drive, and the CD happens
to be in the one without the physical connection. But digital
extraction, DMA access or not, certainly involves /many/ read/copy
actions on the part of the CPU, which is why it is intrinsically more
expensive than analog playback mode (which involves none).

Well, also there's the fact that the CD drive's built-in DAC (which
drives the conversion to the analog signal which gets sent to the sound
card) may not be as great as the software you have in your computer.
Which isn't a small thing.

BTW, see http://www.epanorama.net/documents/pc/cdrom_audio_wire.html for
instructions on building a Y-connector for CD audio cables, which solves
the problem of multiple CD drives...

-- 
Micah J. Cowan
micah at cowan.name