[vorbis] Lossless/lossy hybrid?

[Beni Cherniavsky]

On Thu, 31 May 2001, Chun-Yu wrote:

> On May 31, 2001, Wilson wrote:
> 
> >SACD:  Freaky "DSD" (Direct Stream Digital) format, rather than PCM.
> >I'll quote a Phillips page, since I don't really understand the details
> >of this new format: "DSD is a revolutionary process based on 1-bit
> >sigma-delta modulation, and operates with a sampling frequency of
> 2.8224 MHz."
> >I gather that this makes it capable of an arbitrary number of channels,
> >and roughly 100KHz of sampling rate.
> 
> Nope.  The sampling rate is really 2.8224 MHz.  SACD will record up to
> over 100,000 Hz.Each "sample" is just 1 bit. If you think about it,
> this is better than PCM because PCM requires good DACs to convert from
> digital to analog.  The way DSD works, it is much simpler to perform
> the digital->analog conversion.  A PCM file of all -32768s (in 16-bit
> audio) would be all 0s in DSD.  A PCM file of all 32767s would be all
> 1s in DSD. A PCM file of all 0s would be half 0s and half 1s in DSD.  
> If the digital signal is converted to analog and passed through an
> analog lowpass filter, it will sound like whatever it's supposed to
> be.  It's that simple to do the digital->analog conversion with DSD.  
> I read all this at http://www.superaudio-cd.com/ a while ago. For a
> ton info about SACD, check it out.
> 
> Chun-Yu
> 
Your PC contains one DAC that is simple thanks to this technique: the PC
Speaker!  There are several programs that play arbitrary sound by
alternating the PC speaker between 0 and 1 faster than it can react (or
you can hear).  I wrote one once using one-dimensional error diffusion to
generate the DSD sequence, with quite good quality (read: I could
understand words).  It was funny to run it with the soundcard test files
from Warcraft II setup ("Your sound card works perfectly", "It doesn't get
any better than this", "Enjoying yourself?" ;-).

It's also funny to rediscover the only method to squeeze minimal quality
from the PC speaker at the high-end side :-).


-- 
Beni Cherniavsky <cben at crosswinds.net>
                 (also scben at t2,cben at tx in Technion)

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