From: anbo@wuee1.wustl.edu (Albrecht Jander)
Subject: Re: 1 Bit DAC on CD players
Organization: Washington University, St. Louis Mo.
Date: Wed, 12 Aug 1992 14:59:28 GMT

In article <8135@sirius.ucs.adelaide.edu.au> francis@cs.adelaide.edu.au writes:
>In article <1992Aug10.024538.19830@sactoh0.sac.ca.us>, sap@sactoh0.sac.ca.us (Stephan A. Puga) writes:
>|> Why is a 1 bit DAC on portable CD players better?  I see an awful
>|> lot of ads and such touting the greatness of 1bit DAC.  It seems to
>|> me you could get faster accessing if you had an 8bit DAC, but I
>|> don't know.  If anyone could shed some light on this matter I would
>|> be very grateful.
>
>Very briefly:  all players decode the 16 bit x 2 information on a CD.
>The one bit DAs are simply just one of a number of ways to make a
>DA that converts the 44.1kHz x 16 bit signal into sound. 
>
>Although the data format of a CD does at one point involve eight bit
>data this is a long way from where it gets turned into sound.  Speed
>of accessing makes litte sense in the context of audio CDs.
>
>One-bit/bitstream/MASH designs are simply ones that involve a different
>set of compromises to some other designs.  They require more
>computational power in the digital filtering section and uses a simpler
>analogue part.  Since we are getting better and better at making cheap
>fast digital bits, but progress is a lot slower on the analogue design
>front the one-bit designs seem to have the edge in the tradeoff game.
>They are cheaper to make and produce better results; for the moment.
>
>The ultra high end of hifi still has adherents to the more conventional
>16 bit or wider converters, claiming that if you put the money and effort
>onto these designs they sound better than the one-bit designs.  Serious money
>and effort, and you would buy an argument about the technical merits.
>High end hifi does not go by the same rules as the rest of us.
>
>Of course it is newer and therefore better so far as advertising copywriters
>go.  Next year it will be some other techno-flash technology.  At least
>one-bit DA design has some merit unlike a lot of the other hype we see.
>
>
>						Francis Vaughan

One bit DAC's are inherently linear.  (You can't put two points in space
without being able to draw a straight line through them.)  The problem with
traditional designs of high resolution DAC's has been to achieve linearity,
or even monotinicity (a bigger number in always should produce a bigger
voltage out).  Most traditional DAC's use some kind of switch to switch 
current sources onto the output pin.   The problem is that it's really 
hard to match current sources in one part in 2^15 unless you laser trim
after manufacture.  This is what makes good 16 bit converters expensive.

It turns out that with proper filtering, you can trade bits for sample rate
without losing any of the signal.  A one bit DAC is very course, but it is
run at a couple of megahertz rather than 44.1KHz.  All you need is fast
digital circuitry and some simple analog processing to make a very linear
16 bit DAC.  That's alot cheaper than laser trimmed analog circuits.

I am by no means an expert on the subject, but I hope that clears up some
questions.

Albrecht



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