From: DPierce@world.std.com (Richard d Pierce)
Subject: Re: 20 or 24 bit digital recording?
Organization: The World Public Access UNIX, Brookline, MA
Date: Wed, 22 Jul 1992 12:02:04 GMT

In article <1353@eagle.ukc.ac.uk> wgd@ukc.ac.uk (Warren Day) writes:
>>Would increasing the resolution of CD increase its transparency?
>
>As it is unlikely that complete sonic transparency is just related to
>resolution, I suspect the answer to your questio is no.  Increasing
>resolution will give more detail though.

As it has never been demonstrated the detail is just related to resolution,
I strongly suspect that this is not the case either.

>Current vinyl is apparently about twice the resolution of CD (assuming
>you've got a clean record, but we've talked about that).  If you did
>increase the CD's resolution you need to invent a new completely format
>basically.  Even though you are only changing the 16 bits to 24 bits you
>need to increase the size of your CD or have only a 55 minute limit.  Also
>you need to completely change your error correction software.  Increasing
>your resolution just gives you more detail.  However every so often people
>come up with new plastics which can form more complex shapes and these can
>be used to press records without changing your turntable.

The discussion about "LP vs CD" resolution that has gone flying back and forth
has to be one of the more amusing and ridiculous discussions I have recently
been saddened to see on this newsgroup (I guess that's saying a lot!). I
have no idea where the basis for your assertion about comparitive resolution
comes from, but I would certainly be interested in seeing both some measurments
and references to the relevent literature. But enough of that for the moment.

>The 1-bit
>players however use the 1 bit (delta sigma) system to reconstruct
>the sound without having large errors in the resistors that multi-bits
>can have.
>
>16384 ohms +/- 5% is about 800 ohms error.  In a multi-bit player there
>are also resistors of 1 ohm, 2 ohm, ... , 256 ohm , 512 ohm , 1024 ohm.
>So the errors really corrupt the sound produced.

Well, you've made an assertion here which is easily and unequivocably testable.
That is that CD using parallel DACS ought to have whopping transfer errors.
This should lead to easily measurable converter errors, even non-monotonic
performance. The sorts of errors you describe here should be obvious and gross
yet not a single CD, DAT or, for that matter DAC of any kind produced in the
last 30 years exhibits the kind of errors you contend must exist.

Most D/A converters do not use the binary weighted resistors you discuss
here, they use what's refered to as a "R/2R" scheme. This prevents just
exactly the sort of problem you describe here (some do use it, but they
can be laser-trimmed to an accuracy far better than the 5% you assert).
So instead of a ladder that looks like:

    +V ----+--32768--
           |
           +--16384--
           |
           +--8192---
             . . .
           +---4-----
           |
           +---2-----
           |
           +---1-----

We end up with:

     +V----+--20K--
          10K
           +--20K--
          10K
           +--20K--
            . . .
           +--20K--
          10K
           +--20K--
          10K
           +--20K--

(I may have the pair swapped, If so, fix it yourself, I ain't gonna do it
in emacs at 2400 baud! :-))

The type of gross non-linearity is a problem that was solved in DACS long
before the CD player ever came along.

There are other real advantages to single bit DACS (for example, no glitch
at the zero crossing, etc.), but the issue you metioned is certainly not
one of them.
-- 
|                Dick Pierce                |
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