Path: utzoo!attcan!uunet!island!rich
From: rich@island.uu.net (Rich Fanning)
Newsgroups: sci.electronics
Subject: Re: Digital/Analog converter help needed
Summary: 32 bits not needed
Message-ID: <1847@island.uu.net>
Date: 21 Jul 90 01:10:30 GMT
References: <3550.26a1d2d7@ccvax.ucd.ie> <1839@island.uu.net> <1168@sirius.ucs.adelaide.edu.au>
Reply-To: rich@island.uu.net (Rich Fanning)
Lines: 44

In article <1168@sirius.ucs.adelaide.edu.au> francis@cs.ua.oz.au writes:
>Uh No. If you take the top 16 bits off a 32 bit result you will only get
>the full 16 bit resolution of the source material at full volume.

Exactly the point, at full volume.  You get out exactly what you put in.

>If you
>play the system at ordinary listening levels (say 30db down) you will end
>up discarding nearly half the bits. Leaving you with say 9 bits of music
>and 5 bits of 0s.

True, but this is exactly what happens when the engineer does a fade on the
mixing board.  The signal decreases in amplitude, and the digital values
grow smaller, leaving lots of zeroes at the top.

>The Denon Audio Test disk demonstrates this very well. They have a sequence
>of tracks of orchestral music recorded on the disk at differing levels of
>attenuation. If you normalise the volume on playback for the different tracks
>the difference in sound quality is remarkable. Tracks that are recorded 60db
>down sound worse than telephone lines.

I can only reason, not having the disk, that the recording was not made using
dither.  This will cause obvious distortions in the kind of tests you describe.
A properly dithered recording will simply have random noise added. Of course,
it won't sound better as the attenuation increases, but it ought to sound
much like the original with just noise added.

>To do what you suggest you would need a 32 bit DA. Some DA converters allow
>you to supply an external reference voltage which coresponds to FFFF. If 
>you vary this the output is essentially scaled, but the range of useful
>voltages
>is typicly only 3:1 and therefore of little use as a volume control. 

Professional digital mixers, from what I hear, can sound excellent.
And they only store 16 bits after the mix is complete.  No doubt internally
they use 32 bit accumulators to help prevent rounding errors, but the signal
as it's recorded is still only 16 bits.  

From this, I reason that a turning down a digital volume control which
emits 16-bit codes to the D/A is the same as a fade on a CD.  If you cannot
hear artifacts when a song fades out on a CD, then a digital volume control
driving a 16-bit D/A ought to be adequate.
-- 
	 Rich Fanning         {sun,ucbcad,uunet}island!rich