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From: kube@kokoro.UCSD.EDU (Paul Kube)
Newsgroups: rec.audio.high-end
Subject: Theta DSP digital preamp
Message-ID: <8808@uwm.edu>
Date: 10 Jan 91 14:20:26 GMT
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Originator: tjk@csd4.csd.uwm.edu


I've been reading a bit about the Theta DSPre.  This is a $4000
"digital preamp" containing an 8x oversampling 16-bit D/A that has got
some pretty breathless reviews-- comments of the "I always thought
analog was better than digital but I like CD's through the Theta
better than vinyl through my $10000 phono section" type.  The moral
is, apparently, that you can get very nice performance from a
conventional 44KHz linear 16-bit format and off-the-shelf 176KHz
DAC--- if you have the right interpolation algorithm for generating
the additional 3 samples per tick on playback.  (Maybe 24 bit 88KHz
isn't needed after all...)

But like typical audiophile reviews, these leave a lot of unanswered
questions.  Such as, just what is the hardware here?  The designer
Mike Moffat says two RISC processors, one per channel, each at 38
MIPS.  Has anyone looked inside to see, or otherwise heard, if these
are iPSC/860's?  How much ROM does he use for each?

Also, according to the press releases, the big difference between the
Theta oversampling algorithm and the usual 16-bit algorithm is that it
is optimized for both phase and amplitude fidelity, instead of just
flat amplitude response.  So that leads to more questions.  I haven't
seen a description of the "usual" oversampling algorithm, but why not
just use a symmetric (noncausal, but so what) impulse response in the
interpolation filter, and therefore a real transfer function, so it
will be guaranteed phase invariant?  Then you can just worry about how
to truncate a sinc to give you bandpass ripple and rolloff you can
live with, given how much computer you have.  Or is Theta really
talking about trying to reconstruct phase relationships destroyed by
"brick wall" analog filters used in 44KHz recorders?  That would be
trickier, but then I would think it would sound lousy on heavily
oversampled, digitally filtered CD's like Cheskys, but there's no
mention of this in any of the reviews I've read.

Can anyone shed some light on this for me?  

	Gratefully,

	Paul Kube@ucsd.edu,  ...!ucsd!kube