chowkwan@aerospace.aero.org (07/26/90)
Mike Moffat, the chief designer at Theta Digital, visited
our audiophile club at work yesterday to demo his
DSPre Basic D/A converter and to talk about things digital.
My impressions of the DSPre Basic:
It made all the music we tried sound much smoother and
more pleasant. (The unit has a switch to allow you flip
back and forth between converter and CD player).
Without the converter, the highes sounded etched and
hard. With the converter in, the high end becomes
more rounded, fuller. I can`t speak to imaging as
tonal qualities are my main interest. But some present
claimed that imaging was improved also.
We listened for about .5 hour on the following equipment:
A pair of Bruce Edgar home brew specials: horn loaded speakers.
(he is a contributor to Speaker Builder magazine). Lord only
knows what kind of speaker wire. Home brew power amp.
As one interconnects between converter and amp. Straight Wire
video cable between Phillips CD player and converter
(to carry the digital signal).
Sources:
1. Some dreadful Strauss tone poem.
Couldn't tell too much from this because it starts out with
a series of low level rumblings.
2. Solid Brass at the Opera on Dorian.
You could definitely hear a more rounded and natural horn tone
on the first cut.
3. The Chesky test CD.
The first enjoyable musical cut for me. The last jazz track
before the test stuff comes on sounded delightful. Very
live, warm, and involving.
4. The Opus 3 Depth of Image test CD
Well, I can't tell you if the image was deep, but I did
enjoy the music. Again, very rich and satisfying.
There followed a .5 hour question and answer session with
Mike. I posted, soliciting questions for Mike. Thanks to
Bill Spencer, Sandy Lavine, Brett Martin for responding.
Unfortunately I wasn't able to pose some of your questions
because other club members had to get their questions in too.
Here is a summary of Mike's thoughts:
1. Wadia vs. Krell (2 of Theta's competitors)
Krell does true 64X oversampling. Wadia does a tricky thing
with cascaded converters where a 4X (?) converter feeds into
an 16X (?) converter. This may or may not be considered
true 64X oversampling (depending mainly on whether you're in
the Krell camp or not). Mike didn't want to have any part
of this dispute.
2. Theta vs. (Wadia + Krell)
Theta advocates "brickwall" while Wadia and Kress both
stress oversampling. Now I wasn't too sure what a brickwall
should be doing in my d/a converter but as far as I could
make out it means filtering out all frequencies above
21Khz or so. i.e. Mike said that beyond 21Khz, the Theta
output practically drops to zero whereas the Wadia and Krell
do not. The reason they do not is that they expend their
computing power on all that oversampling business with nothing
left over for brickwalling. (Theta is 8X oversampling).
In short, Mike's motto is "Better music through better DSP".
(Digital Signal Processing).
OK, you can tell I'm out of my technical depth. Well,
I'm here to tell you that I have neither the knowledge or
interest to understand what the brickwall vs. oversampling
dispute is all about. Those of you who know DSP are
welcome to post better explanations.
3. Tubes.
He is not going to make a converter with a tubed analog
stage a la Stax or CAL. :-( He made tube amps 15 years
ago but, in his opinion, you just can't get good tubes
anymore. i.e. he doesn't want all the headaches associated
with people returning their boxes because the tubes failed.
(Having had to take my VTL 90/90 back to the shop for the
second time in six months, he may have a point there.
But I'm sticking to my tubes anyway.)
4. Wires.
Mike isn't interested in digital as a "black art". I take that
to mean unexplainable phenomenon that cause the music to sound
good. i.e. he likes to have a sound technical explanation for
what he hears. Nevertheless, he does hear differences between
cables, without having a good reason why this should be so.
There were some know factors however:
(i) We need a digital wire with the least propagation delay.
Wire that conforms to the RG-64 spec is good. This is
some kind of generic wire spec that came out of the
military. Available (who knows where) at low cost.
He said something about the bandwidth of the wire
has to be so many MHz. Sorry I can't supply the details,
electricity is not my strong suit.
(ii) Whatever the differences between digital wires, it's got
*nothing* to do with dropping bits. All the wires he's
tested deliver all the bits. He suspects something
to do with timing.
5. Language.
He programs in (heavens forfend) assembly language.
6. Production.
They're shipping 130 units a week.
7. Pre and Pro, Senior and basic.
Theta makes Pre and Pro units. With both units, you take the
digitial signal out of your CD player or transport and
input to the Theta. The d/a conversion is done and the
output from the Theta can go to either pre-amp or amp.
The Pre unit has the switch that allows you to flip
between your CD and the converter.
It has an *active* gain control which doesn't add any gain of
its own. The Pro unit is the converter only. This is what I
would get. I don't see the need for the Pre's gain features,
if you have your own pre-amp.
Theta also makes senior and basic models. Both share
the *same* DSP circuitry. The senior model has better
power supplies and superior parts. (Both senior and
basic come in Pre and Pro models).
8. Balanced output.
Furthermore, the senior model is available with balanced output
which, with some suitable handwaving, reduces the noise.
Needs special cables though.
9. Cost.
Theta is at the low end of the scale for high end processors.
Stax has a $12K model. Krell and Wadia both make a range
of models of which the 64X over are the top of the line.
The bottom of the line Wadia and Krell are in line with
the Theta basic models we saw.
So, you're still with me. At this point, I get on my soapbox
and make sure I drive away any readers who may be left.
Someone emailed me asking why it should be necessary to
build a fancy converter when the recordings are made
with crappy Sony a/d converters. My answer is that
when you hear a well-recorded CD like a Chesky, it's worth it.
(I commend Non-Stop to Brazil as worthy of your attention).
It's like anything in life. Garbage In, Garbage Out.
Feed your converter good sources and good things will
come out.
Personally, I feel the quality of the
music overwhelms pure sonic considerations. I can
listen to DG quite happily, if the music is to my liking
whereas the best-recorded Telarc will leave me cold
if the musical quality is not there.
David, you're still here. Well your reward is that you
may reprint this post in its entirety in Positive Feedback.
Just remember, these are the ravings of mind deprived
of its daily VTL fix.
-- ray