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From: 5121cdd@houxm.UUCP (C.DORY)
Newsgroups: net.audio
Subject: Re: Digital companding and dynamic range (followup to Shaun)
Message-ID: <941@houxm.UUCP>
Date: Tue, 16-Oct-84 11:17:39 EDT
Article-I.D.: houxm.941
Posted: Tue Oct 16 11:17:39 1984
Date-Received: Wed, 17-Oct-84 05:37:44 EDT
References: <569@ucbtopaz.CC.Berkeley.ARPA>, <570@ucbtopaz.CC.Berkeley.ARPA>, <509@watdcsu.UUCP>
Organization: AT&T Bell Labs, Holmdel NJ
Lines: 79

It seems as though we need a little clarification on what the dbx 700 is all
about.  True, it employs a form of delta modulation to code waveforms,
however it is not a "12-bit system" as described previously.
(I think what the previous contributor was thinking of was Differential PCM.)
The sampling rate of the unit is about 700K Hz. (By the way, this is about the
same bit rate as current PCM devices, however their sample rate is 1/16
that.)  Each sample (bit), the unit decides whether the waveform went "up" or
"down" in absolute voltage and writes the corresponding "1" or "0".
Now, it's not quite as simple as that in real life -- delta mod has
a few inherent limitations.  The dynamic range is limited as well as the
ability to track transients adequately.  Therefore, dbx uses spectral
companding accross different frequency bands to boost the dynamic range.
(No folks, this is not a dbx 150 in front of their A/D, this companding
works quite well as it is tied in with the A/D through a feedback loop.)
As well, to enable the unit to track transients, elegant prediction
circuitry has been added to the delta-mod coders.  Hence, dbx coins
the acronym CPDM (Companding Predictive Delta Modulation) for their
dbx 700 Digital Audio Processor.  For further reading, I recommend an
article appearing in the current "AES JOURNAL" written by Bob Adams,
the dbx 700's designer.

I know that some of you are just dying to hear the specs, so here they
are (to the best of my recollection):  dynamic range -- >110dB (mid band),
frequency response -- 10 Hz to 20 KHz +-0.5dB, etc.  As well, since this
unit has a sampling rate at 700 KHz (or thereabouts) you don't need those
nasty elliptical anti-aliasing filters ala PCM units (thereby greatly
reducing delay distortion - pinched highs, etc.)  Don't get me wrong,
the dbx 700 also has anti-aliasing filters, however these are Bessel
(linear phase, no overshoot and ring) and the knee (-3dB point) is at
38 KHz!!

What does it sound like?  Glad you asked -- better than the Sony PCM F1.
I've made several recordings in parallel (i.e., from the same pair of
Schoeps mics and custom built mic preamps) with both devices using my wife
(a concert violinist) as a single-blind lintener.  In all cases, she
was able to distinguish the two units and preferred the sound of the dbx.
(Yes, I was careful to match the outputs of both units -- this was
done using a 1 KHz tone.  Matching was good to <.1dB.  However, as
pointed out by L. Miller, this might not be good enough.)  Subjectively,
the dbx 700 reproduces massed string tones and massed voices much
better.  As well, percussive instruments (i.e., triangles, etc.) have a
much cleaner sounding "ting" rather than sounding like "splang" as on the
Sony PCM F1.  This is due, I believe, to the lack of significant delay
distortion in the anti-aliasing filters.  Albeit we were monitoring with
very good equipment:  Stax Lambda Pro earspeakers -- with excellent
loudspeakers (by Nestorovic) the differences were still audible however
not as prevalent.

The unit is built as a professional tool rather than a consumer device
like th Sony PCM-F1, et. al.  In use, it has many advantages over the
Sony.  Inputs and outputs are adjustable and are at line (+4dBm) level.
Also, the inputs and outputs are balanced.  The Sony F1 has three error
states:  error correction, error concealment, and mute (while it
collects its senses).  This is all well and good for non-critical use,
however when mastering a record it leaves a lot to be desired.
Error correction is just that and it seems to work fairly well.
Error concealment can be clearly audible and manifests itself as
strange perturbations of the waveform.  Muting is totally unacceptable,
for 1.5 seconds the unit does a "stable clear" while it gathers its
marbles to go on.  The dbx 700 seems to be much more tolerent to
errors, the most you will get from an uncorrectable error is a
barely-audible "tick".  The ballistics of the meters on the dbx are
much easier to follow than the meters on the F1.  In addition, th dbx
when overloaded behaves a little more gracefully than the F1.

As for the dbx 700's popularity, there are a few hundred in use currently.
The uses are widely varied from on-location recording (like myself)
to use as a mixdown/mastering deck in studios as well as remote links
for WBGH (WGBH?) FM in Boston.  The major complaint seems to be that there
is currently no way to transfer digitally from the dbx 700 to the
Sony PCM 1610 for CDs.  I don't see this as much of a problem -- most
CDs have been through a half a dozen A/D / D/A trips anyway.  At least
the dbx 700 sounds better (and doesn't introduce the gross group-delay
distortions as the PCM units do).  Why doesn't some DSP jock rig
up a way to perform this digital transfer -- they'd probably make a
mint as well as a lot of friends.

Craig Dory
AT&T Bell Laboratories