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From: newton2@ucbtopaz.CC.Berkeley.ARPA
Newsgroups: net.audio
Subject: Re: Sheffield CDs and why they sound bad
Message-ID: <582@ucbtopaz.CC.Berkeley.ARPA>
Date: Mon, 22-Oct-84 03:21:25 EDT
Article-I.D.: ucbtopaz.582
Posted: Mon Oct 22 03:21:25 1984
Date-Received: Tue, 23-Oct-84 05:12:40 EDT
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Organization: Univ. of Calif., Berkeley CA USA
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	I reiterate that a narrow-bandwidth gain-control channel suffices.

	First of all, even if the notion of attack time sufficed to define
the required bandwidth, there is no reason to resolve the gain-control
information to sixteen bits (the implication of "every tenth word a control
*word*"). That's like a floating-point representation with a sixteen-bit
mantissa *and* a sixteen-bit exponent. All this to solve the alleged
problem of "only" sixteen bits of resolution in the present rather impressive
system. If you must have a compander "track", surely gain changes of
a few multiples of 6 dB (i.e., a few bits per control word) should satisfy
the yearning for more than 90-odd dB of dynamic range (personally I think
the present system is all one could reasonably ask for noise/dynamic range
wise, when implemented properly).

	Secondly, on the matter of bandwidth: 

	Regardless of the required instantaneous rate-of-change of channel 
gain, the measure of control-track bit rate is the average, not the peak,
rate of change to be accommodated by the compander. Since the master
encoder that cuts the CD-of-the-future cum control track can easily
survey the signal to be compander-encoded throughout the program's time
history (or at least sufficiently "in advance" of the transient to be
encoded), the gain-control bits that are interleaved with the program 
can appaear sufficently in advance of the gain-change they command to
allow the reproducer channel gain to respond exactly on time.

	I'm not sure I've been clear on this: the recorder (being digital
already) includes a digital program delay line, which allows the gain-
control bits, calculated in real time with respect to the incoming program,
to be interleaved with the delayed program as it emerges. In the
reproducer, the gain control information is extracted and decoded before
the program data to which it applies (and which gave rise to it) arrives.
Thus the level-detectors and VCA's can compensate rather exactly for
the tardiness of response inherent in the low bandwidth of the gain-
control channel.

	This principle is the basis for a patented low-frequency pilot
tone compander compatible with comventional FM stereo broadcasting, and 
this same notion of delaying the program to allow the slow-responding
level-control detector to "catch up" is adapted by Dolby to an 
adaptive delta-mod digital transmission scheme, where the step-size
information is coded within a bandwidth on the order of 100 Hz. Both
these applications take advantage of the economies made possible by the
central transmission nature of their media, which allows the expense of the
delay-line and A/D's at the encoder. CD manufacture offers the same
distribution configuration and thus the same economic practicality.

	Thus, to recapitulate, I don't think you need 30% overhead for
	compander information, I don't think you need 10%, and in fact
	I still don't see why you need an explicit control-track at all,
	since in an instantaneously compandered digital system tracking
	errors are not a problem as with analog noise-reduction systems.
	And I'm unconvinced that 16-bit linear PCM isn't already a
	miraculously generous and ambitious format to offer the mass
	market- after all, this is essentially (except for sample
	rate) the scheme that virtually everyone was/is thrilled to
	use commercially.

	Any, I remain

		Doug Maisel