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From: shauns@vice.UUCP (Shaun Simpkins)
Newsgroups: net.audio
Subject: Re: Sheffield CDs and why they sound bad
Message-ID: <46@vice.UUCP>
Date: Tue, 9-Oct-84 17:29:13 EDT
Article-I.D.: vice.46
Posted: Tue Oct  9 17:29:13 1984
Date-Received: Thu, 11-Oct-84 06:49:46 EDT
References: <460@watdcsu.UUCP>
Organization: Tektronix, Beaverton OR
Lines: 56

Something bothers me about this dynamic range business.  Let's assume that
we have a recording medium with 120dB dynamic range.  Further, let's assume
that MOL (maximum output level) of the recorder will produce a SPL of 130dB,
the threshold of pain to the human ear.  This means that the noise level of
the recording medium will be at 10dB, 10dB above the threshold of hearing in
an anechoic chamber and 30dB below the noise floor of a quiet listening room.

We now substitute a 16-bit digital recorder for our hypothetical system, placing
its MOL at the same point.  Its noise floor will now be at 34dB SPL, still
below the noise floor of a quiet listening room by 6dB.  Put another way, the
difference between ambient noise and a music signal is one bit.

Will a music signal sufficient to twiddle only the LSB of the digital recorder
sound bad?  Maybe, but I claim that you won't notice it, since you'll be
straining so hard to filter out all the ambient noise.

I suspect that the digitization study cited used signals far above the noise
floor of the listening chamber.  What happens when the signals digitized at
4, 6, or 8 bits are played at the SPLs where they would normally occur?  I
suspect that distortion components would be lost in the noise floor.  Even 10
bit quantization noise is lost in the environment in my example - and LSB
dithering would make it even more indistinguishable from random noise.  The
only time such errors become perceptible is in a highly unlikely environment,
the anechoic chamber.

Given this, I find it hard to believe that LPs suffer less from dynamic range
problems than CDs.  Good vinyl, I am led to believe, has a dynamic range of
about 70dB.  Allowing for the ear's adaptive filtering (5dB? 10dB?) this is
still a 16-20dB less than 16 bit PCM.  Is the ear really that bothered by PCM's
inverted distortion vs amplitude characteristics?

Yes, professional recorders need to be better than the consumer environment for
many reasons - Multitrack mixdowns and foresight being two.  It does not mean
that the consumer product must be as good.  I challenge you to make a 20-bit
digitizer with a 10 volt full code output.  The LSB would be 10uV!  Try that on
comtemporary IC processes over the commercial temperature range.

A final comment - I like the idea of compressing CDs, but more for stereo
system protection than for ultimate dynamic range.  If this is done, the
compression should be multiband and the level of compression indicated by an
explicit control track instead of by sensing the level of the compressed output
and supposedly ``undoing'' the change, as is done now.  This would reduce
system distortion to levels approaching -80dB(MOL) rather than the -40 to
-60dB(MOL) possible with today's systems (such as dbx), and could be easily
implemented by an extra control word in the CD bit stream with appropriate
sorting in the player.  Of course, this would entail perhaps a 30% in playing
time, about the same as 20 bit PCM but a whole lot more manufacturable.

The wandering squash,

-- 
				Shaun Simpkins

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