Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 (Tek) 9/26/83; site vice.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxj!houxm!vax135!cornell!uw-beaver!tektronix!tekchips!vice!shauns 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 uucp: {ucbvax,decvax,chico,pur-ee,cbosg,ihnss}!teklabs!tekcad!vice!shauns CSnet: shauns@tek ARPAnet:shauns.tek@rand-relay