Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site clyde.UUCP Path: utzoo!watmath!clyde!saf From: saf@clyde.UUCP (Steve Falco) Newsgroups: net.audio Subject: Re: CD Reflections - 44.1k? Message-ID: <755@clyde.UUCP> Date: Tue, 15-Jan-85 08:31:46 EST Article-I.D.: clyde.755 Posted: Tue Jan 15 08:31:46 1985 Date-Received: Wed, 16-Jan-85 03:57:12 EST References: <15100001@hpfcmp.UUCP> <3411@mit-eddie.UUCP> <1420@hplabs.UUCP> Organization: AT&T Bell Laboratories, Whippany NJ Lines: 32 > It seems to me that the disc itself was created with a given sampling rate. > How can a player change this? > > Bob The disc has a sampling rate of 44.1k. However, you can feed that into a digital filter along with additional "made up" samples (a value of 0 is used). The new sampling rate depends on how many zeros are added. If you add 3 zeros, you now have 4x the rate or 176.4k. According to the article in the Phillips Technical Journal (1982), the net effect is to suppress the aliases of the original spectrum. They achieve about 50db of suppression which makes the analog filter that much easier to build. Note that there are still aliased components at 24.1k (44.1k - 20k) but they are much lower, hence easier to suppress. This stuff looks like out-of-band noise in some sense. By the way, the output of the digital filter is 28 bits, 16 from the original sample, plus 12 that are added by the multiplier coeficients in the filter. This gets averaged down to 14 and then fed to the D/A. Now for the part that bothers me: The Nyquist theorem always seems to be based on continuous signals. I.e, the reason you can sample a 20k signal at just slightly more than 40k is that EVENTUALLY you will get some samples around the peak amplitude - you will also get some around zero. But music isn't like that - notes start and end. In effect, you are multiplying (or modulating) the sine wave by an envelope which also contributes to the spectrum. HOW MUCH ADDED SAMPLING DOES THIS REQUIRE? I don't want to listen to a 20k sine wave, I want transients (anybody remember TIM distortion?). How about it digital types? - any guesses? Steve Falco AT&T Bell Laboratories Whippany, NJ