Path: utzoo!attcan!uunet!microsoft!brianw From: brianw@microsoft.UUCP (Brian Willoughby) Newsgroups: comp.dsp Subject: Re: Adjust-Speed CD player?? Message-ID: <7814@microsoft.UUCP> Date: 23 Sep 89 03:04:09 GMT References: <6028@jpl-devvax.JPL.NASA.GOV> <89255.105143P85025@BARILVM.BITNET> <7767@microsoft.UUCP> <89264.171306P85025@BARILVM.BITNET> <8909@batcomputer.tn.cornell.edu> Reply-To: brianw@microsoft.UUCP (Brian Willoughby) Organization: Microsoft Corp., Redmond WA Lines: 48 In article <8909@batcomputer.tn.cornell.edu> sandell@tcgould.tn.cornell.edu (Gregory Sandell) writes: > I think that the choice of putting speed-variation option on >a CD-player should not be constrained by the requirement that it >be hi-fi. What are people going to be using the feature for, >anyway? My particular use, since I'm a musician, is that I'd >want to use the feature to SLOW DOWN the music in order to >transcribe or learn by ear what a musician is playing. Other >people may want to play spoken CDs at higher speed so they can >assimilate information quicker. In both of these cases, I don't >think the user really is going to care that the audio quality is >distinguishable from normal playback. > >Greg Sandell If the CD playback speed were varied by changing the sample output rate, instead of maintaining a constant output rate and throwing away samples, then this distortion wouldn't occur. I can't see any advantage to dropping samples just to maintain the same conversion rate. You are still left with the more difficult problem of what to do about the rate of data coming *from the disk itself*. If you solve that, then simply changing the conversion rate is trivial. Basically, I'm saying that its too easy to avoid the distortion from dropping samples, so why do it? On a side note, I have heard that someone has developed a compression scheme to fit sixteen times as much data on a CD as is currently done. If you think about the typical audio waveform, you'll understand that it is easy to compress. Just by storing the *difference* between adjacent samples, and assuming that there are no impulses, a great savings in data can be achieved over storing 16 bit *absolute* values. The problem with storing sixteen times as much sound on a CD is that the CD must still be accessed at the data rates it was designed for. In other words, they are getting 16 times too much data at any given time. Solution: read the CD from front to back sixteen times, each time converting a different block of data. Data frames on the CD format are broken into 16 blocks, and the player just cycles through these. It's too bad that that music company Southworth recently went bankrupt. They had announced a Macintosh II-based set of cards which employed similar compression schemes. They cited 30 minutes of stereo audio on a 40 M hard disk with 20 bit samples at a rate of 192 kHz per channel. Brian Willoughby UUCP: ...!{tikal, sun, uunet, elwood}!microsoft!brianw InterNet: microsoft!brianw@uunet.UU.NET or: microsoft!brianw@Sun.COM Bitnet brianw@microsoft.UUCP