Xref: utzoo rec.audio:15437 comp.dsp:3 Path: utzoo!attcan!utgpu!watmath!att!dptg!rutgers!psuvax1!psuvm!barilvm.bitnet!p85025 From: P85025@BARILVM.BITNET (Doron Shikmoni) Newsgroups: rec.audio,comp.dsp Subject: Re: Adjust-Speed CD player?? Message-ID: <89264.171306P85025@BARILVM.BITNET> Date: 21 Sep 89 15:13:05 GMT References: <6028@jpl-devvax.JPL.NASA.GOV> <89255.105143P85025@BARILVM.BITNET> <7767@microsoft.UUCP> Followup-To: comp.dsp Organization: Bar-Ilan University Computing Center, Israel Lines: 47 In response to a question on rec.audio, about the availability of CD players in which the music "speed" can be changed while maintaining the pitch, I posted an article which seems to have stirred a flood of responses. I was out of the office since, so I hadn't had the time to reply to some of the messages. To start with, this article is posted to both rec.audio and the newly born comp.dsp (welcome!), with followups solicited into comp.dsp only. In my posting, I said that the issue of digitally varying the speed of music, while maintaining pitch (or vice versa - same problem), is theoretically hard, is somewhat simpler for speech (and is being done for speech), and probably cannot be solved "perfectly" (i.e., for hi-fi music. Many responses followed. A major part of the responses missed the point of the question, and replied "it's easy", "it's done in turntables and tape decks", "there's a Technics CD that does it" and so forth. Please, read the original question. It is easy - very easy, to change the speed of the reproduction the same way it is done in variable pitch tape decks or turntables. All that's required is a change in signal frequency. Others suggested to drop samples (to change output speed). To change by 1%, drop 1 out of each 100. Of course, this is doable; but what will happen to the music? Try to draw the new curve when you drop 30% of the samples (or double 30% of them to achieve the opposite effect). Is this hi-fi? Not to my opinion... Others suggested spectrum analysis and FFT to move from time domain to frequency domain and vice versa. (1) Can this really be done in real time with today's DSP technology? I would doubt that, although I'm not very familiar with state of the art DSP chips, I must admit. and (2) as I understand it (I might be wrong here - Fourier stuff is not one of my stronger parts), this process should be made on a "quantum" at a time - it's not a continuous process. You will still have distortion when you connect the reconstructed parts in the time domain; either you will introduce new harmonics or you will lose information. This is in the *theoretical* view; I don't know about tolerance - that is, if you can make this process "good enough" for hi-fi music processing. The examples given by some people (dictaphones, speech synthesisers, speech distortion units) do not preserve the sound quality. So again, this does not answer the original question. Regards Doron