🚀 go-pugleaf

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