Subject: RE: [linux-audio-dev] Rollin' and scratchin'
From: MOULET Xavier CNET/DMR/ISS (xavier.moulet_AT_cnet.francetelecom.fr)
Date: ke loka 27 1999 - 03:58:15 EDT
well theoretically ang generally speaking,
if your Johnny cash song is a(t), (in the case of a sine, a(t) =
A*sin(b*t+phi))
then t is the time played, which is a function of the speed of the turntable
and of the REAL time. If you note r the rate between "normal" speed R (i.e.
45 rpm or 33) and the one you give with your hand R', then r=R'/R is varying
during the time, and can be negative if you reverse, zero if you stop the
table and 1 if you speed up.
Then, you get r(t). The output of your turntable (besides the "violin noise"
as said in a preceding message) b(t) is then
b(t) = A*sin(b*t*r(t)+phi), wich is typically a FM signal IF a(t) is a sine,
and more generally,
b(t) = a(t*r(t)) (which means that the time goes "r(t)" times faster ..)
What do you hear ? a FM synth !!
remember r(t) is a rotationnal SPEED .
Well if your r(t) is a pure sine, r(t) = R*sin(Ht+hh), then the frequency
representation of (letting aside the constant phases)
b(t) = Asin(br(t)) = Asin(bsin(Ht+hh)))
is a Bessel function, and the spectrum of your signal has sidelobes (i.e :
supplementary frequencies instead of only one freq. at b) roughly speaking
at b+H and b-H. But you have to scratch like HELL (typically ... er ... 1000
times per second ?) if you need to make an FM style sound like this.
Hope all of that is
1) true
2) understandable
3) gives way to expreimentation and calculation
On to the better than real tuentable !!
> -----Message d'origine-----
> De: reactor/CTPmedia [mailto:reactor_AT_sagv5.gyakg.u-szeged.hu]
> Date: mardi 26 octobre 1999 22:18
> Ŕ: LAD Mail
> Objet: Re: [linux-audio-dev] Re: streaming from disk to terminatorX
> added (via mmap)
>
>
> % Benjamin wrote %
>
> >% Wonder how a sine wave looks like when scratched.
>
> nice :)
>
> i have a Johnny Cash record, which has 2 seconds of sinewave in the
> middle of a song, because Johnny said "bad" words :)
>
> anyway, what do you expect to see?
> it should be just a sinewave with changing frequency.
> i can send you a sample, if you're interested.
>
> --
>
> (reactor/CTPmedia)
> (http://linux.gyakg.u-szeged.hu/> ~reactor/index.html)
> (linux
> audiowarez)
> (http://www.bright.net/~dlphilp/linuxsound/toc.html)
>
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