Forums

CD Mixing - DSP or DAC Clock speed variation?

Started by mr_alex November 7, 2005
HI there,

I'm currently working on a university project and am trying to conduct
some research into how CD DJ players work by changing the speed of a
track playing from a CD.

Some research I've conducted has led me to believe that the sample rate
conversion and the pitch shifting is done via DSP operations - but as
these calculations need to be done in 'real-time' on the fly it would
require a very fast DSP processor to work on 44.1KHz stereo?

I've also read that that 'speed' off the track playing can be changed
by altering the clock rate of the DAC to change from perhaps 40KHz upto
50KHz to cover the standard +-8% range these devices need for DJ's to
mix music with them.

Does anyone on this board have experience of how this is done on
commercial devices? There are many manufacturers who make these CD
mixing players: Pioneer, Denon, Kam, Numark, Technics etc...They must
have some embedded DSP processor which provides the pitch shifting
capabilities and also the time-stretch..

Any help/info/advice appreciated. I have searched the WWW for many an
hour and these are obviously trade secrets so are not easy to come
across!
Thanks, Alex.

> I'm currently working on a university project and am trying to conduct > some research into how CD DJ players work by changing the speed of a > track playing from a CD.
Actually it's not a big deal - I mean, a lot of number crunching but not problematic with today's technology. They do something like determine the component frequencies then play them a little "longer" or shorter to fill up the extra/lesser time. Good algorithms also handle transients (attacks, drums, cymbols, etc) seperately. There are several plugins for winamp that will do this on a modern PC without noticeably loading it: chronotron, I forget the other. There's also amazing slow downer for the mac. I think there might even be source for one floating around. Sometimes these will even work reading directly off a CD (assuming digital audio extraction, obviously it can't work with analog in) Some academic wrote a paper proposing an audio compression scheme that had an inherint pitch shifting / speed modification built in - the compression process tends to include extracting the information you would need anyway, and this compressed format kept it seperate so that you could decode at whatever speed or pitch shift you wanted. Would in theory be better than mp3 for dance music, but in practice mp3 + pitch shift plugin works fairly well.
"mr_alex" <dancedynamix@hotmail.com> wrote in message 
news:1131371573.647670.277030@g49g2000cwa.googlegroups.com...
> HI there, > > I've also read that that 'speed' off the track playing can be changed > by altering the clock rate of the DAC to change from perhaps 40KHz upto > 50KHz to cover the standard +-8% range these devices need for DJ's to > mix music with them.
Changing the DAC clock has the same effect of sample rate conversion (pitch scaling AND inversely proportional time scaling), but without requiring any processing. If the hardware supports this feature, then it is a nice way to do it since there is no loss of quality and no extra SRC processing required. But I would imagine that many of these products would need to mix two or more sound sources at different samples rates or speeds, so unless they have separate DACs for each one (unlikely), it may not be feasible. The processing required for SRC shouldn't be that big of a deal on modern processors anyway.
> Does anyone on this board have experience of how this is done on > commercial devices? There are many manufacturers who make these CD > mixing players: Pioneer, Denon, Kam, Numark, Technics etc...They must > have some embedded DSP processor which provides the pitch shifting > capabilities and also the time-stretch.. > > Any help/info/advice appreciated. I have searched the WWW for many an > hour and these are obviously trade secrets so are not easy to come > across!
You might be able to infer some information about how they work by reading the user's guides and specifications (look especially for limitations). That would be my suggestion.
Jon Harris wrote:

> Changing the DAC clock has the same effect of sample rate conversion (pitch > scaling AND inversely proportional time scaling), but without requiring any > processing. If the hardware supports this feature, then it is a nice way to do > it since there is no loss of quality and no extra SRC processing required.
Yes, but these decks have the ability to independently vary pitch or speed, without affecting the other. This does require signal processing (really you change both, and then use processing to return the other to something like it's original)
<cs_posting@hotmail.com> wrote in message 
news:1131389852.944940.154570@g43g2000cwa.googlegroups.com...
> Jon Harris wrote: > >> Changing the DAC clock has the same effect of sample rate conversion (pitch >> scaling AND inversely proportional time scaling), but without requiring any >> processing. If the hardware supports this feature, then it is a nice way to >> do >> it since there is no loss of quality and no extra SRC processing required. > > Yes, but these decks have the ability to independently vary pitch or > speed, without affecting the other. This does require signal > processing (really you change both, and then use processing to return > the other to something like it's original)
Right. You could use DAC clock frequency shifting to do some of the job, but not all of it. But I doubt that's how the commercial products do it. The DSP requirement for the SRC part is small compared with the time stretching.