From: BlueChip <cs_bluechip_at_webtribe.net>
Date: Wed, 23 Feb 2005 14:15:00 +0000

At 09:22 23/02/2005, you wrote:
> >>Yes, this is pitch shifting ...When you change the speed, the pitch
>implicitly changes (eg. a double of speed will cause a one octave shift)
>...So after you have changed the speed, say "up", you then need to change
>the pitch back "down" ...hope that makes sense.
>
>When you examine their code, you will find the details on how this is
>implemented. It's not that difficult to port it over to Rockbox.
>
>BC
>
>OK, you have the code but I own the real machine. And this doesn't an
>octave shift by doubling speed. This last issue is possible with rockbox
>at the moment, the other not. Or have I misunderstood you?
>
>Joerch

Under normal circumstances, a double of speed causes an octave shift -
quite handy for learning guitar solos on old tape players - halve the
speed, and you are still in tune, just down 1 octave. This is the very
nature of changing the speed of an audio sample - try it in your favourite
sound editor (eg. Sound Forge) Double or halve the sample rate - and
notice the change in pitch.

The maths to shift a few cents or a couple of semitones is far more complex
and uses the decimal point to great effect.

With the recent advent of Fast Fourier Transforms (FFT) (and similar) it is
now realistic to demand pitch shifts WITHOUT changing the speed-of-the-tape
or bitrate-of-the-sample.

"Yeah" I hear you cry "but what about changing speed without changing
pitch" ...think laterally for a moment... Changing the speed inherently
changes the pitch, by it's very nature. BUT we now how to correct the
pitch ...so speed up/down the sample and then pitch shift down/up to
correct what changes.

This process is intensive and requires use of the MAS core, not the
SH1. And this means an "MAS CODEC" ...What we need is someone like
IDCDragon or Amiconn to take over development where Linus & I left off
before I was banned for having an unlikely name and the project was dropped
- the best part of six months of work down the tubes [that should bring a
smile to Bjorns face]

If you need any more information or knowledge about how and why this works,
take a little time to learn all about Fast Fourier Transforms and then
review some speed and pitch shift algorithms, probably easiest to do in
Matlab - for which I know much exists in the way of sample editing source code.

Good Luck,

BC

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Received on 2005-02-23