Print

Print


Mike Richter wrote:
> Jon Noring wrote:

>>This I believe is not correct. Implicit in the assumption of samples
>>is that they represent points along a wave form which is the Fourier
>>sum of sine waves. Thus by doing the D-A conversion (but not really),
>>one can predict with high precision the value a sample should have
>>that's arbitrarily somewhere inbetween any two samples (which is what
>>resampling is intended to do, at least that's how I understand it.)
>>
>>Thus, I am of the understanding, which may be wholly wrong, that the
>>published algorithms for digital resampling are very well developed
>>and universally implemented by the better sound processing tools. And
>>that one can even predict the distortion caused by these algorithms, and
>>that it is typically very small. But then, I've been surprised by what
>>these tools can and cannot do. Anyone?

> You are correct. Except in the simplest cases (e.g., halving the sample
> rate), and not even always there, resampling entails interpolation or
> extrapolation. The quality of the interpolation is selectable; higher order
> means more processing and a better fit.

I think in practical terms this means:

1) Changing the pitch digitally is acceptable (provided the sampling
   rate and bit-depth are sufficient), thus

2) One needs not to do precise pitch control in the analog domain (i.e.,
   the speed of the turntable) IF the resulting digital transfer is to
   be digitally restored afterwards, provided of course that

3) The proper resampling tool is used.

This indicates to me that unless one knows the algorithm in their
sound editing application used to resample (or change pitch which is
essentially the same process), then finding a specific tool to do
this, and which is known to be of high quality, should be used. Such a
tool can be a fairly simple batch operated code, or possibly a plugin
to one of the standard sound editing tools. I am uncertain whether the
resampling/re-pitching algorithms standard in the common sound editing
tools, such as CoolEditPro and Sound Forge, are adequate. Anybody know?
And anyone know of a high-quality standalone (or plug-in) resampling
tool?

With regards to the PDF URL I gave in my prior message regarding the
bandlimited interpolation, there is also an online HTML version:

http://www-ccrma.stanford.edu/~jos/waveguide/Bandlimited_Interpolation.html#17791


Jon Noring