Subject: Re: [linux-audio-dev] Re: Bandlimited interpolation suitable for realtime audio
From: Juhana Sadeharju (kouhia_AT_nic.funet.fi)
Date: to syys 09 1999 - 11:21:42 EDT
Audiality wrote:
>Don't know if they're using something like that, but E-mu claims to have
>"8-point interpolation" in their newer samplers. Unfortunately, I'm afraid
>asking them how they do it wouldn't work! ;-)
Check good papers on the topic:
Dattorro, J., 1997, "Effect Design, Part 1: Reverberator and Other Filters,"
Journal of the Audio Engineering Society, Vol. 45, No. 9
Dattorro, J., 1997, "Effect Design, Part 2: Delay-line Modulation and Chorus,"
Journal of the Audio Engineering Society, Vol. 45, No. 10
I have no idea where is Part 3 but Dattorro is at Stanford...
The part 2 tells: Proteus sampling synth and its relatives all employ
seventh-order interpolation polynomials. They are not exactly "Lagrange"
though. They use a technique in which a Remez exchange is applied to an
"ideal" filter response similar to that of Lagrange, but having lower maxima
in the stopband. This gives the deep notches advantageous in the Lagrange
approach, but also the superior stopband rejection of a sinc-based
design. For more information, see the U.S. patent on the fundamental E-mu
G-chip interpolator; no. 5,111,727, David Rossum.
Check also:
T. Laakso, etc., 1996, "Splitting the Unit Delay," IEEE Signal Processing
Magazine, January 1996
This paper is very informative and lists many references.
Code is available at "http://www.hut.fi/HUT/Acoustics/fdtools.html".
JOS wrote: (hey, I sent you mail to your university address, hopefully it
reaches you sooner than later :-)
>In that case you could upsample once when loading the .wav file
>using a high-quality converter. This would increase the load time, of course,
>which could be a problem if you want to support dynamic loading of wave files
>in response to MIDI "program select" during a musical performance.
There would be pre-upsampled files available, of course. :)
Basically, I'm ready to do the thing the way Julius suggest. While
those papers gives other methods we could in the first place take
what is already working good enough. Distortion? I have already written
a time-varying delay line based on above mentioned papers: if I don't
hear any difference between 2-tap and 4-tap interpolation, it doesn't
matter if I use 2-tap interpolation only. (I might have made some mistage
even I verified the formulas carefully and found a mistage (or my mistage?)
in the paper.)
Yours,
Juhana
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