Re: [Linuxsampler-devel] Sampler quality comparison...

[Mark K. <mk...@co...>]

be...@ga... wrote:
> I agree with Sebastien here, resampling by too big factors causes
> bad shifting of formants even if you use the highest quality 
> resampling method and natural instruments would sound very bad.
> (try to span a piano sample over 2 octaves or so).
> 
> sfz+ uses bandlimited interpolation which is very high quality
> (no aliasing if you upsample high frequency content) but this method
> of interpolation can consume an unlimited amount of CPU as you
> go higher with resampling factors.
> I think cubic interpolation is the best compromise because it is 
> fast and offers good quality when downsampling or upsampling in ranges
> where you don't hit the nyquist frequency. 
> eg a 15KHz sine upsampled by 1 octave (played at speed factor 2) would
> sound good with bandlimited interpolation since it would filter
> the frequencies above 15kHz.
> 15KHz * 2 = 30kHz, apply the samplerate/2 (about 20KHz) filter and
> what you get is silence, which is better than those ghost frequencies
> eg (22 - 15 = 7kHz).
> But as Sebastien said, if you sample each 2-5 semitones you don't run
> into these problems plus you avoid that too much formant shifting causes
> unnatural sound.
> 
> cheers,
> Benno
> http://www.linuxsampler.org

I agree with the both of you. Resampling is generally not the way to go. 
I will relate one problem I ran into a while back with GSt where this 
would have been useful.

I got a message from a guy in Europe somewhere about wanting to 
collaborate on a tune. He sent me a Pro Tools session, both MIDI and 
audio. I didn't notice that the session was at 48K. I do all of mine at 
44.1K, which is the sample rate of all GSt libraries that I own. 
However, since I connect Pro Tools and GSt, which are on different 
computers, through an ADAT digital sync supplied over optical cables, 
GSt started running at 48K, the Pro Tools session speed, instead of 
44.1K, the sample rate of the libraries. It turns out that this 
frequency change is almost exactly a whole note high, being just a bit 
out of tune (try it yourself some time) so I thought he had just made a 
mistake in his guitar tunings. It wasn't until later that I was using 
both GSt and my guitars that I discovered the problems.

It would have been nice, in this specific case, to have a way for GST 
(now LS) to accept a 48K clock and resample the 44.1K library into that 
rate to keep the tuning correct. Granted it would never sound as good as 
if the session had been done at 44.1K, but it would make LS more useful 
to do this.

What I ended up doing was taking the MIDI tracks that went to LS and 
transposing them by a whole step and then creating off-tuning GSt 
session files to get the thing tuned into the audio from Pro Tools. what 
a mess! ;-)

This is not an issue when you use and audio connection as LS will run at 
44.1K in stand alone mode.

- Mark