About samples and harmonics...

[(Ian A. Vine (phone UK 0171 419 3450))]

Hi,

	well this is a pretty big subject
>I've got a question that has been puzzling my mind for quite a while
>now: If you transpose a sampled waveform up or down, do weird things
>happen with the harmonics or not ?

	Yup, as you transpose things up or down the frequency spectrum
	is effectively shifted. Going up higher harmonics go above 20Khz
	and going down the high harmonics are not there eg you sample
	an sound that has harmonics up to say 10Khz, down one octave and
	your highest harmonic is only 5KHz.

	The other effect is length of the sample. Sample a two second sound
	play it one octave higher and it only lasts one second, down an octave
	and it lasts 4 seconds. Most samplers get around this by looping the
	sustained portion of the sound.

>In other words: Why can't you create a decent, say, violin imitation
>from just one sample. Is it because the harmonics of the original
>instrument (i.e. the violin) are different from octave to octave ?

	Right again, ideally you could do with a sample for every key
	(assuming you're using a keyboard of course) this can be taken
	even further having several smaples per key for different levels
	of playing (eg soft, load etc.). Then of course you have all the
	differnet ways of playing a violin.

	Most samplers have an amplitude envelope, filter and some sort
	of modulation to allow the use of less multisamples and also 
	let you be a bit more creative with your source material.

>Are there any good books out there that cover these kind of problems ?
>I would really like to know more about Fourrier analysis etc.

	I don't know of any books off hand. Sounds like you need something 
	that is more aimed at DSP code.

	I just remembered with the mention of fourier analysis, I was 
	recently looking at the Emu line of samplers and they claim
	that the samplers can transpose down 2 octaves and up 10 without 
	distortion. Maybe I've got my figures a bit wrong, anybody know
	how they do this ?

Ian