[sdiy] TX81Z, YMxx chips, and aliasing

Andre Majorel aym-htnys at teaser.fr
Wed Sep 5 17:13:24 CEST 2018

On 2018-09-05 13:14 +0100, rburnett at richieburnett.co.uk wrote:

> However, I believe that synths like the DX-7 reduced audible
> aliasing by employing mappings across the keyboard that
> decreased the modulation index of patches progressively for
> higher pitched notes.  Reducing the modulation index in
> Frequency Modulation puts less energy into the higher-order
> sidebands and more energy into the close-in sidebands from the
> FM process, so it results in a signal that has less potential
> to alias badly.

Right, and that was at the discretion of the person designing
the sound. I don't have any experience with the 4-operator chips
used in the TX81Z but I do know that aliasing was easy to run
into with the 6-operator chips used in the DX7. John Chowning
and David Bristow used this fact to estimate the sampling rate
of the DX7 (page 195 in "FM theory and applications").

André Majorel http://www.teaser.fr/~amajorel/

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