Sinclair QL sound pitch and frequency
December 6th, 2011
The sound “capabilities” of Sinclair QL are controlled through a single call to the IPC. You pass it two pitches, plus numerous strange parameters that distort the sound in more or less useful ways. To my great surprise I couldn’t find any description of the pitch parameter in any book or web page. For example, The SuperBASIC handbook goes to great detail with other parameters, but doesn’t explain how the pitch is calculated. I’m starting to feel like nobody really understood it and just quoted some technical manuals 🙂 Well, here’s my take on it, based on FFT analysis and a bit of math.
Pitch and frequency are inversely related, like on the Amiga, where you use periods. The highest tone you can output is pitch 0, which, according to Audacity’s FFT is 1313 Hz. At the other extreme there’s pitch 255, which produces a 43 Hz tone. Assuming that the formula is of the form a/(x+c), we get approximately the following relationship between frequency (f) and pitch (p):
- f = 11336.256 / (p + 8.634)
- p = 11336.256 / f – 8.634
When dealing with integer numbers you get a good enough approximation with p=11336/f-8. You can download a precalculated list of note pitches and frequencies here.
1 Comment Add your own
1.
Antti Silvast | December 7th, 2011 at 10:57 pm
Thanks for the lab report. 🙂 Motivated to learn the sound chip, I started making instruments and gathering other’s examples here: https://github.com/anttisilvast/QL-Demo/blob/master/examples/sounds.bas
See here for lots of source code on QL beeps: http://www.dilwyn.me.uk/sound/index.html
Kommentin kirjoitus
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