Is there anywhere a FAQ about the individual timbres of the POKEY sound chip on Atari XL?
The POKEY is a really unique sounding device which can do a lot of bizarre timbres (shift register feedback noises) those do not exist on normal analogue or digital synthesizers. It can do e.g. noises like crunching crusty cookies, like a bumblebee in a paper bag or like lighting a match on a matchbox, like scraping a butter knife on a sesame crisp bread, like pulling a pine cone over sand paper - it can sound like overrolling a dry scone with a bike, like peeing on burning sausages on a coal grill or like a leaky gas boiler almost about to explode... Short - it is an as special sound machine like the Roland TB-303
. Unfortunately the behaviour of the most interesting mode ("distortion" mode 12 in Basic) is extremely irregular, i.e. the pitch range from 0..255 is shared among many different waveforms those seem to follow no obvious scheme and break out of the conventional chromatic scale. Is there anywhere a chart that lists all available pitch values per waveform? Is there a mathematical formula that clearly explains which pitch value outputs which waveform (e.g. are certain bits of this register reserved to switch them?).
-Has anybody a complete copy of the POKEY data sheet?
I already tried to download the POKEY chip data sheet. Unfortunately the site that hosted it seems to be dead, thus I could only recover it from the Google cache where all the diagram pictures are missing (was http://homepage.ntlw...ri_hw/pokey.htm
However the formulas in it look very technical and don't obviously explain the irregular order of the waveforms. It seems to be something like that the selected squarewave frequency is bitwise XORed with the output of the shift register noise generator that runs on a fixed clock frequency, and the result depends on the ratio between both frequencies or the like. (It has been very long ago when I learned about polynomic stuff, div() ans mod() operations on the university.) Also the implementation of the "highpass filter" looks very strange; does it basically shorten too long pulses by doing binary operations between 2 sound channels?
I own the German "Atari BASIC Referenzkarten" (reference cards) from my Atari 400, which lists note values for the plain squarewave voice ("distortion" 10); here apparently doubling the value (and adding 1?) halves the pitch frequency. But in mode 12 this scheme does not work. I already experimented a bit with Basic sound loops but could not identify the scheme behind distortion mode 12.
-Too loud volume damages POKEY?
In the "Atari Program Exchange Program Author's Handbook" stands an odd warning: "Never let the sum of the volume levels of the four sound channels exceed 15; doing so damages POKEY (the sound generating chip)." Is this really true? So far I remember, every Basic novice cranked up the volume of all channels to the max without that ever anything bad happened. It may distort and thus might fry a hifi tweeter by the harshness when turned too loud, but certainly not fry a POKEY. I only remember that keyboard and joystick matrix scanning lines could be switched to output and thus short each other when keys/ joystick contacts were closed, but did this cause dangerous currents? Has anybody ever toasted his POKEY chip by too high volume settings?
-About me and the POKEY sound...
I grew up with Atari VCS, 400 and 800XL, thus since my childhood the POKEY tone scale set my melodical perception; I would love to use them to create a software synthesizer based on POKEY sound style, and I am about to modify an old concert zither to imitate the special detuned bass voices. (I found out that nickel wound strings sound brighter than the original copper wound ones and want to re-string the zither alternatingly according to typical POKEY sound structures instead of the original chord pattern. I have to discard some rusty strings anyway those cause splinters in the fingers.)
I am collector of strange music keyboards and sound toys and operate a huge website about them (WarrantyVoid), which includes a detailed circuit-bending FAQ with some basics about squarewave music and POKEY timbres. I also composed some music in that style on Amiga. (See http://Weltenschule....ters/index.html
I also collect historical videogames and homecomputers. By Atari I own various VCS2600 versions, VCS7800, Atari 400, 2x 800XL, 65XE, 260ST, Lynx.
Currently I have not learned Atari 8bit assembly language, but can only do some Atari Basic. Long ago I went through electrical engineering and software engineering studies. It may be that in farer future I will program a software synthesizer for PC based on POKEY sound (with many additions), but at the moment I have enough other things to do. (On PC there is already the "Raster Music Tracker" for making POKEY music, which I haven't examined closer yet.)