ATARI XL XE POKEY

[6sides]

Studying POKEY assembly. Looks like a flexible wavetable synth to me.

Any examples of this technique? Dynamic, crafted data, rather than samples just slapped in.

Looks like built in waveforms were defaults, but widely used.

 

[rdefabri]

No examples that I'm aware of, but I'm not the POKEY expert here.  I know a little about wavetable synthesis (I own an Ensoniq SQ-80), I was still under the impression that the single-cycle waveform source was still a sample.  If that's the case, you would need a source for those sampled waveforms to be referenced.

 

I know POKEY can play sampled sounds obviously - see Chaos Music Composer or Black Magic Composer (I believe both have a source of sampled instruments you can use in song creation).

[Rybags]

Pokey doesn't use complex waveforms.

Ignoring forced volume (sample by software) playback, each voice will only output 0 or 1.

At higher frequencies there can be interaction among voices to produce perceived waveforms such as triangle or sawtooth.

 

In general usage, there's square wave and "poly noise" output.  Poly noise is actually output selection using LFSR.  The longer (17 and 9 bit) ones are perceived as noise.

The shorter ones and combinations of those can resemble waveforms such as sawtooth but are in fact they are all predictable repeating sequences.

[6sides]

Zoom in on a complex waveform and it consists of a series of voltage differences. I believe POKEY is capable of sending variable voltage to the audio output, hence speaker control using data lookup table.

Pump it with efficient algorithm. Populate the lookup table.

 

Volume control toggle.

Edited October 21, 2023 by 6sides

[thorfdbg]

Pokey VOLONLY implements a 4-bit DAC per voice, but since Pokey does not have a DMA channel to supply sample data, it is a very CPU intensive process that is mostly used for special effects (such as software voice) and not so much for music.

[6sides]

Rain is good but, stop raining!   There was a time when it was thought impossible to do rotating starfields on the 64 or AMAZING 3D on Amstrad CPC (Batman Joker demo I think). Oh and XL XE crunching through SID files, the odd one being almost preferable to the original.

Sure, it's not totally faithful; but Research Machines Atari 800XL is on my mind. Nice little puzzler to get on with. Not aiming for SID emu, something dynamic and fresh.

Hard, enjoyable work. For an audience of 200 maybe? Lol😄 

[Rybags]

Something I should have mentioned - if you monitor a pure tone on the audio out pin it should be a square wave.

But the resultant output once it's gone through the external audio circuit - you'll see attack/decay phases in the waveform which probably softens the tone a bit.

Though this is external to Pokey itself (and probably much the same on all computer models)

[6sides]

Some capacitance on the output.

My POKEY musings, Inspired by Chiptunes on a PIC microcontroller, which has surprising results.

And this track.

 

[rdefabri]

@VinsCool what were the major changes to this version of Tarm Ruins from the original I found on the ASMA site?

 

I guess my real question is - how do you keep the instrument sounds intact when playing?  Every time I try to use a 16-bit / linked channel sound, it's degraded by whatever is playing on another channel.  

 

I was surprised to see Tarm Ruins was a 4-channel track - I've been toying with dual POKEY because I can't seem to figure out how to fix the issue above.

Edited October 25, 2023 by rdefabri