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| Electronics » the v3 Z80 project » sound tweaks I'd hadn't really done much with the soundchip since version 2 of the Z80 Project - even then I'd only had it produce a few test beeps. Ideally I wanted it to play music - not the latest bangin' mp3s of course (its soooo not capable and well, that would be dead boring and un-retro anyway:) No, music as in 80's / early 90's game tunes! I'm absolutely *not* a musician myself so I needed it to play existing music files - the most usable and readily available being Amiga Protracker modules (I couldnt really use C64 SID tunes as there's no standard replay routine, PC players have to emulate the C64's CPU as well as the sound hardware). So... there was my challenge, write a Protracker replay routine in Z80 that'd work with my hardware. Slight snag: the Amiga's sound hardware is a 4 channel sample player and my soundchip is more like a 4 channel version of the C64's SID chip, ie: a tone generator. It *can* play sampled sound using interrupts but due it only having a 64 byte internal buffer and no DMA abilities, it can only manage one sampled voice at a time. Some dodgy multiplexing shennanigans could've had it playing x sampled voices of course but that would hog the CPU and be useless for games and demos. Clearly another approach was required.
The first tune I converted was one I'd used in Giddy3 as the game completion tune (called Skogen's Djur 20 written by Daniel Johansson) - I've put an mp3 of the Z80 project playing it below. Its a nice crisp tune that doesnt use anything too fancy in terms of waveforms - it came out quite well I think. Some of the tunes I tried next had distinctive sounding wave shapes and I was having trouble matching them accurately in software so I had another think about playing digital sound. As the waves for such voices were really tiny (about 32 bytes) I realized I could download them to the SX28's internal SRAM (I had 64 bytes to play with -woo!) when the parent module was initalized. From there they could be played by all 4 channels at once if required. So, a new wavetype was added to the soundchip firmware: "Play sample loop from internal registers" - some pointers to the start and end locations were required and then away it could go just like any of the synthetic waveforms. Whilst I was looking into this I remembered how the SID chip did "ring modulation" effects by flipping the output of the triangle wave generator based on the square wave output of another channel (ie the MSB of another channel's accumulator), its probably not going to be much use for music but it makes some interesting FX so I implemented this wave type too. The SX28 has 2048 12-bit locations that can be used for its program. My sound firmware had used about 500 of these words and I wondered if I could employ up the rest usefully. Most modules use drum sounds, so I decided it'd be a reasonable idea to put a micro drumkit in there: a bass drum, a snare and a hi-hat would just about fit with some radical downsampling and 6-bit conversion - and hopefully still sound like drums :) The SX28 IREAD instruction made it easy to read the program word locations as data so I wrote a small routine in Pure Basic to convert the samples to source code and included them as data in the SX28 program. Of course the sounds can also be chopped and mixed by the synth code just like the other voices so a variety of FX can be spawned from the raw samples. Source code etc can be found in the file base.. Here's some mp3s of the tunes I've converted so far:
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