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platkus@en.ecn.purdue.edu (Shawn W Platkus) writes: >I'm not sure what you are talking about. If the GS only has eight output >chanels, then it would seen to me that it could only play eight different >sounds at a time. So how can I digitize fifteen different sound and play >them all back simultaneously if there are only eight output channels. To summarize the following lecture: There are 32 oscillators (16 generators) and 16 channels. Not all are used for actual sound output. Oscillators are "smart voices" Generators are oscillator pairs that can generate extra effects with each other Channels are actual independent output lines like left and right speaker That said, let's start at the top. The DOC is a coprocessor with its own dedicated 64K of RAM. All the sound samples have to be put in this RAM before they can be played. The DOC has 32 'oscillators' which are essentially smart DMA channels. Their basic function is to sweep through areas of the DOC memory reading samples and playing them. They can do so at variable speeds (automatically repeating or skipping sample values as necessary), they can loop on a power of two boundary, they can stop when they read a zero, they have independent volume settings, and various other things that aid in reproducing complex instruments without loading down the main CPU. But when you come down to it, the DOC is capable of playing 32 sounds simultaneously and independent of each other, provided that all the sample data fits in the DOC RAM. The oscillators are not all perfectly identical in operation. For the basic sample playing and looping they are, but for some more complex functions they must be paired. This is where the concept of 'generators' comes from -- the 16 generators ARE operationally identical and that is why software prefers the generator concept. Both oscillators and generators are numbered from 0, so oscillators 0 & 1 are generator 0, oscillators 2 & 3 are generator 1, and so on. Generator 15 (oscillators 30 & 31) is reserved for system use (one oscillator is set to loop slowly at zero volume, to generate tempo; I forget if the other is used by anything, it's probably used to play mono samples). Most software use one generator per voice. Since 15 generators are left over, spec'ing the GS as having "15 voice sound capability" is a fair statement. The actual output that comes out the DOC is a 'time-domain multiplexed' sound output and five digital bits. What happens is this: the DOC services each oscillator in turn, and for each oscillator the current sample value is multiplied by the oscillators' volume setting and a voltage proportional to the product is output on the sound output. Four of the digital bits are set to the 'channel number' setting for the oscillator and the last one simply changes voltage from about 3 volts to about 0.5 (for you EE folks out there, this is the negative edge of an output-valid strobe). External hardware is responsible for splitting off the various channels (4 bits means that there can be 16 of them) and outputting them seperately. The motherboard hardware just ignores the channel setting and mixes all the sound outputs into the speaker/earphone. The sound connector on the motherboard only has room for 3 of the four bits, so expansion cards that plug into the sound connector can only get 8 seperate output channels. Most stereo cards (AE's sonic blaster, for example) only pay attention to the lowest bit, so even numbered channels are left and odd numbered channels are right (or is it the other way round? I forget). No, it isn't simple, but it gives a lot of flexibility -- most of which is largely untapped. Todd Whitesel toddpw @ tybalt.caltech.edu