Yamaha 2149 Verus SID?

[jedinovice]

Just as a comparison in my own mind, how did the Yamaha YM2149 compare with the famous Commodore SID chip? It was, I believe, universally regarded as inferior, but I don't know the details, I played around with the SID chip on the C128 I had but these days I have to do thing sother than program or read tech specs.

 

Overall, how did the two compare?

 

Thanks. Curious.

[NightSprinter]

Well, the YM2149 was really nothing more than a 3-voice sound generator, and mostly output square waves and white noise generation. The SID (6581/8180) was a fully-fledged analog synthesizer in a microchip.

 

The YM chip can sound somewhat crude because it's simple design.. but can be fairly nice-sounding if programmed right.

 

The SID can work just like a normal analog synth (ADSR, distortion, etc.), and can play numerous waveforms.

 

Anyone else whow ants to add to this, feel free.

[supercat]

The Yamaha chip, like many of the time, worked by using frequency dividers to generate square waves. It also had one feedback shift register circuit which could generate a small selection of "white" or "periodic" noise waveforms. Each waveform had a separate volume control.

 

The SID chip used phase accumulators to generate a simple selection of waveforms. Each of three waveforms has a circuit to convert the phase-accumulator output into an output value which is fed into a DAC. In addition, the chip has a programmable analog filter through which any combination of voices may be routed.

 

An extremely sophisticated chip, though Commodore made a number of poor unfortunate decisions in its design. The phase accumulator architecture would allow for some very sophisticated waveforms, but the hardware doesn't capitalize on that ability at all. The envelope generators are fairly sophisticated pieces of hardware, but it would have been more useful to be able to write the voice amplitude registers directly. I think there were some "design by committee" issues at play, since each envelope generator has two 8-bit registers. One controls attack and decay (4 bits each); the other sustain and release (4 bits each). The 4-bit values are fed into 16x8 ROMs to get the actual ADSR values that are then used.

 

The SID does not use all 32 bits of its address space, so I find it curious that the designers went with 4-bit latches feeding 16x8 ROMs instead of using 8-bit latches. The values in the ROMs don't have any particular logic to them--they're just values the designer thought seemed like a nice collection to have.

 

My guess is that there was someone with authority who insisted that the latches be 4 bits, but the designer realized 4 bits didn't have enough resolution, so he snuck in the ROM. The right solution would have been to just use 8-bit latches.

 

Actually, instead of having an ADSR generator, what would have been nicer would have been to just have a "slope rate" register and a "target amplitude". Simpler than ADSR, but more versatile.

[ggn]

I grew up with Pokey and YM sounds, so: Ya boo sucks to SID, YM ftw \o/