The "SID" seems to output a triangle wave ($11) with quite a lot of
noise
in it. Why is this?
/Daniel Kahlin (tlr)
http://www.kahlin.net/daniel/dtv
Since I don't know how serious the noise is, I'll give you two places it could be coming from, but first let me explain how the analog voltages are generated.
Inside the ASIC each voice has a sigma-delta converter, which is made of several adders and special feedback. This outputs a bit train of 1's and 0's that are passed into a low pass cheap R/C filter. The output of the R/C filter is summed/DC isolated with the other channels with a cheap capacitor network. (See the trend of cheap cheap cheap?)
You'll notice more noise in the triangle wave more since it's transistions are softer and have less high frequency edges compared to saw and square waves.
1. Check for defects or incorrectly stuffed parts in the R/C and summing networks.
2. The sigma converter is being clocked at 32mhz, which limits its bandwidth. Increasing the C or R in the low pass will suppress the high frequency conversion errors, but will reduce the sharpness of the saw and square waves. (It's a toss up) Originally there was an internal set of flops that essentially did a low pass on the input to the converters, but had to be abandon to save space (about 40 flops and gating logic)