m00dawg wrote:I'm playing this through a mixer, which also goes out to speakers, but also has a rec-out I use to capture the audio. The mixer, I'm sure, introduces some noise but the noise I am hearing definitely sounds like aliasing now that I've looked into it more - the mixer doesn't add that (I use it to record sources that don't have aliased noise).
Ah, that rules that out then.
m00dawg wrote:From what I understand about the SID, it is a digitally-controlled analog synth at its heart. The precision is probably low, but I don't think it has the aliasing problem in the conventional sense. Plus it has analog filtering caps so I bet that may be helping there, in at least the cases where you have it on low or bandpass (I don't recall hearing any aliasing from the high-pass either though).
Well, you're both right and wrong. The SID has three waveform generators and three envelope generators, all digital with each their own corresponding D/A (the waveform generators have D/As with 12 bit resolution, and the envelopes with 8-bit resolution). The envelopes amplify the waveform in the analog domain using a simple DCA circuit, and the filter circuitry is also analog.
The waveform generators, as I said, are essentially just 24 bit phase accumulators. Waveforms are not generated in the analog domain, nor do they use wavetables. They are simply derived digitally from the topmost 12 bits of the accumulator using some serious why-didn't-I-think-of-that digital circuitry. This also means that you get very high precision frequency control and bit depth, especially compared to other sound chips of the time. It's by the nature of the design that audible folding and other artifacts inherent to sampled sound simply won't happen. It will always output the waveforms with phase length at even divisors of the sample frequency, and the highest tone frequency is 1/256 of the "sample" frequency -- well below the nyquist frequency and then some, for the harmonics.
m00dawg wrote:vgx is probably on the money in regards to the aliasing going on. The characteristics of the spectrum graphs I have produced look strikingly similar to some of the ones I have seen when looking at discussions over downsampling (which causes aliasing).
I still very much doubt it, and I brought out my Game Boy to try it just now. I can tell you right off the bat that my listening conditions aren't the best, but I can't hear _any_ folding or post-nyquist frequency mumbo-jumbo. The harmonics of the square wave, although harsh, are very clear and seem undistorted. The oscillator also sweeps up well into the ultrasound range without any of the folding noise I'd typically identify as an effect of aliasing.
What I can hear, though, is some constant high pitched bus whine, and also some lower frequency zipping noise when using an envelope. The latter noise has some high frequency content, too, which is very obvious and quite annoying. Maybe this is what you are hearing?
Last edited by boomlinde (Jul 4, 2011 6:35 pm)