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Old 04 December 2013, 20:52   #72
Mrs Beanbag
Glastonbridge Software
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Join Date: Jan 2012
Location: Edinburgh/Scotland
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Originally Posted by pandy71 View Post
Nope, Paula decrement SysCLK/8 and SysCLK can be provided different than original i.e. different than 28.37516 MHz for "PAL" or than 28.63636MHz for "NTSC".
Well ok, I thought that was the case but I could only find the one number. Anyway I think you get the point.

If the emulator mixes the sound into a buffer, whatever frequency the buffer plays at it is resampled from 28.x MHz.

Given that, a little interpolation during the periods in which Paula ticks over its counter, would produce a slightly more accurate result. The question is, could anyone hear such a difference? Or is nearest neighbour good enough?

Originally Posted by pandy71 View Post
I think you are confused by analogue, waveform and built-in - 6581 is hardcoded HW design where HW in Amiga is "less" hardcoded.
6581 is digital chip (NCO, hard coded wavetable) with analog filter and digital ring modulator.

More about 6581 internals
Ok so SID produces digital wave forms but not from an internal wave table.

SID had to be entirely self-contained and there was no room at all for a wavetable on the chip.

The Sawtooth waveform was created by sending the upper 12-bits of the accumulator to the 12-bit Waveform D/A.

The Triangle waveform was created by using the MSB of the accumulator to invert the remaining upper 11 accumulator bits using EXOR gates. These 11 bits were then left-shifted (throwing away the MSB) and sent to the Waveform D/A (so the resolution of the triangle waveform was half that of the sawtooth, but the amplitude and frequency were the same)

The Pulse waveform was created by sending the upper 12-bits of the accumulator to a 12-bit digital comparator. The output of the comparator was either a one or a zero. This single output was then sent to all 12 bits of the Waveform D/A.

The Noise waveform was created using a 23-bit pseudo-random sequence generator (i.e., a shift register with specific outputs fed back to the input through combinatorial logic).The shift register was clocked by one of the intermediate bits of the accumulator to keep the frequency content of the noise waveform relatively the same as the pitched waveforms. The upper 12-bits of the shift register were sent to the Waveform D/A.
I would call this a synth.

I would not call Paula a synth because it cannot produce any output without any input wave data.

Last edited by prowler; 04 December 2013 at 21:12. Reason: Back-to-back posts merged.
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