CMS/GameBlaster emulation thread

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Reply 60 of 61, by stripwax

Posted on 2018-11-19, 23:41

stripwax Offline

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Demystifying a couple of other things: different versions of the datasheet have meaningfully different contents I think. At least one version states this (which I didn't realise until just now):

If the frequency channel controlled is off (NE = FE = 0) the envelope will appear at the output, which provides an alternative "non-square" tone capability. In this event the frequency will be the envelope rate, which provided the rate is from the frequency channel, will be a maximum of 1kHz. Higher frequencies of up to 2kHz can be obtained by the envelope resolution being halved from 16 levels to 8 levels. (Rates quotes are based on an 8MHz clock)

So I guess my note above about whether this is by design or undocumented can now be disregarded 😀

There's also a note in the datasheet that says:

The SAA1099 uses pulse width modulation to achieve amplitude and envelope levels ... The amplitude and envelope signals chop the output at a minimum rate of 62.5kHz.

That puzzled me until just now - I think that refers to the following combination:
Frequency enabled, Noise enabled, Amplitude set to 2 and Envelope set to 1.
Under this scenario, per my table above, the combination of the amplitude + envelope patterns ANDed together yields one pulse every 64 cycles; and by mixing both frequency and noise then the pulse will occur only on even periods (since odd periods contribute noise); assuming the noise is generating a zero level then the combination yields just one pulse every 128 cycles, or 62.5kHz at 8MHz clock.

Last edited by stripwax on 2018-11-20, 00:02. Edited 2 times in total.

Reply 61 of 61, by stripwax

Posted on 2018-11-19, 23:57

stripwax Offline

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Posts: 4
Joined: 2018-11-11, 15:05

Just thought about this too:

1HLHHHHHHHHHHHHHL LLLLLLLLLLLLLLLH HLHHHHHHHHHHHHHL LLLLLLLLLLLLLLLH ....
2               ^                ^                ^                ^

So.. that makes me think the tables are wrong by 4 cycles actually. If I shift all the tables by 4 cycles, I think this just works, and then we also know the answer to what was discussed much earlier: "what is the actual alignment of these PDM sequences". It's probably easier to believe that the flipping between tone/noise/tone/noise when mixing both tone and noise occurs at clock cycle 0, rather than at clock cycle 60. I'd be willing to bet that we get the real PDM tables:

1AMPLITUDE PDM SEQUENCES
2 0: 0000000000000000000000000000000000000000000000000000000000000000 = 0
3 1: 0000000011110000000000000000000000000000000000000000000000000000 = 4/64
4 2: 0000000000001111111100000000000000000000000000000000000000000000 = 8/64
5 3: 0000000011111111111100000000000000000000000000000000000000000000 = 12/64
6 4: 0000000000000000000000000000000000001111111111111111000000000000 = 16/64
7 5: 0000000011110000000000000000000000001111111111111111000000000000 = 20/64
8 6: 0000000000001111111100000000000000001111111111111111000000000000 = 24/64
9 7: 0000000011111111111100000000000000001111111111111111000000000000 = 28/64
10 8: 1111000000000000000011111111111111110000000000000000111111111111 = 32/64
11 9: 1111000011110000000011111111111111110000000000000000111111111111 = 36/64
1210: 1111000000001111111111111111111111110000000000000000111111111111 = 40/64
1311: 1111000011111111111111111111111111110000000000000000111111111111 = 44/64
1412: 1111000000000000000011111111111111111111111111111111111111111111 = 48/64
1513: 1111000011110000000011111111111111111111111111111111111111111111 = 52/64
1614: 1111000000001111111111111111111111111111111111111111111111111111 = 56/64
1715: 1111000011111111111111111111111111111111111111111111111111111111 = 60/64

1ENVELOPE PDM SEQUENCES
2 0: 0000000000000000000000000000000000000000000000000000000000000000 = 0/64
3 1: 0000000000001000000000000000100000000000000010000000000000001000 = 4/64
4 2: 0000010000000100000001000000010000000100000001000000010000000100 = 8/64
5 3: 0000010000001100000001000000110000000100000011000000010000001100 = 12/64
6 4: 0011000000110000001100000011000000110000001100000011000000110000 = 16/64
7 5: 0011000000111000001100000011100000110000001110000011000000111000 = 20/64
8 6: 0011010000110100001101000011010000110100001101000011010000110100 = 24/64
9 7: 0011010000111100001101000011110000110100001111000011010000111100 = 28/64
10 8: 1100001111000011110000111100001111000011110000111100001111000011 = 32/64
11 9: 1100001111001011110000111100101111000011110010111100001111001011 = 36/64
1210: 1100011111000111110001111100011111000111110001111100011111000111 = 40/64
1311: 1100011111001111110001111100111111000111110011111100011111001111 = 44/64
1412: 1111001111110011111100111111001111110011111100111111001111110011 = 48/64
1513: 1111001111111011111100111111101111110011111110111111001111111011 = 52/64
1614: 1111011111110111111101111111011111110111111101111111011111110111 = 56/64
1715: 1111011111111111111101111111111111110111111111111111011111111111 = 60/64

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