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papers/doc/7-systems/opll.md
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the YM2413, otherwise known as OPLL, is a cost-reduced FM synthesis sound chip, based on the Yamaha YM3812 (OPL2). thought OPL was downgraded enough? :p
OPLL spawned also a few derivative chips, the best known of these is:
- the myth. the legend. THE VRC7. 6 channels, *rather interesting* instruments sound bank, no drums mode
- Yamaha YM2423, same chip as YM2413, just a different patch set
- Yamaha YMF281, ditto
# technical specifications
the YM2413 is equipped with the following features:

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papers/doc/7-systems/saa1099.md
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# Philips SAA1099
this was used by the Game Blaster and SAM Coupé. it's pretty similar to the AY-3-8910, but has stereo sound, twice the channels and two envelopes, both of which are highly flexible.
this was used by the Game Blaster and SAM Coupé. it's pretty similar to the AY-3-8910, but has stereo sound, twice the channels and two envelopes, both of which are highly flexible. The envelopes work like this:
an instrument with envelope settings is placed on channel 2 or channel 5
an instrument that is used as an "envelope output", is placed on channel 3 or channel 6. You may want to disable wave output on the output channel.
# effects

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papers/doc/7-systems/vrc6.md
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the chip has 2 pulse wave channels and one sawtooth channel.
volume register is 4 bit for pulse wave and 6 bit for sawtooth, but sawtooth output is corrupted when volume register value is too high. because this register is actually an 8 bit accumulator, its output may wrap around.
For that reason, the sawtooth channel has it's own instrument type. Setting volume macro and pattern editor volume setting too high (above 42/2A) will distort the waveform.
pulse wave duty cycle is 8-level. it can be ignored and it has potential for DAC at this case: volume register in this mode is DAC output and it can be PCM playback through this mode.
Furnace supports this routine for PCM playback, but it consumes a lot of CPU time in real hardware (even if conjunction with VRC6's integrated IRQ timer).
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these effects only are effective in the pulse channels.
- `12xx`: set duty cycle (0 to 7).
- `17xx`: toggle PCM mode.
- `17xx`: toggle PCM mode.