Merge branch 'freq-mod-freq-mod-docs2'

papers/doc/7-systems/opll.md

@@ -2,6 +2,11 @@
 
 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:

papers/doc/7-systems/saa1099.md

@@ -1,6 +1,9 @@
 # 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
 

papers/doc/7-systems/vrc6.md

@@ -5,6 +5,8 @@ the most popular expansion chip to the NES' sound system.
 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).
 
@@ -13,4 +15,4 @@ Furnace supports this routine for PCM playback, but it consumes a lot of CPU tim
 these effects only are effective in the pulse channels.
 
 - `12xx`: set duty cycle (0 to 7).
-- `17xx`: toggle PCM mode.
+- `17xx`: toggle PCM mode.