VRC7 audio
The Konami VRC7, in addition to being a mapper chip, also produces 6 channels of 2-operator FM Synthesis Audio. The synthesis core appears to be mostly compatible with the Yamaha YM2413 OPLL.
Registers
Audio Register Select ($9010)
7......0 VVVVVVVV ++++++++- The 8-bit internal register to select for use with $9030
Audio Register Write ($9030)
7......0 VVVVVVVV ++++++++- The 8-bit value to write to the internal register selected with $9010
Internal Audio Registers
The VRCVII appears to have 26 internal registers. Registers $00-$07 define a custom patch that can be played on any channel set to use instrument $0. Registers $10-$15, $20-25, and $30-35 control 6 channels for FM synthesis. Other register values appear to be ignored.
Custom patch
| Register | Bitfield | Description |
|---|---|---|
| $00 | TVSK MMMM | Modulator tremolo (T), vibrato (V), sustain (S), key scaling (K), multiplier (M) |
| $01 | TVSK MMMM | Carrier tremolo (T), vibrato (V), sustain (S), key scaling (K), multiplier (M) |
| $02 | KKOO OOOO | Modulator key scale level (K), output level (O) |
| $03 | KK-Q WEEE | Carrier key scale level (K), unused 0 ? (-), carrier waveform (Q), modulator waveform (W), feedback (E) |
| $04 | AAAA DDDD | Modulator attack (A), decay (D) |
| $05 | AAAA DDDD | Carrier attack (A), decay (D) |
| $06 | SSSS RRRR | Modulator sustain (S), release (R) |
| $07 | SSSS RRRR | Carrier sustain (S), release (R) |
Channels
| Register | Bitfield | Description |
|---|---|---|
| $10-$15 | LLLL LLLL | Channel low 8 bits of frequency |
| $20-$25 | --ST OOOH | Channel sustain (S), trigger (T), octave (O), high bit of frequency (H) |
| $30-$35 | IIII VVVV | Channel instrument (I), volume (V) |
Each channel x is controlled by three registers at $1x, $2x, and $3x.
The 8 bits of $1x with a 9th bit from bit 0 of $2x create a 9-bit frequency value (freq). This is combined with a 3-bit octave value from $2x (octave) to define the output frequency (F):
49722 hz * freq
F = -----------------
2^(19 - octave)
Writing to register $2x either begins a key-on or key-off event, depending on the value in the trigger bit (T). If the trigger bit is 1, a new note begins, if it is 0 it will begin the release portion of its envelope which will eventually silence the channel. If it was already 0, no new key-off will be generated. Note that the octave and high frequency bit cannot be altered between key-on and key-off, but the low 8 bits of frequency in $1x can be used to apply vibrato and pitch bends.
Register $3x selects the instrument patch to use, and chooses a volume. Note that volume value is inverted; $F the lowest volume, and $0 is the highest.
Internal patch set
There are 16 different instrument patches available on the VRC7. With the exception of instrument $0, which can be controlled by registers $00-$07 (see above), these are hardwired into the chip and cannot be altered.
Exact values for the fixed patch set are not yet known, the following table is conjectural:
00 01 02 03 04 05 06 07
-----------------------
0 | -- -- -- -- -- -- -- --
1 | 31 22 23 07 F0 F0 E8 F7
2 | 03 31 68 05 F2 74 79 9C
3 | 01 51 72 04 F1 D3 9D 8B
4 | 22 61 1B 05 C0 A1 F8 E8
5 | 22 61 2C 03 D2 A1 A7 E8
6 | 31 22 FA 01 F1 F1 F4 EE
7 | 21 61 28 06 F1 F1 CE 9B
8 | 27 61 60 00 F0 F0 FF FD
9 | 60 21 2B 06 85 F1 79 9D
A | 31 A1 FF 0A 53 62 5E AF
B | 03 A1 70 0F D4 A3 94 BE
C | 2B 61 E4 07 F6 93 BD AC
D | 21 63 ED 07 77 F1 C7 E8
E | 21 61 2A 03 F3 E2 B6 D9
F | 21 63 37 03 F3 E2 B6 D9
Differences with OPLL
- Register layout is the same
- VRC7 has 6 channels, OPLL has 9
- VRC7 has no rhythm channels, OPLL does (the last 3 channels are either FM or Rhythm on OPLL)
- VRC7 built-in instruments are NOT the same as OPLL instruments (except for instruments 4,5,7,8 which might be the same on both)
- VRC7 has no readily-accessible status register, under normal circumstances it is write-only; OPLL has an undocumented, 2-bit 'internal state' register.
- VRC7 has an internal state output pin (may be serial version of the 2 bit internal state register) and has one output pin for audio, multiplexed for all 6 channels; OPLL has two output pins, one for FM and one for Rhythm, and has no special status pin.
References
- VRC7 chip info by Kevin Horton: http://kevtris.org/nes/vrcvii.txt
