FM Note Calculation
FM notes generated on the YM2612 are specified by two numbers: a block number (Block) and a frequency (Fnum) number. Block selects an octave and Fnum selects a note within that octave. Fnum is 12 bits wide and Block is 3 bits wide.
where:
- Fnote = the frequency of the desired note (i.e. A4 = 440Hz)
- Clock-YM is the clock frequency of the YM2612 (NTSC = 7,670,453Mhz - PAL = 7,600,428Mhz)
- Block is the desired Octave (0 is a valid octave)
Fnum is 12 bits wide and Block is 3 bits wide and are concatenated together when writing data to the YM2612. The 3 Block bits occupy the upper nibble of the most siginificant byte. The data bus to the YM2612 is only 8 bits wide, therefore, these two numbers must be concatenated and then split accordingly as follows:
| |
D7 |
D6 |
D5 |
D4 |
D3 |
D2 |
D1 |
D0 |
| 0xA0+ |
Fnum7 |
Fnum6 |
Fnum5 |
Fnum4 |
Fnum3 |
Fnum2 |
Fnum1 |
Fnum0 |
| 0xA4+ |
|
Block2 |
Block1 |
Block0 |
Fnum11 |
Fnum10 |
Fnum9 |
Fnum8 |
Conversely, to calculate the actual frequency being output from a particular combination of Fnum and Block:
Example YM2612 NTSC Note TableBased on A4 = 440Hz, the following octave can be obtained. For higher octaves, all Fnum values will be identical, simply increment the block value.
Example: - C1 -> Block = 0, Fnum = 644 (0x0284)
- C2 -> Block = 1, Fnum = 644 (0x1284)
- C4 -> Block = 3, Fnum = 644 (0x3284)
Therefore, software only needs to retain a table of values for a single octave, higher octaves just add 16 (0x10) on the high byte.
| Name |
Freq |
Block |
Fnum |
Actual |
MSB (h) |
LSB (h) |
| C1 |
32.703 |
0 |
644 |
32.715 |
0x02 |
0x84 |
| #/b |
34.648 |
0 |
682 |
34.645 |
0x02 |
0xAA |
| D1 |
36.708 |
0 |
723 |
36.728 |
0x02 |
0xD3 |
| #/b |
38.891 |
0 |
766 |
38.912 |
0x02 |
0xFE |
| E1 |
41.203 |
0 |
811 |
41.198 |
0x03 |
0x2B |
| F1 |
43.654 |
0 |
859 |
43.637 |
0x03 |
0x5B |
| #/b |
46.249 |
0 |
910 |
46.227 |
0x03 |
0x8E |
| G1 |
48.999 |
0 |
965 |
49.021 |
0x03 |
0xC5 |
| #/b |
51.913 |
0 |
1022 |
51.917 |
0x03 |
0xFE |
| A1 |
55.000 |
0 |
1083 |
55.016 |
0x04 |
0x3B |
| #/b |
58.270 |
0 |
1147 |
58.267 |
0x04 |
0x7B |
| B1 |
61.735 |
0 |
1215 |
61.721 |
0x04 |
0xBF |
PSG Note Calculation
PSG notes generated on the SN76489 are specified by a single 10 bit divider.
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