see Sound

Registers

AUDF0 $17
AUDF1 $18

Stella

These addresses write data into the audio frequency divider registers.
D4	D3	D2	D1	D0	30KHz divided by
0	0	0	0	0	no division
0	0	0	0	1	divide by 2
0	0	0	1	0	divide by 3
...	...	...	...	...	...
1	1	1	1	0	divide by 31
1	1	1	1	1	divide by 32