Frequency Control ($9003)

Normally you should write $00 to this register on startup to initialize it, and not make any further writes to it. This is what all three original VRC6 games do.

$9003 controls the overall frequency of the VRC6 audio.

7 bit 0
---- ----
.... .ABH
|||
||+- Halt
|+-- 16x frequency (4 octaves up)
+--- 256x frequency (8 octaves up)

H - halts all oscillators, stopping them in their current state
B - 16x frequency, all oscillators (4 octave increase)
A - 256x frequency, all oscillators (8 octave increase)