every pattern is converted to a string of values, which represent frequency, control (type) and volume (and duration).
the string is read sequentially as follows:
0: bit 7: change V/C bit 6: change F bit 0-4: freq/length
if change F is set, bits 0-4 are interpreted as frequency change. if all bits are zero, this marks the end of the pattern.
if change V/C is set, the next byte is read:
1: bit 4-7: control (type) bit 0-3: volume