B is for… Beeper music
The original ZX Spectrum's sound output was an extremely crude 1-bit 'beeper' that could be set to an 'on' or an 'off' state - flipping rapidly and regularly between the two gives a steady tone.
It's not really a sound chip at all, as it requires the CPU to be constantly feeding data to it, and it was only really intended for sound effects. That didn't stop crafty programmers pushing it to the max, though - witness Tim Follin's multi-channel sound routines, as heard in games like Agent X 2 and Chronos. And Brian Marshall's ahead-of-its-time banger of a tune for 1986's Prodigy has to be heard to be believed - breakcore/classical mash-up, anyone? Today's most active 1-bit maniac is probably Mister Beep.
B is also for... Bits!
Chip music is often referred to as '8-bit' music. This is a reference to the 8-bit CPUs used by the vast majority of late ‘70s and early ‘80s home computers and games consoles. It does not refer to the bit-depth of the audio they produce, however.