Assembly: https://worldofspectrum.net/legacy-info/spectrum-rom-load-ro...
TL;DR:
> A '0' bit is encoded as 2 pulses of 855 T-states each.
> A '1' bit is encoded as 2 pulses of 1710 T-states each (ie. twice the length of a '0')
> A 'pulse' here is either a mark or a space, so 2 pulses makes a complete square wave cycle.