LSDJ provide a sync feature, similar to sync24 (24ppqn, or “pulses per quarter note”), http://en.wikipedia.org/wiki/DIN_Sync, with the difference that the gameboy do not wait for pulses, but bytes (so 24 bytes per quarter notes)
So converting Sync 24 to Lsdj is quite simple : Every Sync 24 pulses must be changed into bytes (0xff, or b11111111)
Check this gwEm page to learn how to generate “bytes” with a NE555 : http://preromanbritain.com/gwem/lsdj_pitch/index.html
Thanks to gwEm and Johan for details.
Lsdj to sync24 demo video : http://www.youtube.com/v/KOIxR6RWC_M -- bill