[old_tv_nut]

Originally Posted by David Roper

Baird's description of how phonic coils operated sounds like a sync pulse to me.

I believe as described it was a blanking pulse rather than a sync pulse, that is, it did not go below black level, and thus would disappear on a black scene. The signals used by NBTV these days have a true sync pulse for lines, and one missing sync pulse (blanking only) between frames.