Origins of the 405 line system.

[Gryphon]

ISTR reading that a mechanical SPG was used at Ally Pally at one point. Is this true?

Depends on what you mean by "mechanical SPG".

AAUI, there was no equivalent of a central SPG in the Baird system, as the sources made their own syncs, so in this sense each source had its own mechanical SPG.  As an aside, this has interesting implications for the idea of having co-timed sources at a mixing point, and therefore here is another reason why the Baird system was a dead-end.

The M-EMI SPG is in Birkenshaw and is electronic.  However, I believe that, at one time, mains lock was achieved by comparing a pulse at frame rate with local mains and using the integrated error to drive a d.c. servomotor whose shaft was ganged to a component controlling the frequency of the master oscillator - so in this sense, there can be said to have been a mechanical element to the SPG.