Origins of the 405 line system.

[Gryphon]

There were electronic switches of a sort - for example, you could use two pentodes with a common anode load, and switch them by altering the potential applied to the screen-grids.  This is how the DF signal switching is done in the R1155 receiver, designed by Marconi just before the start of the war.  I find it difficult to believe that the technique, or something similar, was neither known to, nor shared with, EMI.

I also recall that there was an issue with time-constants in the vision system somewhere which made cuts very difficult until the equipment was improved after the war.  I don't think it's in Birkenshaw, but I know I have read it somewhere.  Sadly, I can no longer remember where.

Lift and syncs were added to the waveform very late in the M-EMI processing chain.  I assume that the intention was to give the black-level clamps as much black level as possible to work on.