A fascinating item here from a BBC programme shown in April 1970, looking back at the production of the first televised play in 1930. At the time of this item most of the original cast and crew were still around to re-enact it... in colour!
https://www.youtube.com/watch?v=oaYe1eLFCxw
I'm pretty sure when 'The Man With The Flower In His Mouth' was broadcast, the procedure was to show a few minutes of vision, followed by a blank screen with the audio as both could be not be transmitted at the same time. Is that correct?
Steve