Ta for the replies :-)
I've been doing some fiddling and can report back...
At 25 & 50, pulse lasts 1 second, but happens every 30 seconds.
Both iframe and FPS at 25, the pulsing happens every 2 seconds.
At 15 & 15, pulsing every 10 seconds.
Bumping bit rate to 8192 and max had no effect, and...