I remember a summer job that I had in the mid 1980s while I was at
university. It involved being the poor sod who digitised a lot of
U-Matic tapes of <censored> for a defence company. Tedious and
repetitive work. They had a slow motion recorder that used what looked
like an 8-inch floppy disk.
They had a computer which triggered the slo-mo to record one frame every
n frames of video from the U-matic, and then played it back frame by
frame, with the computer triggering frame advance after it had finished
digitising one of the still frames. I think it took about 5 seconds to
digitise a frame - it was very definitely not real-time.
The slo-mo was broadcast-spec, a cast-off of BBC or ITV sport. Its own
inherent logic (ie without being driven by the computer) allowed it to
record normal video and then play it back at 1/2, 1/3, 1/4 etc speed, as
well as slightly more esoteric rates like 1 frame of slo-mo taking 3
fields, 5 fields etc (1.5, 2.5 etc frames) rather than the normal 4, 6,
8 fields (2, 3, or 4 frames). I found a manual for it, and there was a
hand-written note that someone at BBC/ITV had made, saying that
mixed-field rates (one frame displayed for 1.5, 2.5, etc frames) was not
to be used if a certain make of VTR was used to record the output
because it confused the VTR's sync track. I'm not sure why a frame made
up of two fields taken at the same time was any better than a frame made
up of two fields taken with a time interval between them.