On 1/1/22 8:48 PM, Tom Van Baak wrote:
Hal Murray wrote:
> where do you get the time?
The new years celebrations last night remind me of another source of
time: time balls.
Although a bit of nostalgia these days or even a joke, time balls were
clever, precise, and a critical part of naval infrastructure in the
19th century, lasting well into the 20th century, and still
operational daily in Greenwich (not right now due to refurbishment).
It falls into the category of "time dissemination"; you know, WWV,
GPS, UTC, NTP, and all.
The sound of church bells gave approximate time, partly because their
use case didn't require high precision and partly because sound
travels only one foot per millisecond. Related: starter pistols at
track events; clapperboards on movie sets; timing thunderstorms with
lightning.
Clapper boards even automatically compensate for the acoustic time
delay, especially in a multi channel recording scenario. The board is
placed near the people being recorded, the camera and sound gear is
started (Rolling, Speed,...)
Then the clapper board is actuated (electronically today, with timecode
displayed on an electronic board) (a process called "slating"). So the
*sound* of the clap has to propagate to the sound recording equipment
(some ten miliseconds away if the mic is on a fishpole or boom). In
editing, the sound and picture are lined up, (i.e. the sound is started
later) using the sync from the clapper.
Obscure note - if they forgot to slate at the beginning, or they
couldn't do it for some reason (focus, zoom, blocking), they do it at
the end of the take, and hold it upside down.
Slating is done even if there's no sound, so you can identify the piece
of film. And, if there's multiple cameras, there will be multiple slating.
I sort of left the business before modern digital cameras were
universal, I assume they still do it, even if there's no need to
physically identify a piece of film.
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