NewTek Tricaster TCXD850, User Manual, Page: 304 / 336

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B.3 IMAG AND LATENCY
What’s IMAG? It’s a compression of the expression “image magnification.” Typically in modern
IMAG applications, video cameras supply live imagery to projection systems, magnifying speakers
and performers so that audience members further back in large venues can still see what’s going
on.
IMAG is a very tricky task at the best of times, one that calls for excellent planning, and where
possible, testing. Those designing an IMAG installation have, not just one, but
two inter-related
broadcasts to consider – in the form of the audio and video streams.
B.3.1 RELATIVITY AND THE SPEED OF LIGHT
Wouldn’t it be nice if audio and video travelled from their respective broadcast devices at the
same speed? Then, wherever you were seated in the audience, the sound from hypothetically
perfect speakers and the video image from huge video displays co-located at the front of the
auditorium would reach your ears and your retinas at precisely the same moment!
This is not the case, however. Sound travels quite slowly – so slow, in fact, that even in relatively
small venues it reaches those in the rear of the audience noticeably later than those in the front.
In loose terms, for a mid-size auditorium 600 feet long, it takes around a half-second for the audio
to reach those in the back.
For this reason, audio engineers often position speakers throughout the ‘house’, then introduce
carefully considered delays by electronic means - to ensure ‘late sound’ from front speakers does
not arrive after sound from the nearest speaker to those further back.
Light, on the other hand, travels so much faster that for all intents transmission can be
considered instantaneous in the same setting. So a person in the rear will see the image on a
screen at the front long before sound from a
co-located speaker arrives.
If transmission of the video signal from the cameras lens right through to the projection screen
were instantaneous (it’s not, mind you), we’d likely need to
find a way to delay it (as otherwise,
the carefully timed delays the audio engineers induced would ensure a mismatch between video
and late arriving audio). Viewed in this light, a certain amount of latency is actually “A Good
Thing!”