Alesis ADAT Manuel du propriétaire télécharger pdf (Page 295)

The Basics

295

Owner’s Manual

Version 1.0 – English -

Time-Referenced synchronization

Time-referenced synchronization originates from the ﬁeld of

ﬁlm synchronization, but nowadays it is also used for audio

work. This is why it divides a second not into tenths and

hundredths, but into frames. One frame was originally the time

it took for a single frame of ﬁlm (i.e. one image) to pass through

a ﬁlm camera, or projector.

Unfortunately, the number of frames used per second varies

according to country, norm and usage. For ﬁlm, the interna-

tional norm is 24 frames per second (fps). American black and

white television uses 30 fps. With the introduction of color tele-

vision, the frame rate of the NTSC norm, used in America and

Canada had to be reduced to 29.97 fps for technical reasons. In

Europe, a lower frame rate of 25 fps was used from the start,

and with the introduction of color television this was adopted

by the European PAL TV standard, as used in Europe today.

 The original reason for the differing ﬁlm rates, incidentally, derives from the different

rates of alternating current used on the different continents (USA: 60Hz, Europe: 50

Hz), which corresponds to the number of half-frames of ﬁlm passing through a

camera/projector per second.

SMPTE/EBU

It was the American Society of Motion Picture and Television

Engineers (SMPTE) which ﬁrst laid down a norm for encoding

the individual frames. This designates exactly 80 bits per frame

for encoding the hour (0-23), minute, second, and frame (frame

number within the second). Some of the surplus bits are used

to indicate the frame rate, i.e. the number of frames per second.

This encoded data stream of 80 bits per frame is known as

SMPTE time code. Because the individual bits themselves

have a deﬁnite time spacing, they are also used as a further

subdivision of a frame, called a ›subframe‹.

This code was adopted without alteration by the European

Broadcasting Union (EBU), for use with the European frame

rates, and renamed ›EBU Time Code‹. In practice this time

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Alesis ADAT Manuel du propriétaire Page 295