MOTU micro express-USB, Руководство пользователя

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A P P E N D I X B : S M P T E S Y N C H R O N I Z A T I O N B A S I C S
146
WHAT IS LTC?
Longitudinal Time Code (LTC) is the audio form of
SMPTE Time Code. LTC consists of an audio signal
that oscillates between two frequencies (approxi-
mately 2 and 4 kHz) to form an 80-bit word of 1’s
and 0’s for each frame on the tape. The 80 bits in
each SMPTE frame describe, in binary form (1’s
and 0’s), the location of that frame in hours,
minutes, seconds, and frames.
WHAT IS VITC?
Vertical Interval Time Code (VITC, pronounced
“Vit-see”) is SMPTE time code that is encoded in
the video signal in the vertical blanking segment at
the top edge of each frame. A video signal consists
of 525 scan lines, which the rotating heads of a VTR
scan as the tape rolls past them. The first couple
dozen of the scan lines at the edge of each frame are
blank; they do not contain any part of the video
picture. VITC is recorded on several of these blank
scan lines.
You can actually see VITC in the vertical blanking
segment portion of a video picture by adjusting the
vertical hold on a video screen. The 90-bit binary
VITC signal appears as a series of white dots in the
black strip between the top and bottom of the
picture.
VITC is part of the video signal; it does not have its
own “track” on the video tape. It is therefore not
possible to stripe VITC by itself onto video tape.
VITC can only be recorded at the same time as the
video picture.
THE BENEFITS OF VITC OVER LTC
Of the two forms of SMPTE time code, LTC has
become much more widely used as a synchroni-
zation standard in the audio production industry
because VITC synchronizers in the past have been
extremely expensive. So, the term
SMPTE or
SMPTE time code has become a common
expression for LTC in recording studios, post-
production houses, MIDI hardware and software
manuals, and so on.
Mark of the Unicorn has developed the Digital
Timepiece, an affordable VITC synchronizer and
video character generator. It is both an LTC
and
VITC synchronizer, so the distinction between
LTC and VITC becomes important. So, if you are
used to saying “SMPTE”, ask yourself “What type
of SMPTE? LTC or VITC?” This will help prevent
confusion as you work with the Digital Timepiece.
The primary advantage that VITC has over LTC is
that synchronization can be achieved at very slow
tape speeds––even when shuttling the video tape
backwards or forwards one frame at a time. VITC
allows for this because it is part of the video signal,
which is continuously scanned by the VTR’s
rotating heads even when the tape is stopped. LTC
cannot be read at slow tape speeds because it is an
audio signal in one of the audio tracks, which can
only be read when the tape is moving at a constant
speed.
Another benefit of VITC is that it does not eat up
any audio tracks.
SHOULD I USE LTC OR VITC?
Since VITC only works with video, you must use
LTC for synchronizing a multi-track tape deck. But
don’t fret: LTC is affordable and more than
adequate for tape synchronization.
If you compose music for film or video, you too can
use LTC. We strongly recommend, however, that
you consider VITC because of the benefits noted
above. Contact Mark of the Unicorn for more
information about the Digital Timepiece.
FRAME RATES
In either form (LTC or VITC), SMPTE time code
has several basic formats for counting frames per
second (fps): 24, 25, 29.97 non-drop, 29.97
!USB Interfaces Manual Book Page 146 Tuesday, October 10, 2000 12:43 PM