MOTU Digital Timepiece Instruction Manual download pdf (Page 148)

APPENDIX B: SMPTE SYNCHRONIZATION BASICS

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 LT C 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

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