Appendix 1

Appendix 1: Understanding the NTSC Video Format and Digital Video
On the earliest systems, a spinning drum with the heads attached was placed perpendicular to the
tape movement. The heads would spin across the width of the tape, which was two inches wide (see
Figure A1.8). Because the tape was moving forward at a slow speed, each head crossed the tape at
a slight angle creating slightly angled bands of signal across the tape. Thirty-two of these bands could
hold one frame of analog video signal.
Figure A1.8 Two inch quad videotape track layout. Two inch quad was the “gold standard” for
decades
On later systems, the tape was wound around the head drum in a helix, spiraling down from the top
of the drum to the bottom. This made the signal bands much longer, several inches long even on a
much narrower tape. Now, each fi eld could be recorded on one band or two bands per frame.
While the video was recorded on the helical bands, audio was recorded on the edge of the tape with
a fi xed head. A “control track” was recorded on the opposite edge also with a fi xed head. The control
track is used to keep the speed of the moving tape interlocked with the video signal. Without it, the
tape speeds up or slows down causing the video heads to loose alignment with the video tracks.
Time code can also be recorded by a fi xed head on one of these “linear” or “longitudinal” tracks
running along the edge of the tape.
On some machines, time code and even audio can be interleaved into the video on the helical tracks.
Time code placed into the sync is called Vertical Interval Time Code (VITC). It is also possible to
interleave copyright information and copy guard.
When digital recorders came on the scene, they used this same basic tape transport system. The
helical heads could record the extremely high bandwidth needed when recording a digital data stream.
One important difference between digital and analog recording is that an analog recording needs to
be a constant stream of successive signal. However, digital data can be shuffl ed, compressed, and
even mixed with other data. Digital recorders do not “waste” bandwidth recording sync information
like an analog recorder does. The sync information is created in playback. A digital fl ag causes
the sync video signal to be created by the playback machine at the proper time. Yet some digital
recorders can still record and play VITC and copyright data by recording six lines of the sync signal
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