DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Mechanical synchronization
is called servo lock, or lock.
Certain tape formats have no control
track – its function is accomplished
by sophisticated processing of
special tracking signals recorded on
the helical tracks.
tension is controlled more crudely, often by a passive
friction mechanism.
Editing
I mentioned that when recording blank tape, the CT is
recorded. Assemble edit refers to switching (at a field
boundary) from synchronized playback into record, such
that the resulting pattern of tracks on tape is continuous.
Subsequent playback is continuous across the
edit. An assemble-mode edit records the control track.
When assembly recording ends, if the tape was previously
recorded, there will almost certainly be a discontinuity
between the track just recorded and the tracks
previously recorded. Subsequent playback across this
point would suffer a disruption in playback video while
the playback circuits reestablished synchronization.
An insert edit begins identically to an assemble edit;
however, the control track is not recorded. Instead, it is
played back – the playback CT information locks the
capstan servo, exactly as in play mode, so that the tape
advances in accordance with the previously recorded
tracks. When insert recording ends, the boundary
between new material and previously recorded material
is continuous; playback across that boundary will
suffer no disruption.
You might be tempted to insert-edit all the time.
However, insert editing requires previously recorded
video: You’ll have to use prerecorded tape (typically
recorded with black) instead of blank tape.
Many studio recorders are capable of confidence play,
whereby video is read immediately upon being
recorded on tape. Confidence playback is achieved
through a set of playback heads different from the
record heads. Often the playback heads can be
deflected for track-following in trick modes. (Record
heads are always affixed to the scanner.)
In most consumer VCRs, audio is recorded on
a longitudinal track. This severely limits audio performance,
and prevents digital audio recording. Studio
416 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES