560 Chapter 17
In practice the same heads can also be used for recording, even though they are too wide.
As can be seen in Figure 17.27 , the excess track width is simply overwritten during the
next head sweep. Erase heads are unnecessary, as the overlapping of the recorded tracks
guarantees that the whole area of a previous recording is overwritten. A further advantage
of the system is that more than one track width can be supported by the same mechanism
simply by changing the linear tape speed. Prerecorded tapes made by contact duplication
have lower coercivity coatings, and to maintain the signal level the tracks are simply
made wider by raising the tape speed. Any RDAT machine can play such a recording
without adjustment.
In any rotary head recorder, some mechanism is necessary to synchronize the linear
position of the tape to the rotation of the heads, otherwise the recorded tracks cannot be
replayed. In a conventional video recorder, this is the function of the control track, which
requires an additional, stationary head. In RDAT the control track is dispensed with, and
tracking is obtained by reading patterns in the slant tracks with the normal playback heads.
Figure 17.28 shows how the system works. The tracks are divided into fi ve areas. PCM
audio data are in the center and subcode data are at the ends. Audio and subcode data
are separated by tracking patterns. The tracking patterns are recorded and played back
along with data. The tracking is measured by comparing the level of a pilot signal picked
up from the tracks on each side of the wanted track. If the replay head drifts toward one
side, it will overlap the next track on that side by a greater amount and cause a larger pilot
signal to be picked up. Pilot pickup from the track on the opposite side will be reduced.
The difference between the pilot levels is used to change the speed of the capstan, which
has the effect of correcting the tracking.
Ordinarily, azimuth effect prevents the adjacent tracks being read, but the pilot tones are
recorded with a wavelength much longer than that of data. They can then be picked up by
a head of the wrong azimuth.
Track
width
Head
widthOverlapFigure 17.27 : In azimuth recording, the tracks can be made narrower than the head pole by
overwriting the previous track.