Accom ASWR8150, Technical Manual

Page: 210 / 236

"Preread" and the 8150
Some digital VTRs offer a "preread" or "read before write" feature. This
function takes advantage of the fact that, in a digital VTR, the "stunt
motion" playback heads are physically ahead of the erase and record
heads for each video track.
In normal Play mode, the stunt motion playback heads are used as the
playback heads. In Record or Insert Edit modes, they actually play back
the video and audio data on the tape (the so-called "preread") before it
is erased and modified video and/or audio is recorded in its place. The
playback heads are typically located at points on the scanner as far in
advance of their corresponding erase heads as possible. Since the
scanner is rotating at a known speed, there is a certain predictable
amount of time between when the playback heads are in contact with a
certain point on the tape, and when the record heads are in contact with
the same point on the tape. It is this time interval that lets preread work
by routing the preread video out of the VTR to another processing device
(in this case, the 8150), and back into the VTR before the record heads
get to the point on the tape where the video originally came from.
The preread video from the VTR can now be combined with a new video
signal (usually by keying the new video over the off-tape signal), or
otherwise processed, and sent back to the VTR input. If the processing
time outside the VTR is within the time interval between the playback
heads and the record heads, minus processing time inside the VTR, the
modified video can be re-recorded in the same location on the tape. In
the case of the Digital Betacam

, this time interval is approximately 40
TV lines in the 525-line standard. Since the maximum delay through the
8150 is 34 lines, the 8150 can be used to process the off-tape video in
Preread mode.
Of course, when you perform a Preread edit, you erase and overwrite
the original (unmodified) video so that it no longer exists on that tape.
You must process the off-tape video correctly the first time – there are
no second chances. For this reason, it would be good practice to make a
digital protection "clone" or "dub" of the master tape before editing it in
Preread mode.
The 8150 needs no special setup to work with the Preread feature.
Connect the 8150 and the digital VTR to the same reference; connect the
8150 Program output or one of the Aux outputs to the VTR digital video
input; and connect the VTR digital video output to one of the 8150 digital
inputs.
"Preread" and the 8150 8150 Technical Guide
5-74
Engineering Setup
9100-0212-04 - May 2000