Sony BVU-950 - Page 26

What comes out of the sync separator can be represented as in Figure
B. It is only the sync information, which you will recall contains
both the horizontal sync info (to turn the electron beam off at the
end of a line, and return it to the left side of the screen to start
the next horizontal line,) and the vertical syne (i.e. to turn the
beam off after the 262.5th line, and tell it to return to the top of
the screen to start the next field.)
What we want to do next is just pull out the vertical sync info, which
will occur every 1/60th of a second: this is the role of the Vertical
Drive Detector, and it gives us a signal such as that in Figure C; a
pulse every 1/60th second. This figure C signal goes into either a
mono-stable multi-vibrator or a bi-stable multi-vibrator (commonly
called a flip-flop). Some Sony VTR's use one, some use the other,
Since either type works.
What the MM or FF gives us is the signal in Figure D, which is the
control signal laid down longitudinally as mentioned above. This
signal is either on (in which case it is about a 12 volt signal) or
off (a 0 volt signal, obviously). Every. time the vertical sync pulse
in Figure C goes through the MM/FF, the control signal (Figure D)
changes from on to off to on, etc. When it goes from off to on, we
call it a positive-going pulse. It is these positive going pulses
which are used as a reference signal.
So where does the framing servo fit into this?
You've noticed by now, I am sure, that (1) the on and off states of
the control signal last exactly 1/60th of a second, the same as one
field, and (2) together, the on and off states last exactly 1/30th
of a second, the same as one frame. Indeed, this is where we started
off in the first place.
The next question is, which field does the on or off state refer to?
This is exactly what the framing servo function is; to tell us this
information.
A framing servo:
1. Identifies the odd and even fields of the incoming video signal.
2. Bases the reference signal for the VTR servo system on the frame
identification which is possible because of this odd/even identi-
fication.
3. Records this reference signal onto the CTL track.
How does it do this?
First of all, we'll add a “half-horizontal line reject" circuit and a
gate circuit to the circuits we showed you earlier. Chart 4 indicates
this addition to the circuits shown in Chart 3.
4-10