Hot switch handling – Grass Valley 8981FS User Manual
Page 19
8981FS Instruction Manual
13
Configuration
Gain can be adjusted ±40%. (See
, Function Switch posi-
tions 1 though 6, Bank 2) Offset and gain adjustment values set in Variable
mode will be retained if the mode setting is changed to UNITY. If the
setting is later changed back to VARIABLE these offset and gain settings
will return.
Hot Switch Handling
The 8981FS module has a number of mode setting options for handling
switching transients from a routing switcher feed. If the input signal con-
tains non-synchronous switches, the module will briefly provide a field
freeze until it can lock to the new input timing. These automatic freezes can
be inhibited by setting the Auto-freeze mode to DISABLE. (See
, Function Switch position 8, Bank 1).
The automatic freeze circuitry looks for vertical timing jumps and hori-
zontal timing steps in the input signal. Changes in horizontal timing can be
detected on the line immediately following the step.
Vertical timing jumps cannot be reliably detected until the next vertical
interval. This means that a full field may have to be written to memory
before the module can detect that the vertical timing is no longer as it was.
Depending upon the relative video input to output timing, the signal
written to memory may have already been read out before the synchro-
nizer can detect that the vertical timing is incorrect. This can result in a one
field vertical jump in the output picture. If the horizontal timing also con-
tained a step, the module would enter the freeze mode without having to
wait for the next vertical interval. The output would then have a momen-
tary freeze and no vertical picture jump would occur.
The magnitude of the horizontal timing step required to trigger the auto-
matic freeze mode can be set to one of two values using the Auto-freeze
Sensitivity mode setting. When the sensitivity is set HIGH, a timing step of
a single 27 MHz clock cycle will trigger a momentary freeze. With the sen-
sitivity is set LOW, a horizontal timing step of greater than 20 ms is
required to trigger a freeze. This mode is intended for use in applications
where the synchronizer is downstream of switching equipment which has
its inputs timed within 20 µs of each other. As long as the switching equip-
ment has its inputs timed vertically and horizontally within this window,
the synchronizer will not produce momentary freezes at signal switches.
This provides clean active picture switches at the module output. The
module can recover from vertical interval timing steps of this magnitude if
they occur on or before line 12/275 (525) or line 9/322 (625). Horizontal
blanking interval ancillary data (HANC) may be disturbed for up to a
frame following the switch point. This will also apply to any embedded
active line time vertical blanking interval ancillary data (VANC) from the
switch point up to line 14/277 (525) or line 11/324 (625).