Line Sync (ADC-8032A & ADC-8032A-S)
Line Sync
mode uses the external reference to generate the output timing on a line-by-line basis, but
uses the input signal timing to decide when a frame begins.
Whenever a signal is received, the output-timing generator waits for the next falling edge of H Sync
on the external reference before displaying the incoming line. The advantage of this mode is that as
long as all input sources are timed to be on the same line, the output timing stays constant, regardless
of which input is selected. This is very useful in that all sources to the
ADC-8032A
(for example,
from a routing switcher) only need to be lined up to the same line.
The
Line Sync
mode has internal latency, so that using minimum delay still means that the output
video will not be lined up with the external reference. To do so would have added even more latency,
which would not be desirable. However, output timing can be delayed by up to two lines from the
minimum delay point on, in sub-pixel increments, so that the output video can be lined up as required.
Note
If two input sources are not on the same line, then switching
from one source to the other causes a jump in the output timing
by an exact number of lines. For example, if the two sources
are 2.5 lines apart, the output will jump by either 2 or 3 lines
depending on the position of the sources with respect to the
external reference.
Threshold Point
As the input signal is delayed in time, there comes a point where the output timing will have to jump
by exactly one line (the output must always come after the input). This point is called the
threshold
point
. When comparing the input signal with the external reference, the threshold point is 4.25
microseconds before the falling edge of the external reference. In other words, if the input signal’s
falling H sync edge is 4.25 microseconds before the external reference’s falling H sync edge, the input
signal is said to be on the threshold point.
In addition, there is built in hysteresis so that if an input source is on the threshold point, the output
timing will be constant and will not jump back and forth by a line if the input signal varies slightly.
However, if the input signal is exactly on the threshold point, the output timing may vary from one
power up sequence to the other. In other words, the card may power up with
different timing
than the
previous time it was powered-up. To avoid this condition, it is recommended to keep the input signal
away from the threshold point. One microsecond can be considered a safe distance.
Note
In
Line Sync
mode, you must keep the falling H sync edge of
the incoming video away from the threshold point.
Frame Delay (ADC-8032A-S)
The
Frame Delay
mode is identical to the
Line Delay
mode, except that the output timing can be
delayed by up to a full frame of video. By using almost a full frame of delay, it is possible to set
apparent negative timing, making it look like the output occurs before the input, although delayed by 1
frame.
Frame Sync (ADC-8032A-S)
The
Frame Sync
mode makes it possible to use asynchronous sources. The output timing is generated
entirely based on the external reference. Whether a synchronous or an asynchronous source is used,
output timing is constant. The delay through the card will be the minimum latency (in the order of
microseconds) to a full frame of delay.
4-14
•
Using Banks
ADC-8032A User Manual (Iss. 04)