7700 MultiFrame Manual
7700ADA7-EQ Equalizing Analog Video Distribution Amplifier
Page - 8
Revision
1.2
6.4.
DC RESTORING (CLAMP) OPERATION
A clamp pulse is generated from the sync information and a clamp signal is delivered to a DC restorer
circuit that positions the blanking level to ground. This feature will work with standard definition analog
NTSC and PAL-B sync structures. Analog tri-level sync 1125-line high definition video (SMPTE 240M) will
also be detected and DC restored. In addition, most versions of bi-level sync that have been applied to
1125-line high definition video will be supported.
"
The clamp position and size will successfully DC restore HD signals with bi-level
sync generated by Evertz DACs for applications with computer monitors. Other
manufacturer’s DACs may not have the same sync size and the DC restoring may not
work with these signals.
The clamp action can be enabled with a jumper. The time-constant of the clamping action may be fast or
slow and the DC level may be set with a card edge POT. For non-video applications disable the clamp
with the jumper (J7). For removing low frequency distortions (<1kHz) from clean video, use the clamp
time-constant (J8) in the FAST position. If the video has a sizeable amount of noise (i.e. a low quality off-
air signal) the fast clamp will generate a line-to-line level distortion (also called "piano-keying") because of
the noise. In this situation, use the SLOW clamp position of (J8). The
CLAMP LEVEL
POT on the card
front edge allows you to adjust the DC level that the back porch is being clamped to. This allows you to set
the output DC level to values other than back porch to ground.
"
Do NOT turn on the clamp when distributing Pb and Pr signals. Standard Definition
and High Definition component Pb and Pr signals have waveforms that extend below
blanking level. The right level and duration of a negative going video signal may be
interpreted as a sync signal, enabling a clamp pulse in active video. This will cause
the DC restorer circuit to distort the video, creating a large Average Picture Level
(APL) change.
6.5. GAIN
ADJUSTMENT
The
GAIN
POT on the cards front edge allows you to adjust the input signal level. Turning the POT
clockwise will increase the gain.
"
There is enough range on this control to counteract the video level error due to a
missing or double terminated coaxial connection. This will create frequency
response problems due to the mis-termination.
TIP:
Set the DC LEVEL before adjusting the gain. The Gain control amplifies the signal around ground.
By setting the back porch to ground first, you may avoid re-adjusting the gain.
