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* AGC
An AGC system controls the IF amplifier gain such that the video output amplitude is constant.
The demodulated video signal is supplied, via a low pass filter, to an AGC detector with external
decoupling capacitor. The AGC detector voltage controls directly the IF gain stages.
Negative/positive modulation:
For optimal AGC behavior the charge and discharge current of the AGC are chosen so that both, a
relative fast AGC, as well a low tilt are possible for positive and negative modulated signals with the
same AGC capacitor.
A SECAM-L speed-up circuit improves the AGC settling time after IF-signal loss.
With tilt is mend the video amplitude variation due to less "memory" function of the AGC capacitor). In
Fig 2 the tilt is given during a field period for positive modulated signals. For negative modulated
signals the tilt is line frequent. For negative modulated signals the AGC is a top sync detector.
Positive modulation:
For positive modulated signals the AGC is a top white AGC including black level clamp which making
the video amplitude independent of video contents.
A top white AGC requires a 100% white reference pulse in order to be independent of video contents.
Suitable for that purpose is the white pulse (Video Insertion Test Signal, VITS) in line 17 and 330 - see
Fig 2.
Because the time constant is large (AGC current is decreased for positive modulation) the AGC will be
independent of the video contents between the reference pulses.
The maximum tilt per field is defined by the external capacitor, the AGC steepness and the small
discharge current in this mode.
100% white
Fig 2: Positive modulated signal with top white reference pulse
This top white AGC however is not optimal for those signals that have neither a white reference pulse
or white video information (e.g. with some VCR signals). Due to the top level AGC principle a gray
scene becomes white and a dark scene becomes gray, see Fig 3. This behavior can be avoided by
means of the black level clamp.
one field
5
tilt
Reference
pulse
//
3DQDVRQLF
AGC voltage
Video output
signal
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