The charge (or attack) current is 1mA, the discharge (or decay) current is 200nA. This equals a current ratio of 5000.
The time constant is defined by the AVL capacitor.
The DC voltage (1-5V) across the AVL capacitor controls a gain stage that stabilizes the audio output level.
Horizontal and vertical synchronization
The main functions are :
- Horizontal sync separator
- Horizontal oscillator and calibration system
- PHI-1 detector
- PHI-2 dector and sandcastle generation
- Horizontal output with slow start/stop facility
- Coincidence detector
- Noise detector
- Vertical sync separator
- Vertical divider system
Y Horizontal sync separator
The horizontal sync separator is supplied from the CVBS/Y inputs (chosen video source). For horizontal synchroni-
sation the sync separator slices in the middle of the sync pulse and the slicing level is independent of the sync pulse
amplitude. For the vertical synchronisation the sync pulse is sliced at level of about 30% (closer to the black level).
This ensures optimal output signals for a stable horizontal and vertical deflection under various video input conditions.
The top sync level is clamped at the CVBS input. The black level is stored internally.
Y Horizontal oscillator and calibration system
The horizontal oscillator requires no external components and is fully integrated. The adjustment for nominal fre-
quency is derived automatically by a calibration circuit.
The oscillator generates a sawtooth signal with double horizontal frequency. This sawtooth signal is used to derive
several other gating and timing signals.
After calibration the horizontal oscillator is controlled by the PHI-1 loop for synchronisation with the incoming video
input signal.
The calibrator is responsible for the automatic adjustment of the horizontal oscillator. One of the colour crystals is
being used as reference. For that reason a correct crystal selection by XA, XB(Xtal selection)is very important during
power-on. Calibration occurs during the vertical retrace period and only under following conditions :
- At power-on/initialization
- After power dip (shoutdown detection), re-initialization is required.
- After loss of synchronization (e.g.after channel switching)
Y PHI-1 detector
The PHI-1 detector is a PLL circuit that synchronizes the horizontal oscillator with the incoming video signal. The PLL
compares the output of the H-sync separator with the horizontal oscillator. The PLL output current is converted to a
voltage by means of the external loop filter. This voltage controls the horizontal oscillator. The loop filter is connected
externally so the time constant can be defined according to the customer requirements. Because the static loop gain
is very high there will be no phase shift when switching between input signals with different line frequencies (e.g.Fh
with 50 and 60 Hz. systems).
Y PHI-2 detector and sandcastle
As described, the Horizontal PLL(PHI-1 loop) synchronizes the horizontal oscillator with the incoming video signal.
The PHI-2 loop provides a stable picture position on screen.
This is necessary because due to beam current variations the storage time of the line transistor varies and, due to
that, the picture position on screen.
The PHI-2 detector compares the horizontal oscillator signal (reference)with the horizontal flyback input pulse, pin41.
This flyback pulse is related to the horizontal deflection.
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