Philips Semiconductors
Preliminary specification
PAL/NTSC/SECAM video decoder with adaptive PAL/NTSC
comb filter, VBI-data slicer and high performance scaler
SAA7114H
8.1.2.1
Clamping
The AGC (automatic gain control for luminance) is used to
amplify a CVBS or Y signal to the required signal
The clamp control circuit controls the correct clamping of
the analog input signals. The coupling capacitor is also
used to store and filter the clamping voltage. An internal
digital clamp comparator generates the information with
respect to clamp-up or clamp-down. The clamping levels
for the two ADC channels are fixed for luminance (60) and
chrominance (128). Clamping time in normal use is set
with the HCL pulse at the back porch of the video signal.
amplitude, matched to the ADCs input voltage range.
The AGC active time is the sync bottom of the video signal.
Signal (white) peak control limits the gain at signal
overshoots. The flow charts (see Figs 7 and 8) show more
details of the AGC. The influence of supply voltage
variation within the specified range is automatically
eliminated by clamp and automatic gain control.
8.1.2.2
Gain control
The gain control circuit receives (via the I2C-bus) the static
gain levels for the two analog amplifiers or controls one of
these amplifiers automatically via a built-in Automatic Gain
Control (AGC) as part of the Analog Input Control (AICO).
TV line
analog line blanking
handbook, halfpage
controlled
ADC input level
handbook, halfpage
analog input level
255
maximum
+3 dB
GAIN
HSY
CLAMP
HCL
0 dB
range 9 dB
0 dB
60
1
(1 V (p-p) 18/56 Ω)
−6 dB
minimum
MHB325
MGL065
Fig.4 Analog line with clamp (HCL) and gain
range (HSY).
Fig.5 Automatic gain range.
2000 Mar 15
14
NXP [ NXP ]
