VPC 323xD, VPC 324xD
ADVANCE INFORMATION
2.3.1. IF-Compensation
2.3.2. Demodulator
With off-air or mistuned reception, any attenuation at
higher frequencies or asymmetry around the color
subcarrier is compensated. Four different settings of
the IF-compensation are possible (see Fig. 2–3):
The entire signal (which might still contain luma) is
quadrature-mixed to the baseband. The mixing fre-
quency is equal to the subcarrier for PAL and NTSC,
thus achieving the chroma demodulation. For SECAM,
the mixing frequency is 4.286 MHz giving the quadra-
ture baseband components of the FM modulated
chroma. After the mixer, a lowpass filter selects the
chroma components; a downsampling stage converts
the color difference signals to a multiplexed half rate
data stream.
– flat (no compensation)
– 6 dB/octave
– 12 dB/octave
– 10 dB/MHz
The subcarrier frequency in the demodulator is gener-
ated by direct digital synthesis; therefore, substan-
dards such as PAL 3.58 or NTSC 4.43 can also be
demodulated.
The last setting gives a very large boost to high fre-
quencies. It is provided for SECAM signals that are
decoded using a SAW filter specified originally for the
PAL standard.
dB
2.3.3. Chrominance Filter
The demodulation is followed by a lowpass filter for the
color difference signals for PAL/NTSC. SECAM re-
quires a modified lowpass function with bell-filter char-
acteristic. At the output of the lowpass filter, all luma
information is eliminated.
The lowpass filters are calculated in time multiplex for
the two color signals. Three bandwidth settings (nar-
row, normal, broad) are available for each standard
(see Fig. 2–5). For PAL/NTSC, a wide band chroma fil-
ter can be selected. This filter is intended for high
bandwidth chroma signals, e.g. a nonstandard wide
bandwidth S-VHS signal.
MHz
Fig. 2–3: Frequency response of chroma IF-com-
pensation
Notch
Filter
Luma / CVBS
Luma
Chroma
1 H Delay
CrossSwitch
ACC
IF Compensation
Lowpass Filter
Phase/Freq
MIXER
Demodulator
DC-Reject
Chroma
ColorPLL/ColorACC
Fig. 2–4: Color decoder
10
Micronas
MICRONAS [ MICRONAS ]
