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VDP3108

Single-Chip Video Processor

Micronas 

VDP 3108

ADVANCE INFORMATION

2.2. Color Decoder

In this block the entire luma/chroma separation and multi

standard color demodulation is carried out. The color de-

modulation uses an asynchronous clock, thus allowing

a unified architecture for all color standards.

Both luma and chroma are processed to an orthogonal

sampling raster. Luma and Chroma delays are matched.

The total delay of the decoder is adjustable by a FIFO

memory. Thus, including the delay of the display proces-

sing, exactly 64 µsec processing delay can be obtained.

The output of color decoder is YCrCb in a 4:2:2 format.

2.2.1. IF-Compensation

With off-air or mistuned reception any attenuation at

higher frequencies or asymmetry around the color sub-

carrier is compensated. Three different settings of the

IF-compensation are possible:

flat (no compensation)

6 dB /octave

12 dB /octave

Fig. 2–4: Frequency response of chroma IF-com-

pensation

2.2.2. Demodulator

The entire signal (which might still contain luma) is now

quadrature-mixed to the baseband. The mixing frequen-

cy 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 quadrature

baseband components of the FM modulated chroma.

After the mixer, a lowpass filter selects the chroma com-

ponents; a downsampling stage converts the color dif-

ference signals to a multiplexed half rate data stream.

The subcarrier frequency in the demodulator is gener-

ated by direct digital synthesis; therefore substandards

8

such as PAL 3.58 or NTSC 4.43 can also be demodu-

lated.

2.2.3. Chrominance Filter

The demodulation is followed by a lowpass filter for the

color difference signals for PAL/NTSC. SECAM requires

a modified lowpass function with bell-filter characteristic.

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 (narrow,

normal, broad) are available for each standard. The filter

passband can be shaped with an extra peaking term at

1.25 MHz.

dB

0

–10

PAL/

NTSC

–20

broad

normal

–30

narrow

–40

–50

MHz

0

1

2

3

4

5

dB

0

–10

SECAM

–20

–30

–40

–50

MHz

0

1

2

3

4

5

Fig. 2–5: Frequency response of chroma fil-

ters

2.2.4. Frequency Demodulator

The frequency demodulator for demodulating the SE-

CAM signal is implemented as a CORDIC-structure. It

calculates the phase and magnitude of the quadrature

components by coordinate rotation.

The phase output of the CORDIC processor is differen-

tiated to obtain the demodulated frequency. After a pro-

grammable deemphasis filter the Dr and Db signals are

scaled to standard CrCb amplitudes and fed to the cross-

over-switch.

MICRONAS INTERMETALL

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