VDP3108B datasheet(11/72 Pages) MICRONAS

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PRELIMINARY DATA SHEET

VDP 31xxB

Micronas

2.3.2. 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

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

lated.

2.3.3. 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. For PAL/

NTSC, a wide band chroma filter can be selected. This

filter is intended for high bandwidth chroma signals, e.g.

a nonstandard wide bandwidth S-VHS signal.

Fig. 2–6: Frequency response of chroma filters

PAL/NTSC

SECAM

2.3.4. 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 the

deemphasis filter, the Dr and Db signals are scaled to

standard CrCb amplitudes and fed to the crossover-

switch.

2.3.5. Burst Detection

In the PAL/NTSC-system the burst is the reference for

the color signal. The phase and magnitude outputs of

the CORDIC are gated with the color key and used for

controlling the phase-lock-loop (APC) of the demodula-

tor and the automatic color control (ACC) in PAL/NTSC.

The ACC has a control range of +30 ... –6 dB.

For SECAM decoding, the frequency of the burst is mea-

sured. Thus, the current chroma carrier frequency can

be identified and is used to control the SECAM proces-

sing. The burst measurements also control the color kill-

er operation; they can be used for automatic standard

detection as well.

2.3.6. Color Killer Operation

The color killer uses the burst-phase / burst-frequency

measurement to identify a PAL/NTSC or SECAM color

signal. For PAL/NTSC, the color is switched off (killed)

as long as the color subcarrier PLL is not locked. For SE-

CAM, the killer is controlled by the toggle of the burst fre-

quency. The burst amplitude measurement is used to

switch-off the color if the burst amplitude is below a pro-

grammable threshold. Thus, color will be killed for very

noisy signals. The color amplitude killer has a program-

mable hysteresis.

2.3.7. PAL Compensation / 1-H Comb Filter

The color decoder uses one fully integrated delay line.

Only active video is stored.

The delay line application depends on the color stan-

dard:

– NTSC:

1-H comb filter or color compensation

– PAL:

color compensation

– SECAM: crossover-switch

In the NTSC compensated mode, Fig. 2–7 c), the color

signal is averaged for two adjacent lines. Thus, cross-

color distortion and chroma noise is reduced. In the

NTSC combfilter mode, Fig. 2–7 d), the delay line is in

the composite signal path, thus allowing reduction of

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