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VDP313XY

Video Processor Family

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VDP 313xY

ADVANCE INFORMATION

2.4.2. Demodulator

The entire signal (which might still contain luminance)

is now quadrature-mixed to the baseband. The mixing

frequency is equal to the subcarrier for PAL and NTSC,

thus achieving the chrominance demodulation. For

SECAM, the mixing frequency is 4.286 MHz giving the

quadrature baseband components of the FM modu-

lated chrominance. After the mixer, a lowpass filter

selects the chrominance components; a downsam-

pling stage converts the color difference signals to a

multiplexed half rate data stream.

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.

2.4.3. 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

luminance 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.

For PAL/NTSC, a wide band chrominance filter can be

selected. This filter is intended for high bandwidth

chrominance signals, e.g. a nonstandard wide band-

width S-VHS signal.

2.4.4. Frequency Demodulator

The frequency demodulator for demodulating the

SECAM signal is implemented as a CORDIC-struc-

ture. It calculates the phase and magnitude of the

quadrature components by coordinate rotation.

The phase output of the CORDIC processor is differ-

entiated to obtain the demodulated frequency. After

the deemphasis filter, the Dr and Db signals are scaled

to standard CRCB amplitudes and fed to the cross-

over-switch.

2.4.5. Burst Detection / Saturation Control

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 demodu-

lator and the automatic color control (ACC) in PAL/

NTSC.

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

Color saturation can be selected once for all color

standards. In PAL/NTSC it is used as reference for the

ACC. In SECAM the necessary gains are calculated

automatically.

For SECAM decoding, the frequency of the burst is

measured. Thus, the current chrominance carrier fre-

quency can be identified and is used to control the

SECAM processing. The burst measurements also

control the color killer operation; they are used for

automatic standard detection as well.

PAL/NTSC

2.4.6. 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

SECAM, the killer is controlled by the toggle of the

burst frequency. The burst amplitude measurement is

used to switch-off the color if the burst amplitude is

below a programmable threshold. Thus, color will be

killed for very noisy signals. The color amplitude killer

has a programmable hysteresis.

SECAM

Fig. 2–4: Frequency response of chrominance filters

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