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VCT38XXA

Video/Controller/Teletext IC Family

Micronas 

VCT 38xxA

ADVANCE INFORMATION

2.4.2. Demodulator

The entire signal (which might still contain luma) is

now quadrature-mixed to the baseband. The mixing

frequency 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 low-pass filter selects the

chroma components; a downsampling 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 low-pass filter for

the color difference signals for PAL/NTSC. SECAM

requires a modified low-pass function with bell-filter

characteristic. At the output of the low-pass filter, all

luma information is eliminated.

The low-pass 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 chroma filter can be

selected. This filter is intended for high bandwidth

chroma signals, e.g. a non-standard wide bandwidth

S-VHS signal.

2.4.4. Frequency Demodulator

The frequency demodulator for demodulating the

SECAM signal is implemented as a CORDIC structure.

It calculates the phase and magnitude of the quadra-

ture 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 crossover

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-locked-loop (APC) of the demod-

ulator 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 chroma carrier frequency

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 stan-

dard 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. 2–5: Frequency response of chroma filters

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