SAF7118H datasheet(25/173 Pages) PHILIPS

25 / 173 page

2004 Jul 22

Philips Semiconductors

Product speciﬁcation

Multistandard video decoder with adaptive

comb ﬁlter and component video input

SAF7118

8.1.2.1

Chrominance path

The 9-bit CVBS or chrominance input signal is fed to the

input of a quadrature demodulator, where it is multiplied by

two time-multiplexed subcarrier signals from the subcarrier

generation block 1 (0

° and 90° phase relationship to the

demodulator axis). The frequency is dependent on the

chosen colour standard.

The time-multiplexed output signals of the multipliers are

low-pass filtered (low-pass 1). Eight characteristics are

programmable via LCWB3 to LCWB0 to achieve the

desired bandwidth for the colour difference signals (PAL,

NTSC) or the 0

° and 90° FM signals (SECAM).

The chrominance low-pass 1 characteristic also influences

the grade of cross-luminance reduction during horizontal

colour transients (large chrominance bandwidth means

strong suppression of cross-luminance). If the Y-comb

filter is disabled by YCOMB = 0 the filter influences directly

the width of the chrominance notch within the luminance

path (a large chrominance bandwidth means wide

chrominance notch resulting in a lower luminance

bandwidth).

The low-pass filtered signals are fed to the adaptive comb

filter block. The chrominance components are separated

from the luminance via a two-line vertical stage (four lines

for PAL standards) and a decision logic between the

filtered and the non-filtered output signals. This block is

bypassed for SECAM signals. The comb filter logic can be

enabled independently for the succeeding luminance and

chrominance processing by YCOMB (subaddress 09H,

bit D6) and/or CCOMB (subaddress 0EH, bit D0). It is

always bypassed during VBI or raw data lines

programmable by the LCRn registers (subaddresses

41H to 57H); see Section 8.3.

The separated CB-CR components are further processed

by a second filter stage (low-pass 2) to modify the

chrominance bandwidth without influencing the luminance

path. Its characteristic is controlled by CHBW

(subaddress 10H, bit D3). For the complete transfer

characteristic of low-passes 1 and 2 see Figs 12 and 13.

The SECAM processing (bypassed for QAM standards)

contains the following blocks:

• Baseband ‘bell’ filters to reconstruct the amplitude and

phase equalized 0

° and 90° FM signals

• Phase demodulator and differentiator

(FM-demodulation)

• De-emphasis filter to compensate the pre-emphasized

input signal, including frequency offset compensation

(DB or DR white carrier values are subtracted from the

signal, controlled by the SECAM switch signal).

The succeeding chrominance gain control block amplifies

or attenuates the CB-CR signal according to the required

ITU 601/656 levels. It is controlled by the output signal

from the amplitude detection circuit within the burst

processing block.

The burst processing block provides the feedback loop of

the chrominance PLL and contains the following:

• Burst gate accumulator

• Colour identification and colour killer

• Comparison nominal/actual burst amplitude (PAL/NTSC

standards only)

• Loop filter chrominance gain control (PAL/NTSC

standards only)

• Loop filter chrominance PLL (only active for PAL/NTSC

standards)

• PAL/SECAM sequence detection, H/2-switch

generation.

The increment generation circuit produces the Discrete

Time Oscillator (DTO) increment for both subcarrier

generation blocks. It contains a division by the increment

of the line-locked clock generator to create a stable

phase-locked sine signal under all conditions (e.g. for

non-standard signals).

The PAL delay line block eliminates crosstalk between the

chrominance channels in accordance with the PAL

standard requirements. For NTSC colour standards the

delay line can be used as an additional vertical filter.

If desired, it can be switched off by DCVF = 1. It is always

disabled during VBI or raw data lines programmable by the

LCRn registers (subaddresses 41H to 57H); see

Section 8.3. The embedded line delay is also used for

SECAM recombination (cross-over switches).

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