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TDA9181

Integrated multistandard comb filter

Philips Electronics 

Philips Semiconductors

Integrated multistandard comb filter

Objective specification

TDA9181

FUNCTIONAL DESCRIPTION

Input configuration

The Y/CVBS1 and Y/CVBS2 input signals are clamped by

means of an internally generated clamp pulse which is

derived from the sandcastle input signal (pin SC). If no

sandcastle signal is available, a clamp pulse signal may be

applied to pin SC. External clamp capacitors are needed.

The buffered and clamped Y/CVBS1 and Y/CVBS2 signals

are then applied to the input switch. The input switch select

signal (INPSEL) determines whether Y/CVBS1 or

Y/CVBS2 is passed through to the anti-alias low-pass filter.

This 3rd-order low-pass filter is optimized for best

performance with respect to step response and clock

suppression. The filtered signal is sampled at a clock

frequency of four times the colour subcarrier frequency

(fSC).

A colour subcarrier frequency signal is applied to pin FSC.

The colour subcarrier select input signal (FSCSEL)

indicates whether the colour subcarrier frequency (fSC) or

twice the colour subcarrier frequency (2 × fSC) is being

applied at the FSC input. An external coupling capacitor is

needed for the colour subcarrier input signal.

Comb filter

The sampled CVBS signal is applied to two delay lines.

Depending on the applied standard, one delay line delays

the signal over 1 or 2H for NTSC and PAL respectively

(1H = one line-time). The standard select inputs SYS1 and

SYS2 indicate which standard, PAL B, G, H, D, I, M, N or

NTSC M, is being applied.

The direct and delayed signals are applied to an adaptive

comb filter. The adaptive comb filter performs band-pass

filtering around the colour subcarrier frequency and

compares the contents of adjacent lines. In this way the

combing of signals with different information is prevented

and artifacts such as hanging dots are avoided.

Both the combed chrominance and the combed luminance

signals are passed through a reconstruction low-pass filter

to obtain continuous-time signals. These low-pass filters

are 3rd-order, optimized for best performance with respect

to step response and clock suppression. The

reconstructed signals are applied to the output switches.

Output configuration

The luminance output switch selects between the

reconstructed combed luminance signal and one of the

buffered and clamped input signals, Y/CVBS1 or Y/CVBS2.

The chrominance output switch selects between the

reconstructed combed chrominance signal and the

chrominance input signal (CIN). An external coupling

capacitor is needed for CIN. The selected signals are

applied to the outputs Y/CVBSOUT and COUT respectively

via a buffer stage. The output switch signal (OUTSEL)

determines whether the output switches select the internal

combed signals or the external Y/C signals.

Clock generation and filter tuning

The clock generator is driven by a Phase-Locked Loop

(PLL) circuit which generates a reference frequency of four

times the colour subcarrier frequency. This PLL circuit is

phase-locked to the colour subcarrier input signal (FSC).

Several internal clock signals are derived from the 4 × fSC

reference.

The filter tuning ensures the automatic alignment of the

anti-alias and the reconstruction low-pass filters. A 4 × fSC

clock signal is used as a reference for the alignment. The

tuning takes place each line during the line blanking and is

initiated by means of an internally generated signal which

is derived from the sandcastle input signal.

If the output switches select external Y/C signals the

oscillator of the PLL circuit is stopped regardless of the

FSC input and no internal clock signals are generated. The

filter tuning is also stopped.

2000 Nov 22

5

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