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Fabricante

TDA4671

Picture Signal Improvement (PSI) circuit

Philips Electronics 

Philips Semiconductors

Picture Signal Improvement (PSI) circuit

Product specification

TDA4671

FUNCTIONAL DESCRIPTION

The TDA4671 contains luminance signal processing and

colour-difference signal processing. The luminance signal

section comprises a variable, integrated luminance delay

line with luminance signal peaking and a noise reduction

by coring.

The colour-difference section consists of a transient

improvement circuit to decrease the rise and fall times of

the colour-difference signal transients. All functions and

parameters are controlled via the I2C-bus.

Y-signal path

The video and blanking signal is AC-coupled to the input

pin 16. Its black porch is clamped to a DC reference

voltage to ensure the correct operating range of the

luminance delay stage.

The luminance delay line consists of all-pass filter sections

with delay times of 45, 90, 100, 180 and 450 ns

(see Fig.1). The luminance signal delay is controlled via

the I2C-bus in steps of 45 ns in the range of 20 to 1100 ns,

this ensures that the maximum delay difference between

the luminance and colour-difference signals is ±22.5 ns.

An automatic luminance delay time adjustment in an

internal control loop (with the horizontal frequency as a

reference) is used to correct changes in the delay time,

due to component tolerances. The control loop is

automatically enabled between the burst key pulses of

lines 16 (330) and 17 (331) during the vertical blanking

interval. The control voltage is stored in capacitor CDL

connected to pin 2.

The peaking section is using a transversal filter circuit with

selectable centre frequencies of 2.6 and 5.0 MHz.

It provides selectable degrees of peaking of −3, 0, +3 and

+6 dB and noise reduction by coring, which attenuates the

high-frequency noise introduced by peaking.

The output buffer stage ensures a low-ohmic VBS output

signal on pin 12 (<160 Ω). The gain of the luminance

signal path from pin 16 to pin 12 is unity.

An oscillation signal of the delay time control loop is

present on output pin 12 instead of the VBS signal. It is

present during the vertical blanking interval of the burst key

pulses in lines 16 (330) to 18 (332). This sync should not

be applied for synchronization.

Colour-difference signal paths

The colour-difference input signals (on pins 3 and 7) are

clamped to a reference voltage.

Each colour-difference signal is fed to a transient detector

and to an analog signal switch with an attached voltage

storage stage.

The transient detectors consist of differentiators and

full-wave rectifiers. The output voltages of both transient

detectors are added and then compared. The comparator

controls both following analog signal switches

simultaneously.

The analog signal switches are in open position at a

certain value of transient time; the held value (held by

capacitors) is then applied to the outputs. The switches

close to rapidly accept the actual signal levels at the end

of these transients. The improved transient time is

approximately 100 ns long independent of the input

transient time.

Colour-difference paths are independent of the input

signal polarity and have a nominal unity gain.

The CTI functions are switched on and off via the I2C-bus.

1996 Dec 11

5

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