TDA9171 datasheet(5/20 Pages) PHILIPS

5 / 20 page

1996 Jun 17

Philips Semiconductors

Preliminary speciﬁcation

YUV picture improvement processor based

on histogram modiﬁcation and blue stretch

TDA9171

As the miscount detector shortens the effective

measurement period and, because of spreads of internal

and external components, the current source is controlled

in a closed-loop to provide a constant value of the sum of

the segment voltages. The dominant time constant of the

closed-loop is external and can be tuned with an

appropriate capacitor value at pin TAUHM (pin 8).

Processing of the measured histogram value

FIELD AVERAGING OF HISTOGRAM VALUES

With very rapid picture changes, also related to the field

interlace, flicker might result. The histogram values are

averaged at the field rate thus reducing the flicker effects.

The time constant of the averaging process is adapted to

the speed of the histogram changes.

ADAPTIVE WHITE-POINT STRETCHING

For dominant HM4 and HM5 voltages, or large white parts,

the histogram conversion procedure makes a transfer with

large gain in the white parts, however the amount of light

coming out of the scene is considerably reduced. The

white stretcher introduces additional overall gain for

increased light production and, as a result, violates the

principle of having a full scale reference.

STANDARD DEVIATION

For scenes, in which segments of the histogram

distribution are very dominant with respect to the others,

the non-linear amplification should be reduced in

comparison to scenes with a flat histogram distribution.

The standard deviation detector measures the spread of

the histogram distribution in the segments HM1 to HM5

and modulates the user setting of the non-linear amplifier.

Non-linear ampliﬁer

The stored segment voltages relative to their average

value, averaged over two fields, determine the individual

gain of each segment in such a way that continuity is

guaranteed for the complete range. The maximum and

minimum gain of each segment is limited. Apart from the

adaptive white-point stretching the black and white

references are not affected by the non-linear processing.

The amount of linearity can be controlled externally by the

NLC pin (Non Linearity Control).

Colour compensation

Non-linear luminance processing influences the colour

reproduction, mainly the colour saturation. Therefore, the

U and V signals are also processed for saturation

compensation.

By convention

−U and −V signals must be supplied to the

TDA9171. The

−U and −V input signals are clamped

during the logic HIGH period of CLP, defined by the

sandcastle reference. In YC-applications just one colour

difference channel is required for processing the chroma

signal. However, external decoupling capacitors should be

applied to both inputs UIN and VIN. The external coupling

capacitor value should be such that the burst period of the

chroma signal is very softly clamped.

The processing is dependent on the amplitude and sign of

the colour difference signals whenever the blue stretch

circuitry is activated. Therefore, both the polarity and the

nominal amplitude of the colour difference signals are

relevant when using the blue stretch facility.

Blue stretch

The blue stretch circuit is intended to shift colours near

white, with sufficient contrast values, towards more blue

coloured white to give a brighter impression. The

chromaticity shift is proportional to the excess of the

contrast value of a white video signal with respect to a user

adjustable minimum level, defined by a voltage at

pin BLM. In this way blue shift in, for instance, human

faces can be prevented. The global amount of blue shift is

defined by the voltage level at pin BLG. The direction of

shift in the colour triangle is fixed by hardware.

It should be noted that the colour shift is different with a

wrong polarity of the colour difference signals. The

preferred BLG and BLM settings will be related to the

actual nominal amplitudes of the colour difference signals.

The blue stretch facility must be disabled in

YC-applications by setting both BLG and BLM to ground.

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