TDA9178 datasheet(6/36 Pages) PHILIPS

6 / 36 page

1999 Sep 24

Philips Semiconductors

Preliminary speciﬁcation

YUV one chip picture improvement based on luminance

vector-, colour vector- and spectral processor

TDA9178

FUNCTIONAL DESCRIPTION

Y input selection and ampliﬁcation

The gain of the luminance input amplifier and output

amplifier can be adjusted to signal amplitudes of

0.315 and 1.0 V typically (excluding sync) by I2C-bus

bit AMS. The sync part is processed transparently to the

output, independently of the feature settings.

The Y, U and V input signals are clamped during the

burstkey period, defined by the sandcastle reference and

should be DC-coupled (the circuit uses internal clamp

capacitors). During the clamp pulse (see Figs 7, 8, 9

and 10) an artificial black level is inserted in the Y input

signal to correctly preset the internal circuitry.

Luminance vector processor

In the luminance vector processor the transfer is controlled

by a black stretch, the histogram processing and a gamma

control circuit. The luminance vector processor also

creates the cue flash signal.

BLACK STRETCH

A black detector measures and stores the level of the most

black part of the scene within an internal defined fixed

window in each field into a time constant. The time

constant and the response time of the loop are internally

fixed. Any difference between this value and the value

measured during the clamp is regarded as black offset.

In a closed loop offsets until a predefined value of the full-

scale value are fed back to the input stage for

compensation. The loop gain is a function of the histogram

and variable gamma settings. The black offset correction

can be switched on and off by the I2C-bus bit BON.

Related to the corrected black offset the nominal signal

amplitude is set again to 100% full scale through an

amplitude stretch function. Luminance values beyond full

scale are unaffected. Additionally, the measured black

offset is also used to set the adaptive black stretch gain

(see also Section “Adaptive black stretch”).

HISTOGRAM PROCESSING

For the luminance signal the histogram distribution is

measured in real-time over five segments within an

internally defined fixed window in each field. During the

period that the luminance is in one segment, a

corresponding internal capacitor is loaded by a current

source. At the end of the field five segment voltages are

stored into on-board memories. The voltages stored in the

memories determine the non-linear processing of the

luminance signal to achieve a picture with a maximum of

information (visible details).

Each field the capacitors are discharged and the

measurement starts all over again.

Parts in the scene that do not contribute to the information

in that scene, like sub or side titles, should be omitted from

the histogram measurement. No measurements are

performed outside the internal fixed window period.

Very rapid picture changes, also related to the field

interlace, can result in flicker effects. The histogram values

are averaged at the field rate thus cancelling the flicker

effects.

Adaptive black stretch

The so-called adaptive black stretch gain is one of the

factors that control the gamma of the picture. This gain is

controlled by the measured black offset value in the black

stretch circuit and the I2C-bus adaptive black stretch DAC:

bits BT5 to BT0. For pictures with no black offset the black

stretch gain equals unity so the gamma is not changed and

the DAC setting has no influence. In case of a black offset,

the black stretch gain is increased so the gamma of the

picture is reduced. This procedure results in a maximum of

visible details over the whole range of luminances.

However, depending on personal taste, sometimes higher

values of gamma are preferred. Therefore the amount of

gamma reduction can be adjusted by the DAC.

Adaptive white-point stretching

For pictures with many details in white parts, the histogram

conversion procedure makes a transfer with large gain in

the white parts. The amount of light coming out of the

scene is reduced accordingly. The white stretcher

introduces additional overall gain for increased light

production, and so violating the principle of having a

full-scale reference. The white-point stretching can be

switched on or off by means of the I2C-bus bit WPO.

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

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

Non-linear ampliﬁer

The stored segment voltages determine the individual gain

of each segment in such a way that continuity is granted

for the complete luminance range.

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