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TDA9178T

YUV one chip picture improvement based on luminance vector-, colour vector- and spectral processor

Philips Electronics 

Philips Semiconductors

YUV one chip picture improvement based on luminance

vector-, colour vector- and spectral processor

Preliminary specification

TDA9178

SKIN TONE CORRECTION

Skin tones are very sensitive for transmission (hue) errors,

because we have an absolute feeling for skin tones.

To make a picture look free of hue error, the goal is to

make sure that skin tones are put at a correct colour.

The dynamic skin tone correction circuit achieves this goal

by instantaneously and locally changing the hue of those

colours which are located in the area in the UV plane that

matches skin tones (see Fig.4).

The correction is dependent on luminance, saturation and

distance to the preferred axis and can be done towards

two different angles. The preferred angle can be chosen by

bit ASK in the I2C-bus settings. The settings are

123° (ASK = 0) and 117° (ASK = 1). The enclosed

correction area can be increased to 140% with the I2C-bus

bit SSK (so-called: Size). The enclosed detection ‘angle’ of

the correcting area can be increased to 160% with the

I2C-bus bit WSK (so-called: Width). The skin tone

correction can be switched on or off with the I2C-bus

bit DSK.

GREEN ENHANCEMENT

The green enhancement circuit (see Fig.5) is intended to

shift low saturated green colours towards more saturated

green colours. This shift is achieved by instantaneously

and locally changing those colours which are located in the

area in the UV plane that matches low saturated green.

The saturation shift is dependent on the luminance,

saturation and distance to the detection axis of 208°.

The direction of shift in the colour is fixed by hardware.

The amount of green enhancement can be increased to

160% by the I2C-bus bit GGR. The enclosed detection

‘angle’ of the correcting area can be increased to 160%

with the I2C-bus bit WGR (so-called: Width). The enclosed

correction area can be increased to 140% with the I2C-bus

bit SGR (so-called: Size). The green enhancement can be

switched on or switched off with the I2C-bus bit DGR.

BLUE STRETCH

The blue stretch circuit (see Fig.6) is intended to shift

colours near white towards more blueish coloured white to

give a brighter impression. This shift is achieved by

instantaneously and locally changing those colours which

are located in the area in the UV plane that matches

colours near white. The shift is dependent on the

luminance and saturation. The direction of shift (towards

an angle of 330°) in the colour is fixed by hardware.

The amount of blue stretch can be increased to 160% by

the I2C-bus bit GBL.

The enclosed correction area can be increased to 140% by

the I2C-bus bit SBL (so-called: Size). The blue stretch can

be switched on or off by the I2C-bus bit DBL.

SATURATION CORRECTION

The non-linear luminance processing done by the

histogram modification and variable gamma, influences

the colour reproduction; mainly the colour saturation.

Therefore, the U and V signals are linear processed for

saturation compensation.

Noise measuring

A video line which is supposed to be free from video

information (‘empty line’) is used to measure the amount of

noise. The measured RMS value of the noise can be used

for reducing several features, by the I2C-bus interface,

such as luminance vector processing and spectral

processing. For the TDA9178 the empty line is chosen

three lines after recognition of the vertical blanking from

the sandcastle pulse input. Figures 7, 8, 9 and 10 show

the measurement locations for different broadcast norms.

The noise detector is capable of measuring the

signal-to-noise ratio between −45 and −20 dB. The output

scale runs linearly with dB. The noise samples are

averaged for over 20 fields to reduce the fluctuations in the

measurement process. It is obvious, that for signal

sources (like VCR in still picture mode) that re-insert the

levels of the retrace part, the measurement is not reliable

(see Section “Feature mode detector”). The result of the

averaging process will update the contents of the I2C-bus

register: bits ND5 to ND0 at a rate of 1⁄32 of the field

frequency. If a register access conflict occurs, the data of

the noise register is made invalid by setting the flag bit DV

(Data Valid) to zero.

Feature mode detector

A detector is available for detecting signal sources (like

VCR in still picture mode) that re-inserted the levels of the

retrace part. For this kind of signals the noise

measurement of the TDA9178 is not reliable, but this

detector sets bit FM in the ND-register to logic 1.

For normal video signals bit FM is set to logic 0.

This circuit measures transients (like synchronization

pulses) on the luminance input during the internal V-pulse.

The feature mode detector is setting bit FM to logic 1 when

no transients are present during 2 lines in the vertical

retrace part over 3 fields (like the synchronization pulses).

1999 Sep 24

9

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