TDA9170 datasheet(4/22 Pages) PHILIPS

4 / 22 page

October 1994

Philips Semiconductors

Preliminary speciﬁcation

YUV picture improvement processor based

on histogram modiﬁcation

TDA9170

FUNCTIONAL DESCRIPTION

Y input selection and ampliﬁcation

The dynamic range of the luminance input amplifier can be

switched between 0.3 and 1 V (excluding sync) either

externally (pin AMPSEL) or by I2C-bus (AMPSEL bit).

Amplitudes that exceed the corresponding specified range

(e.g. the sync) will be clipped internally. The input is

clamped during the logic HIGH period of the clamp which

is defined by the sandcastle reference and should be

DC-decoupled with an external capacitor.

Black offset detection and correction

The black detector measures and stores the blackest part

of the picture within a defined window in each field. Any

difference between this value and the value measured

during the black clamp period is regarded as black offset.

In a closed loop configuration, the black offset is held until

a predefined value of the full scale (FS) value is fed back

to the input stage where it is partly compensated for.

Depending on the loop gain, 30% to 50% of the offset

value is counteracted. The loop gain is also a function of

the adaptive and variable gamma settings. The black

offset correction mechanism can be switched on and off by

the I2C-bus via the BON bit (see Table 6), or externally with

the black offset on/off switch (BOF pin 11).

Two external time constants are required to ensure correct

performance of the black detector; a loop filter time

constant (TAUBL) for the loop dynamics and a time

constant for memorizing the darkest parts of the picture

(TAUBP) in just one field. During the field retrace the time

constant TAUBP is first sampled and then preset to a value

that corresponds to the maximum black offset.

The corrected black offset is related to the nominal signal

amplitude which is reset to 100% FS via an amplitude

stretch function. Luminance values beyond FS are not

affected. Additionally, this offset is also used to set the

adaptive gain (see Section “Adaptive gamma”).

Histogram measurement

The histogram distribution is measured in real time over

five segments (HM1 to HM5) within a defined window

period of each field. During the window period, the video is

in one segment, a corresponding external capacitor CHMx

is loaded via a current source. At the end of the field five

segment voltages are stored from the external capacitors

into on-board memories. The external capacitors are

discharged and the measurements are restarted.

Any part of the picture that does not contribute to the

information within the total picture should be omitted from

the histogram measurement. The miscount detector

disables measurements until it detects changing parts.

Additionally, luminance values close to FS (or white) do

not contribute sufficiently in order to maintain the absolute

light output. This procedure is allowed because the eye is

less sensitive to details in white.

As the miscount detector shortens the effective

measurement period and, because of spreads of internal

and external components, the current source is controlled

within in a closed loop so as to maintain a constant

average 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 connected to

TAUHM (pin 22).

Processing of the measured histogram values

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 to reduce these flicker effects.

The time constant of the averaging process is adapted to

the speed of the histogram changes.

ADAPTIVE GAMMA

The output voltage of the first segment is fed to a variable

gain amplifier with a gain between 1 and 3. In this way

luminance values in the ‘black’ segment have a larger

weight. In our perception black parts are expanded, as

occurs with gamma control. However, the effective

contribution to the non-linear gain is only relevant for

moderate segment voltages and hence the term adaptive

gamma.

The adaptive gamma gain is a max-function of a fixed gain

part and a dynamic gain part. The fixed gain part can be

set externally with the adaptive gamma gain control

(ADGAM) or via the I2C-bus.

The dynamic part of the adaptive gamma gain is controlled

by the measured black offset value from the black

detector.

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