OM8373 datasheet(49/108 Pages) PHILIPS

49 / 108 page

2004 Jan 15

Philips Semiconductors

Tentative Device Speciﬁcation

TV signal processor-Teletext decoder with

embedded

µ-Controller

OM8373; OM8378

Chroma, luminance and feature processing

The chroma band-pass and trap circuits are realised by

means of gyrators and are tuned to the right frequency by

comparing the tuning frequency with the reference

frequency of the colour decoder. The luminance delay line

and the delay cells for the peaking circuit are also realised

with gyrators.

The circuits contain the following picture improvement

features:

• Peaking control circuit. The ratio of the positive and

negative overshoots of the peaking can be adjusted by

means of the bits RPO1/RPO0 in subaddress 2EH.

• Video dependent coring in the peaking circuit. The

coring can be activated only in the low-light parts of the

screen. This effectively reduces noise while having

maximum peaking in the bright parts of the picture.

• Black stretch. This function corrects the black level for

incoming signals which have a difference between the

black level and the blanking level.

• Blue-stretch. This circuit is intended to shift colour near

‘white’ with sufficient contrast values towards more blue

to obtain a brighter impression of the picture.

• Dynamic skin tone (flesh) control. This function is

realised in the YUV domain by detecting the colours

near to the skin tone.

Colour decoder

The ICs can decode PAL and NTSC signals. The

PAL/NTSC decoder does not need external reference

crystals but has an internal clock generator which is

stabilised to the required frequency by using the 12 MHz

clock signal from the reference oscillator of the

µ-Controller.

Under bad-signal conditions (e.g. VCR-playback in feature

mode), it may occur that the colour killer is activated

although the colour PLL is still in lock. When this killing

action is not wanted it is possible to overrule the colour

killer by forcing the colour decoder to the required standard

and to activate the FCO-bit (Forced Colour On) in

subaddress 21H.

The Automatic Colour Limiting (ACL) circuit (switchable

via the ACL bit in subaddress 20H) prevents that

oversaturation occurs when PAL/NTSC signals with a high

chroma-to-burst ratio are received. The ACL circuit is

designed such that it only reduces the chroma signal and

not the burst signal. This has the advantage that the colour

sensitivity is not affected by this function.

The base-band delay line is integrated. This delay line is

also active during NTSC to obtain a good suppression of

cross colour effects. The demodulated colour difference

signals are internally supplied to the delay line.

RGB output circuit and black-current stabilization

In the RGB control circuit the signal is controlled on

contrast, brightness and saturation. The ICs have a linear

input for external RGB/YUV signals. Switching between

RGB and the YUV/YPRPB mode can be realised via the

YUV0/YUV1 bits in subaddress 2BH. The signals for OSD

and text are internally supplied to the control circuit. The

output signal has an amplitude of about 2 V black-to-white

at nominal input signals and nominal settings of the

various controls.

To obtain an accurate biasing of the picture tube the

‘Continuous Cathode Calibration’ (CCC) system has been

included in these ICs. When required the operation of the

CCC system can be changed into a one-point black

current system. The switching between the 2 possibilities

is realised by means of the OPC bit in subaddress 2BH.

When used as one-point control loop the system will

control the black level of the RGB output signals to the

‘low’ reference current and not on the cut off point of the

cathode. In this way spreads in the picture tube

characteristics will not taken into account. A further

consequence is that the RGB output signals have a fixed

amplitude (2 VP-P under nominal conditions) and that the

‘cathode drive level’ bits (CL3-CL0) have no effect on

these amplitudes. For this reason the gain of the RGB

output stages has to be adapted to the required drive level

of the cathodes.

A black level off-set can be made with respect to the level

which is generated by the black current stabilization

system. In this way different colour temperatures can be

obtained for the bright and the dark part of the picture.

In the Vg2 adjustment mode (AVG = 1) a certain black level

is inserted at the RGB outputs and the black current

stabilization system checks the feedback current of the 3

channels and indicates whether the current level is in a

certain window (WBC-bit) or below or above this window

(HBC-bit). This indication can be read from the status byte

01 and can be used for automatic adjustment of the Vg2

voltage during the production of the TV receiver. During

this test the vertical scan remains active so that the

indication of the 2 bits can be made visible on the TV

screen.

OM8373 Datasheet(PDF) 49 Page - NXP Semiconductors

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