Daewoo DTC-20T2, DTZ-29U7/U3, DTZ-2881/28W8, DTY-21U7/21B4, DTC-21Y1/21Y2, DTZ-2481 Colour decoder

CP-520/520A/520F Service Manual

The circuit contains the following picture improvement features:

nPeaking control circuit. The peaking function can be activated for all incoming CVBS, Y/C and RGB/YPrPb signals. Various parameters of the peaking circuit can be adapted by means of the I2C-bus. The main parameters are:

-Peaking centre frequency (via the PF1/PF0 bits in subaddress 19H).

-Ratio of positive and negative peaks (via the RPO1/RPO0 bits in subaddress 47H). The peaks in the direction “white” are the positive peaks.

-Ratio of pre- and aftershoots (via the RPA1/RPA0 bits in subaddress 47H).

nVideo 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.

nBlack stretch. This function corrects the black level for incoming signals which have a difference between the black level and the blanking level. The amount of stretching (A-A in Fig. 72) and the minimum required back ground to activate the stretching can be set by means of the I2C-bus (BSD/AAS in subaddress 45H).

nGamma control. When this function is active the transfer characteristic of the luminance amplifier is made non-linear.Thecontrolcurvecanbeadaptedbymeans of I2C-bus settings (see Fig. 74). It is possible to make the gamma control function dependent on the picture content (Average Picture Level, APL). The effect is illustrated in Fig. 75. Previously this function was mentioned under the name “white stretch function”.

nBlue-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.

nDynamic skin tone (flesh) control. This function is realised in the YUV domain by detecting the colours near to the skin tone.

nScan-Velocity modulation output. Also the SVM function can be activated for all incoming CVBS, Y/C and RGB/YPrPb signals. The delay between the RGB output signals and the SVM output signal can be adjusted (by means of the SVM2-SVM0 bits in subaddress 48H) so that an optimum picture performance can be obtained. Furthermore a coring function can be activated. It is possible to generate Scan Velocity Modulation drive signals during the display of ‘full screen’ teletext (not in mixed mode). Another feature is that the SVM output signal can be made dependent on the horizontal position on the screen (parabola on the SVM output).

5.2.9 Colour decoder

The ICs decode PAL, NTSC and SECAM 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 clock signal from the reference oscillator of the TCG u -Controller.

Under bad-signal conditions (e.g. VCR-playback n 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 subaddress3CH. The sensitivity of the colour decoder for PAL and NTSC can be increased by means of the setting of the CHSE1/CHSE0 bits in subaddress 3CH.

The Automatic Colour Limiting (ACL) circuit (switchable via the ACL bit in subaddress 3BH) prevents that oversaturation occurs when 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.

DTV R&D Europe 39