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TW9903

Multi-standard Video Decoder With High Quality Down Scaler

Unspecified 

TW9903

Vertical sync processing

The vertical sync separator detects the vertical synchronization pattern in the input video signals.

In addition, the actual sync determination is controlled by a detection window to provide more

reliable synchronization. It achieves the functionality of a PLL without the complexity of a PLL. An

option is available to provide faster responses for certain applications. The field status is

determined at vertical synchronization time. When the location of the detected vertical sync is inline

with a horizontal sync, it indicates a frame start or the odd field start. Otherwise, it indicates an

even field. The field logic can also be controlled to toggle automatically while tracking the input.

Color Decoding

Y/C separation

The color-decoding block contains the luma/chroma separation for the composite video signal and

multi-standard color demodulation. For NTSC and PAL standard signals, the luma/chroma

separation can be done either by comb filter or notch/band-pass filter combination. For SECAM

standard signals, only notch/band-pass filter is available. The default selection for NTSC/PAL is

comb filter. The characteristics of the band-pass filter is shown in the filter curve section.

In the case of comb filter, the TW9903 separates luma (Y) and chroma (C) of a NTSC composite

video signal using a proprietary 2H adaptive comb filter. The filter uses a two-line buffer. Adaptive

logic combines the upper-comb and the lower-comb results based on the signal changes among

the previous, current and next lines. This technique leads to good Y/C separation with small cross

luma and cross color at both horizontal and vertical edges. Due to the 90-degree phase difference

on adjacent lines of a PAL chroma signal, the 2H line memory are used optionally to provide an

adequate PAL comb filter performance.

Due to the line buffer used in the comb filter, there is always one line processing delay in the output

images in general except the PAL comb filter case, which has no line delay.

If notch/band-pass filter is selected, the characteristics of the filters are shown in the filter curve

section.

Color demodulation

The color demodulation for NTSC and PAL standard is done by first quadrature mixing the chroma

signal to the base band. The mixing frequency is equal to the sub-carrier frequency for NTSC and

PAL. After the mixing, a low-pass filter is used to remove carrier signal and yield chroma

components. The low-pass filter characteristic can be selected for optimized transient color

performance. For the PAL system, the PAL ID or the burst phase switching is identified to aid the

PAL color demodulation.

For SECAM, the mixing frequency is 4.286Mhz. After the mixer and low-pass filter, it yields the FM

modulated chroma. The SECAM demodulation process therefore consists of low-pass filter, FM

demodulator and de-emphasis filter. The filter characteristics are shown in filter curve section.

During the FM demodulation, the chroma carrier frequency is identified and used to control the

SECAM color demodulation.

The sub-carrier signal for use in the color demodulator is generated by direct digital synthesis PLL

that locks onto the input sub-carrier reference (color burst). This arrangement allows any sub-

standard of NTSC and PAL to be demodulated easily with single crystal frequency.

TECHWELL, INC.

7

REV. 0.92 ( B )

06/02/2002

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