TW2802 datasheet(9/80 Pages) ETC

9 / 80 page

Preliminary

TW2804/TW2802 Multiple Video Decoder

Techwell, Inc.

www.techwellinc.com

Datasheet Rev. 2.4

09/09/2003

Sync Processing

The sync processor of TW280X detects horizontal synchronization and vertical synchronization

signals in the composite. The TW280X utilizes proprietary technology for locking to weak, noisy,

or unstable signals such as those from on air signal and fast forward or backward of VCR

system.

Video Level Adjustment

A patented digital gain and clamp control circuit restores the ac coupled video signal to a fixed

dc level. The clamping circuit provides line-by-line restoration of the video pedestal level to a

fixed dc reference voltage. In no AGC mode, the gain control circuit adjusts only the video sync

gain to achieve desired sync amplitude so that the active video is bypassed regardless of the

gain control. But when AGC mode is enabled, both active video and sync are adjusted by the

gain control. The range of AGC is from –6dB to 18dB approximately.

Horizontal Sync Processing

The horizontal synchronization processing contains a sync separator, a PLL and the related

decision logic. The horizontal sync separator detects the horizontal sync by examining low-pass

filtered video input whose level is lower than a threshold. Additional logic is also used to avoid

false detection on glitches. The horizontal PLL locks onto the extracted horizontal sync in all

conditions to provide jitter free image output. In case the horizontal sync is missing, the PLL is

on free running status that matches the standard raster frequency.

Vertical Sync Processing

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

signals. 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.

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