ANALOG DEVICES AN-850 Service Manual

AN-850

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: [phone] • Fax: [phone] • www.analog.com

Adaptive Digital Line Length Tracking

by Frank Kearney

INTRODUCTION NOISE-INDUCED TIME-BASE INACCURACIES

Despite the rapid advancement of digital TV, analog TV
remains dominant for both transmission and display. Analog
video formats, like digital formats, have precise specifications
for how to format video. These specifications include how the
luma, chroma, and synchronization information is packaged to
provide the line, field, and frames of video information that
recreate the images observed on TV screens. To display the
video information accurately, each of these components must be
correctly extracted from the video signal. Similarly, the timing
and phase information must be maintained or recreated as it
was when the video signal was first encoded at the source.

This application note outlines the challenges encountered when
decoding and restoring a correct time base to nonstandard
input video sources. This technology is known as Adaptive
Digital Line Length Tracking (ADLLT™).

Video signals consist of various components, some of which can
be altered or corrupted within the transmission path, resulting
in distortion of some video package aspects. For RF transmitted
signals, the synchronization information is normally present on
the recovered signal, but its detection and extraction can be
difficult or impossible because of excessive noise. It is important
to note that even when recovering the synchronization is
possible, its detection can be offset due to noise, which in turn
introduces jitter on the recovered synchronization information.

Figure 1 shows a representation of a typical stream of line
synchronization information.

Figure 2 shows that slicing the
synchronization information from this stream results in a stable
video display.

≈ 64µs≈ 64µs≈ 64µs

SYNC

SLICE

LEVEL

Figure 1. Line Synchronization Stream Without Noise at Source

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SLICE

50% TIMING

SLICED HSYNC

Figure 2. Stable Synchronization Extraction with No Vertical Jitter on Displayed Image

BLANK
LEVEL

PICTURE HAS STABL E

VERTICAL EDGES

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TABLE OF CONTENTS

Introduction ...................................................................................... 1

VCR-Induced Time-Base Errors .................................................5

Noise-Induced Time-Base Inaccuracies........................................ 1

Revision History ............................................................................... 2

REVISION HISTORY

7/06—Revision 0: Initial Version

ADLLT............................................................................................6 T

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Figure 3 is a representation of how noise can distort the synchronization information, resulting in a misinterpretation of the synchroniza-

Figure 3 shows the resulting stripped synchronization that can result from the noise. Note that the time base is corrupted and jitter

tion.
introduced. The effect of this jitter on the displayed video is serrations at the start and end of each line (see

SLICE

VISIBLE JITTER ON

50% POINTS

CORRUPTED

BY NOISE

VERTICAL EDG ES

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Figure 3. Noisy Synchronization Information Resulting in Extracted Synchronization Jitter

SYNC

SLICE

LEVEL

≈ 63µs≈ 64µs≈ 65µs

Figure 4).

Figure 4. HSYNC Jitter Resulting in Line-to-Line Shifting of Displayed Image

A typical noisy input from a tuner source is shown in
information.

Figure 6 illustrates the decoder output re-encoded into an analog format.

2

1

CH1 200mV CH2 200mV M10µs VID CH2 Ο/110

Figure 5. Actual Video Output from Tuner Source

Measured on Input to Decoder

Figure 5

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. It demonstrates the difficulty in determining the synchronization

2

CH2 200mV M10µs VID CH2 Ο/110

Figure 6. Decoder Output with Correct Noise-Free Synchronization Restored

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