Maxim > Design Support > Technical Documents > Application Notes > General Engineering Topics > APP 4287

Keywords: analog, video, reconstruction, moiré, picture, artifacts, digital, quantization, low, pass, filter, anti-

alias, converter, ADC, DAC, clock, sample, boxcar, Nyquist, Standard, Definition, SD, PAL, NTSC, High, HD,

ATSC, MPEG

APPLICATION NOTE 4287

Reconstructing Analog Video with the Maxim Video

Filter Family

By: Bill Laumeister, Strategic Applications Engineer

Sep 18, 2008

Abstract:

This application

note provides an overview of video digitization, and the spectrum and sidebands

produced. The article examines the process of converting digital video back into analog video and explains

how video reconstruction requires proper filtering techniques. Examples are provided to avoid or minimize

moiré effects and picture artifacts.

Introduction

Once analog video is captured and digitized, the big challenge remains: restoring the video image without

artifacts or moiré effects so that the best possible viewing experience is delivered. To do this, designers must

examine the process of converting digital video back into analog video (i.e., video reconstruction), and the

various filtering techniques and implementation examples that can be used to avoid or minimize moiré effects

and picture artifacts.

Maxim video filters make digital - to - analog video conversion simple and reliable. The filter family consists of 36

filters (number of devices as of July 2008) and covers standard- (SD) and high -definition (HD) standards.

Many devices have multiple channels matched in frequency response, group delay, and gain. The typical

channel is not only automatically tested, but is also inexpensive. At low volume (1000k pieces, FOB USA, web

price) the cost at the time of this publication is much less than one dollar (USD) per channel.

Why Convert Digital Video to Analog?

Television is literally seeing at a distance. Analog light is captured by a camera and typically the video is

digitized for convenient transmission. Eventually our analog human eyes require analog light to view the image

(Figure 1). The process for turning digital video back into analog video is called reconstruction. Due to

quantization and other issues caused by moving from the digital to analog domains, moiré effects and picture

artifacts have to be removed through various filtering techniques to deliver top-quality video.

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Figure 1. Television starts as analog light, becomes a digital transmission, and returns to analog light for our

eyes.

Digital video has many advantages. It can be compressed with predictable quality and, once digitized, it does

not degrade with storage and transmission. Transmission includes many ways of delivering the image such as

DVD, satellite, cable, and over-the-air systems. To view digital video with our analog human eyes, it must be

reconstructed into an analog video signal and into analog light.

Like digital video, a jigsaw puzzle emerges from the box: jumbled, out of order, and with strange spurious

shapes or artifacts. The shapes have little coherence with the puzzle's image. By following the basic rules we

learned as children, we sort out the corners and edges to start reconstructing the image. In a similar way

digital video transmission is comprised of picture pieces which may be transmitted out of order and contain

spurious artifacts. However, by following a set of rules we can reassemble (reconstruct) the picture whose

quality corresponds to the original analog video input signal.

At the end of the digital reassembly process both video and the jigsaw puzzle need some "analog smoothing."

For the jigsaw puzzle, that analog smoothing can be squinting our eyes or moving back far enough so that the

lines between puzzle pieces are not objectionable. For digital video, this analog smoothing is accomplished by

an analog lowpass filter.

Video Reconstruction Process

An analog video signal delivered by a camera or other capture device is digitized in an analog- to -digital

converter (ADC). The ADC "memorizes" the value in an instant of time at each clock edge (Figure 2). The

arrows at the upper left of Figure 2 show the clock instant that the analog signal data is stored. The analog

signal is continuously changing, but the digital representation is sampled periodically. After digital processing

and transmission, the digital signal is converted back to analog video by a digital- to - analog converter (DAC).

The DAC's output is shown in the upper right portion of Figure 2, with the arrows again representing the

clock.

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