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Under the hood of DTV standard

May 7, 2001  •  Post A Comment

One of the problems in getting digital television off the ground is getting DTV technology solutions focused on real-world problems.

“Eventually 8VSB can be made to work essentially everywhere,” said Mark Schubin, a TV technology consultant and standards critic. “The biggest problem is that DTV doesn’t replicate NTSC [standard TV] coverage.”

Mr. Schubin noted that despite assurances otherwise, he cannot receive New York City DTV stations reliably at his Manhattan apartment using an indoor antenna.

Analyzing DTV signals

Until now, public entities had never recorded actual signals from the field; the few tests that were conducted outside the lab simply measured whether DTV signals could be received. And the results of more detailed field tests conducted by manufacturers were never shared.

The Advanced Television Technology Center recently completed the first public effort to capture real-world radio frequency DTV signals, providing the industry with a repeatable source of applicable material to test and optimize DTV receivers. The ATTC is a private, nonprofit corporation organized by broadcasters and consumer-product manufacturers to test and recommend solutions for the delivery and reception of 8VSB digital transmissions.

“The majority of DTV problems stem from low signal strength compounded by multiple echoes [multipath],” said Jim Kutzner, senior technology director at ATTC. “The problem tends to be worse in an urban environment such as an apartment on the Upper West Side of Manhattan.”

That’s because the proliferation of buildings further reduces signal strength and increases the number of times a signal bounces around before it reaches a receiver.

Now that the data are in, the ATTC is developing “test ensembles” (sets of characteristics that create distortion) to create new demodulation chips to measure where reception breaks up. The group hopes to parlay its investigations into suggestions for 8VSB enhancements to the Advanced Television Systems Committee, the country’s standards-setting body.

All in all, Mr. Kutzner is heartened by the progress made in getting 8VSB in shape to be a viable solution. “The proposed enhancements-at least some of them-look pretty good,” he said, an assessment he shares with the more skeptical Mr. Schubin.

“The ATSC RF Task Force report is very good and provides a good analysis of the problems and possible solutions,” Mr. Schubin said. “It should be required reading at the FCC and Congress.”

In addition to creating high-profile tests, the ATTC also assesses the interoperability of encoders and decoders using a wide variety of bit streams. One provider of those bit streams is the Sarnoff Corp., a combination private research lab and high-tech product development start-up engine.

Diagnosing video problems

Sarnoff has also been busy targeting new technologies to evaluate-for example, the actual performance of a DTV encoder and better tailoring image quality to real consumers. Its Encoder Stress Pattern (see photo) is a motion sequence of tests for various aspects of video processing. The technology pinpoints design problems in DTV encoders running at various bit rates by means of an on-screen display. Technicians can actually see the problems instead of reviewing sheets of data.

For example, the happy face in the lower right hand corner (see photo) simulates the presentation of a TV station logo. Its smooth dissolve into and out of the screen display indicates that the encoded stream passes tests designed for things such as rate control; the face’s more erratic entry and departure indicate a problem called “judder” (shakiness). The technology has been licensed by Sencore for production of its HD292 HDTV Reference Signal Source chassis.

Sarnoff’s JNDmetrix-IQ is a software tool that uses psychophysical algorithms to calibrate how people will perceive the quality of an image or video to determine what part of each video frame is off. Comparisons with signal-to-noise ratio and absolute error quality measures show JNDmetrix-IQ to be more reliably attuned to perceptions by the human eye. Available for Windows and Unix platforms, it allows for a wide variety of input video types, file formats and output measurement modes.