FCC Advisory sub-group sets standard-TV specifications

Electronic News, July 24, 1995 by David Hack

WASHINGTON - The Technical Sub-Group of the Advisory Committee on Advanced Television Services (ACATS) - advisor to the Federal Conmunications Commission (FCC) - met last week and accepted video scanning parameters that were recommended recently by its Expert Group on Scanning Formats and Compression.

These parameters are to be used for standard-definition television (SDTV) transmission within a digital high-definition television (HDTV) broadcast system (EN, June 5, 1995).

The HDTV system was developed by the Grand Alliance, the combination of the four joint-venture contestants whose proposed systems were tested in 1992 without any one system emerging a clear winner over the others.

The SDTV scanning parameters that were accepted last week for the broadcast signal are: 480 lines by 704 pixels; aspect ratios 3:4 and 9:16; picture rates 60 per second (interlaced) and 24, 30 and 60 (progressive). Also: 480 lines by 640 pixels; aspect ratio 3:4; picture rates the same as above. The Technical Sub-group also accepted the recommendation to eliminate all of the 360- and 240-line progressive-scanning schemes that had earlier been considered for inclusion.

In the Expert Group a week before, the broadcast industry had receded from the 360-line formats while the television manufacturing industry receded from the 240-line formats.

Video scanning parameters for digital HDTV broadcast were adopted by the full Advisory Committee last year. Bench testing of those scanning parameters, and of the radio frequency modulation systems, was completed last week at the television industry's Advanced Television Test Center (ATTC) in Alexandria, Va. Results are still being evaluated and reports are yet to be written.

Field trials of the same prototype equipment are beginning this week at Charlotte N.C., under special FCC permission to broadcast on UHF channel 53 using the modulation scheme known as Grand Alliance vestigial-sideband (GA-VSB).

The field trials also include testing the 64QAM modulation scheme for cable. Testing still to be conducted by other laboratories, using tapes generated by the ATTC, includes tests for asynchronous-transfer-mode (ATM) compatibility (by Bell South) and tests for Moving-Picture-Experts-Group (MPEG) compliance (by Hitachi and IBM).

The ACATS Technical Sub-Group is also considering a "dark-horse" modulation scheme as a possible substitute for the GA-VSB modulation now being field-tested. This scheme, known as coded orthogonal frequency-division modulation (COFDM), is under serious consideration in Europe, and is advocated in this country most notably by MIT Professor Emeritus of Electrical Engineering William Schreiber, who is directing the doctoral research of two graduate students in simulating some of COFDM's properties.

In theory, COFDM has some properties superior to those of VSB modulation, but shoestring restarch budgets for advanced television have held back its development for North American application.

Late last week, another ACATS working group was meeting with European engineers - who have developed prototype COFDM hardware - to decide whether to recommend the prototype system for testing at the ATTC. If testing is recommended, developing and carrying out a testing plan would seriously threaten FCC Chairman Reed Hundt's goal of choosing an advanced television system this year. On the other hand, Professor Schreiber said that one problem with the ACATS over its 7-1/2 years of existence has been overly optimistic timetables, which have precluded a serious search for the "best" system.

The ACATS was established in 1987, at the request of the television broadcast industry, to plan an advanced-television system using previously reserved but unassigned TV-broadcast radio frequency spectrum. Another objective of some participants then was to foster a U.S.-developed advanced-TV standard, to counter an analog HDTV standard then being promoted by Japanese electronics companies and their U.S. affiliates. By 1992, U.S. television engineers had persuaded their peers around the globe that a digital advanced TV system was both feasible and superior.