Standards for analog video
WILLIAM B. GARDNER, AT&T
Standards specific to lightwave technology have generally been created from scratch. By contrast, analog television pre-dates the advent of fiber-optic technology, and therefore generated its own history of standards.
Although digital video transmission may be the television wave of the future, analog video television is still a major enterprise today. As such, its standards requirements for transmission over fiber must be addressed. Lightwave standards bodies are thus trying to implement analog video standards without re-inventing the wheel.
Carrier-to-noise ratio, along with such nonlinear distortions as composite second order, composite triple beat and cross modulation, is used to characterize the quality of multichannel analog video signals. The Federal Communications Commission has set minimum performance standards for these parameters, and the National Cable Television Association has issued methods for measuring them. These FCC and NCTA documents pre-date those for optical media.
Several years ago, the Japanese delegation to the International Electro-technical Commission SC 86C Working Group 1 on Fiber Optic Sub-Systems proposed test procedures based on the NCTA Recommended Practices for Measurements on Cable Television Systems. The working group chairman, Allen Cherin at AT&T, submitted these test procedures to the IEC SC 86C plenary in Istanbul last September.
Further editing is being done to make the procedures consistent with the second edition (1993) of the NCTA publication. There are presently five procedures--1280-3-2 on carrier-to-noise ratio, 1280-3-3 on composite second order, 1280-3-4 on composite triple beat, 1280-3-5 on cross modulation and 1280-3-6 on receiver sensitivity (the minimum optical power required to operate at a specified carrier-to-noise ratio).
Thomas McIntosh at AT&T has submitted similar documents to the Telecommunications Industry Association for letter balloting as Optical Fiber System Test Procedures. The procedures include--OFSTP-21 on receiver sensitivity; OFSTP-22 on carrier-to-noise ratio; OFSTP-23 on cross modulation; OFSTP-25 on composite triple beat; and OFSTP-26 on composite second order. Initial results of this balloting were reported when the TIA FO-2.1/6.6 Joint Subcommittee on Singlemode Systems, also chaired by Cherin, met in Albuquerque, NM, in January 1995.
These fiber procedures concern multichannel or cable-TV transmission. A separate set of standards for single-channel video has developed during the last 50 years for long-haul transmission of broadcast television signals over the telecommunications network. The Electronic Industries Association set forth transmission requirements in RS-250B in 1976 and RS-250C in the 1980s to ensure FCC picture-quality standards would be met.
After AT&T`s divestiture of its local telephone companies in 1984, this responsibility shifted to the Exchange Carrier Standards Association T1 Committee on Telecommunications, which generated American National Standards Institute standard T1.502 in 1988. This standard established loss budgets and distances for copper and microwave transmissions, but not for fiber.
Subcommittee T1A1, formed by the merger of Subcommittees T1Q1 and T1Y1, has committed to rectify this situation. Working Group T1A1.5, chaired by Eric Hauch of the State of Tennessee Telecommunications, created an ad hoc committee chaired by Frank Taylor at BellSouth Telecommunications to update T1.502. The committee will extend T1.502 to cover transmission over fiber, and may also extend the standard to digital television and multichannel applications. These possibilities were scheduled for discussion at the Working Group T1A1.5 meeting in Boulder, CO, last April. q
William B. Gardner represents AT&T Bell Laboratories, Norcross, GA, on several fiber standards committees. He received a B.S. from the University of Alabama and a Ph.D. from Johns Hopkins University, both in physics.