![[Photo of Linc Reed-Nickerson]](reed.jpg)
This month's speaker, Linc Reed-Nickerson of Tektronix, gave an
informative and enlightening talk on 8VSB, the modulation scheme used
for digital television broadcasting. After some anecdotes about early
analog television standards and a review of various modulation
techniques for analog signals, he described various digital modulation
methods, illustrating them with constellation diagrams and
discussing suitability factors for each.
Emphasizing the absolute need for maintaining the fidelity of the digital signal during transmission, Linc used a block diagram of a DTV transmitter to show various places where distortion can cause bit errors. Reference demodulation is used to generate corrective predistortion to accomodate for antenna system nonlinearities such as icing and line deformities. An overview of the signal processing and coding system followed, including the significance of randomizing, Reed-Solomon encoding, interleaving and trellis coding. Data multiplexing, pilot insertion and modulation were outlined.
All of this points to the need to maintain the signal in as sound a condition as possible to avoid problems in the fringes of coverage, said Reed-Nickerson. Minor transmitter problems can cause loss of coverage, especially in indoor-antenna situations where small changes in multipath, such as a person moving around a room, can cause sudden loss of picture at the receiver.
Linc went over testing and monitoring, including the FCC bandwidth emission requirements, signal-to-noise and modulation error ratio measurement, group delay and error vector magnitude. He showed constellation diagrams of signals afflicted with typical problems, such as noise, clipping, expansion and phase shift, and how to recognize them. Finally, he explained the use of eye diagrams, spectrum sweeps and peak-to-average power monitoring as diagnostic tools to aid in keeping a system clean, and demonstrated the use of a Tektronix RFA-800 8VSB Test Set.
Prior to Linc's talk, John F. X. Browne, P.E., of John F. X. Browne and Associates, updated section members on FCC matters relating to DTV, including an overview of the Final Report and Order, which formalized power levels, channel allotments and the emissions mask. He also described the results of reception tests made in the Washington, D.C. area using receivers equipped with typical consumer-grade indoor antennas. Out of more than 200 sites located up to 20 miles from the transmitters, only 35 percent on average were able to get acceptable reception, compared with an 84 percent success rate with outdoor antennas.
Also, Central Region Governor Harold Miller reported on the progress in establishing a student section at Specs Howard School of Broadcast Arts in Southfield, and invited Detroit section members to participate in their programs.
![[Photo of Ameritech TOC]](toc.jpg)
Following the meeting, Wallace Murray of Ameritech took several
members on a tour of Ameritech's Television Operations Center.