Published: TV Technology magazine

Issue: 6 April 98



DTV Transmission Debate Continues

Broadcasters Look at COFDM as a Alternative to 8-VSB



by Joe Fedele



\WASHINGTON

  This February, UHF broadcasters expressed a sigh of relief when the FCC increased the minimum power levels for 

DTV transmissions as part of the final allotment table. In the table, released Feb. 17, the commission decided to raise 

the lower limits of the DTV power table from 50 kW to 200 kW. It also agreed to allow ERPs (effective radiated 

power) of up to 1 mW when appropriate beam-tilt measures maintain interference to their newly specified levels (see 

"FCC Approves Core Spectrum" in the March 9 issue of TV Technology ).

  The twentyfold increase in power, attainable through the use of beam-tilted antennas, was meant to quell concerns by 

those not satisfied that a mere 50 kW of power would sufficiently supply enough signal strength to meet the needs of 

viewers using indoor TV antennas.

  But the jubilation could be premature. According to Nat Ostroff, vice president of technology for Sinclair Broadcast 

Group, a group owner based in Maryland, additional problems exist that could complicate matters for viewers using 

indoor TV antennas for DTV reception.



COMPLICATIONS IN RECEPTION

  Raw power may not be enough to supply viewers with a good signal. The problem can be traced back to the transmis-

sion standard itself.  According to Ostroff, 8-VSB may not be the right answer for indoor reception woes.

  In Ostroff's opinion, the commission may have chosen the 8-VSB transmission standard prematurely and without 

giving Coded Orthogonal Frequency Division Multiplexing (COFDM) a good look.  

  Like 8-VSB, COFDM is a transmission standard that has also been considered for use in DTV applications.  But 

COFDM differs from 8-VSB in a number of ways.  Those differences, and their respective strengths and weaknesses, 

have been the topic of numerous debates worldwide.  And some of its proponents in the United States believe COFDM 

was not thoroughly evaluated as a replacement for 8-VSB. 

  Did the commission pass up COFDM in its zeal to drive the DTV train down the tracks? Not so, believes Wayne 

Luplow, executive director of consumer products and HDTV for Zenith Electronics, the developers of 8-VSB.  

"COFDM was thoroughly tested," he said, and found to be less effective than 8-VSB. 

  "Like any system there are trade-offs," Luplow continued. "COFDM does do better with ghosting in some instances, 

but the carrier-to-noise (C/N) ratio is not as good [as 8-VSB]."  Luplow cited tests that showed C/N levels of 19 dB 

for COFDM while 8-VSB  provided levels as low as 15 dB.

  He believes that the 4 dB difference between COFDM and 8-VSB is significant enough to warrant 8-VSB as the sys-

tem of choice. He also noted that COFDM provides for a much-lower capacity for handling data than 8-VSB. That 

translates into slower bit-rate capabilities than the 19.3 Mbps stream currently specified under 8-VSB.  

  And slower bit-rates mean less information (audio, video and data) that will be pushed through "the pipe" at any 

given time. This directly impacts how much information, or individual signals, can be broadcast on one channel.

  "Our tests showed that the COFDM bit-rate was closer to 17 Mbps," Luplow added.  With broadcasters' hopes of 

cramming as many bits as they can into the DTV signal, Luplow does not believe COFDM will fit the industry's needs 

or expectations.

  In Ostroff's opinion "a fundamentally egregious mistake was made by the commission" when it tested and designed 

the DTV transmission standard and assumed that consumers would use outdoor antennas.   

  He explained that the 8-VSB system transmits the entire 19.3 Mbps data-rate payload on a single carrier. COFDM, on 

the other hand, consists of as many as a thousand or more carriers, which effectively divide up the load in parallel 

form, rather than the serial configuration of 8-VSB.



MULTIPATH CONCERNS

  DTV design engineers have long recognized how highly susceptible digital transmissions are to multipath effects.  

"Indoor reflections," noted Ostroff, "tend to be very fast due to inherently short reflection paths."  That can cause a 

DTV signal to break down at the receiver. He also pointed to the fact that COFDM is currently being used in Europe 

and is likely to be chosen as the transmission system in Japan.

  Luplow countered that the Europeans are faced with an entirely different dilemma: the transmission of multiple 

channels across many borders using single frequency networks, which do not provide for insertion of local program-

ming.  

  In the United States, individual affiliates only duplicate network broadcasts for part of the day and insert local com-

mercials and programming the remainder of the day. European stations transmit the same signal 100 percent of the 

time throughout each country and over multiple transmitter sites.  

  The result is 100 percent ghosting of the signal in overlapping areas.  COFDM was specifically designed to work un-

der 100 percent ghosting conditions while 8-VSB cannot handle more than 90 percent effectively.

  This type of scenario is more compatible with COFDM, he said. As for Japan, Luplow believes that country, too, will 

eventually commit to 8-VSB. 

  Ostroff qualifies his support of the COFDM standard, saying "We are not proponents of  COFDM." However, he 

counters, "nobody is prepared to tell us [unequivocally] that 8-VSB is robust enough  for indoor antennas."  n

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