Comments

Should the DTV Industry Switch to CODFM Modulation?

By Larry Bloomfield
Extract from DTV Tech Note #028, April 11, 1999

I have been following, with a great deal of interest, the concerns of several who are of the opinion that we, the American broadcast industry, have made a very serious and grievous error in selecting 8VSB as our method of modulation. If that is true, the time is now, not in the future when most of the country has made the switch, to take whatever corrective action that may be necessary.

Several questions come to mind: First, can we fix what we've got or, second, are there any ways to salvage it? You don't know how I wished I had an answer to those questions. My fellow engineers with whom I've spoken don't seem to think so or are of the same frame of mind as am I.

Most all of us realize that the method of modulation is at the very core or heart of any transmitter system. We have to either fix it or change the type of modulation from 8-VSB (eight level Vestigial SideBand) to something else. Either path will have a serious impact on our industry, not to mention those who have invested heavily in the migration (both at home and at the TV stations). Any changes won't arrest, but will seriously push back, the migration from analog to digital by a significant amount of time. The ripple effect will be incalculable! It will impact multichannel development and any of the perceived Datacasting (content-casting) services that are in development. As you know, these technologies are only possible in the digital world of television.

Many have suggested such modulations techniques as Coded Orthogonal Frequency Division Multiplexing (COFDM), like is specified in the DVB-T standard, but there have been some indication that it may have its' own set of problems. One thing for sure, we don't what to embark on a change only to find out that we need to change horses a second time in midstream. That could all but kill the industry.

The broadcast community is like an inductor: it will oppose any change in current flow, so it's not likely that any of this will happen, but, as stated, there has been a lot of fuss, from a number of different sources about 8VSB not being very robust. The exchange in coverage broadcasters will go through when they move from NTSC coverage to the coverage that appears to be what comes with 8VSB is not equitable in most cases, especially if indoor antennas are considered. The DTV (8VSB) broadcaster is going to come up short.

Sinclair Broadcasting has already raised this issue and has demonstrated that the initial power allocations given DTV Broadcasters was insufficient. They got the FCC to up the power levels and change the signal levels out at specified distances from the transmitter. Examples of the kind of problems they are discovering are with high power levels in the city grade contours that create very objectionable multipath, which is disastrous.

Sinclair has even taken it further and demonstrated that reception with the 8VSB system is only good typically when the viewer has an antenna (nominally 30 feet), with a clear shot to the transmitter site, outside his house (or whatever). Not everyone lives where this is possible. Apartment dwellers are in deep trouble. Rabbit-ear antennas just don't work, according to their survey and a number of others that have taken place in the "real world." This was discovered only after a few DTV stations had gotten on the air.

COFDM, which came out after 8VSB, appears to be more robust when it comes to using indoor antennas, antennas in a mobile environment and is not nearly affected by multipath. The need for tall antenna structures at your home may still be necessary in the distant areas, but not close in, as is the case with 8VSB.

I wrote an article for the May 99 issue of BE that reports on a survey I made of a number of Chief Engineers across the country who believe that this move to DTV with 8VSB will relegate terrestrial television to the role that AM radio stations now play. Most feel that viewers will get their signals either via Satellite or some form of cable.

The issue boils down to the type of modulator that is used in the very flat (over its bandwidth) transmitter. The retrofit would be more reasonable now than if we wait until more stations have come on to the digital airwaves.

I believe if Sinclair and the others who are pushing this really serious issue are successful, the whole idea of DTV will go into limbo and get put on hold for about a year or so. Are we ready for that? Yes or no--we're at a crossroads and the signals are flashing. We've got to do something. As the man once said, lead, follow or get the hell out of the way. Digital television will make its visitation on us in one form or another, sooner or later, and that you can bet on.

One redeeming factor is that most of the DTV-ready TV sets accept video, audio and data bitstreams only after they have been demodulated, having no tuners or RF sections. Many DTV ready TV's today do not have their tuners built in and rely on a STBs (set top boxes) to demod their signals.

A concluding thought: assuming this is all true and we do nothing, you can bet cable penetration will increase and the need for terrestrial broadcasters will diminish accordingly. If you think that cable tries to be in the driver's seat now, standby: they'll have you by the you know what! If nothing else, all of us should be considering what impact this will have and can we weather the storm? Good luck.

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Edited by Gary Stigall. Posted 12-Apr-99.