Roberta "Bobbi" Barmore, KB9GKX, is a broadcast engineer with Channel 13 in Indianapolis, Indiana. Recently, Bobbi mentioned that her company has started to transmit digital television. On hearing this, I wrote to her asking for more details. My questions are preceded with "JP". Her response is denoted as "RB".
First, it would be prudent to review today's TV broadcast standard which is abbreviated as NTSC (National Television System Committee). The raster or picture in the NTSC signal comprises of 525 lines. First, the 262.5 odd lines are "painted up" then the remaining 262.5 even lines are painted. The process is called "interlaced scanning" in order to reduce flicker in the picture. Thirty complete pictures are painted each second. The video portion (luminance information) modulates an AM carrier. The colour information (chrominance) is sent on a sub-carrier which is modulated both in phase and amplitude. Sound is sent on an separate FM carrier and usually in stereo these days. Each TV channel occupies 6 MHz of spectrum space. Over the years, there have been other minor "add ons" to the NTSC standard to support such functions such as automatic colour correction and close-captioned video for the hard of hearing.
JP - MAY I SAFELY INTERCHANGE THE TERMS HDTV (HIGH DEFINITION TV) WITH DIGITAL TV (DTV)?
RB - NO. The Direct Broadcast Satellites (the technology that provides 400-odd channels with an 18" dish) is digital but not high-definition. Conversely, Japan had implemented an on-the-air, analog, high-definition TV service for a few years already. In the US, the high-definition broadcast TV service is digital, using the MPEG-2 (Motion Picture Experts Group) standard for generating the video and sound "payload" and an encoding scheme known as 8VSB transmission to encode the signal. The broadcast signal from the transmitter would closely resemble the data stream as sent by a very fast analogue modem attached to a home computer. That format uses 8 discrete levels of modulation. Since communications standards in North America are established by international agreement, it will just be a matter of time before DTV broadcast arrives in Canada.
The DTV system is far more complex than the current NTSC system. A DTV station can transmit one high-definition picture, plus 5.1 channels of sound (Left, Right & Center Front, Left & Right Back, plus a sub-woofer channel, which is the ".1") in any of several sound formats, or four simultaneous channels of low-definition video with stereo sound. "DTV" is probably the best generic term to describe the new North American system.
The four-picture format is intended to be viewed one "sub-channel" at a time for the time being. As an example, each video sub-channel could carry one type of programming such as soap operas, sports, sitcoms etc., but no one is set up to do that yet. For now, the intention is to broadcast one, high-definition, high grade picture which will produce the initial revenue streams.
JP - IN WHAT FREQUENCY BANDS WILL DIGITAL TV OPERATE?
RB - DTV will operate in the same bands as regular TV except for the assignments currently used by the VHF-Low channels 2 through 6. UHF includes channels 14 to 57.
JP - IF DTV BECOMES SUCCESSFUL, ARE THERE ANY PLANS TO DISPLACE NTSC TYPE TRANSMISSIONS IN TODAY'S TV BANDS?
RB - In the US, the plan is to have everybody broadcasting digital and NTSC by the year 2006, then start shutting down the NTSC format. Network-affiliated stations in the top 25 markets in the US (Indianapolis is 24th!) will have to broadcast a DTV signal by November, 1999, so we jumped the gun a little. Public Broadcast System (PBS) stations, mostly with limited budgets, are the last to be subjected to DTV migration.
JP - WHAT IS THE BANDWIDTH OF A DTV TRANSMISSION AND HOW MANY LINES ARE THERE IN THE PICTURE?
RB - The signal takes up just about every last bit of the standard 6 MHz bandwidth allotment, however, it must be 50 dB down at +/-3MHz from the channel center, which takes some serious filtering and signal processing in every stage of the transmitter.
The present top-level high-definition picture is 1,080 active lines using interlace scanning. There are other picture scanning formats available in DTV. One such example is "720p" . This is 720 lines, scanned progressively (non-interlaced) like it's done in many computer monitors. As new and better formats are developed, DTV will be able to easily adapt to them in a similar manner to the multisync capability of a computer monitor.
JP - ALTHOUGH THE COST OF DTV SETS WILL INITIALLY BE HIGH, MASS PRODUCTION SHOULD DRIVE DOWN THE PRICE. WHAT'S YOUR OPINION?
RB - Maybe in the long term the price of DTV will come down. The first DTV's must be designed to receive both the NTSC and DTV standards during the transition period. The cost to broadcasters will also be higher since they will have to simulcast in both formats.
There are still some small technical glitches to work out. As an example, the 8VSB encoding is supposedly very robust...to a point. It really hates signal reflections, but this can usually be countered by re-orienting the receiving antenna to a small degree. The transmission format also "scrambles" the data over time, to reduce the effects of "burst noise" from lightning, auto ignitions, and similar sources. It takes also some powerful hardware and software to just to switch to a new channel. The mechanism must be robust and fast enough to handle "channel hopping", or sometimes known as "channel surfing". With today's DTV technology, it takes around one-half second in order to capture a new channel selection. With the NTSC standard, channel selection is instant and 'very fast' with the direct-broadcast satellite digital technology. There are other technical issues which are too complex to discuss in this forum.
JP - WILL THE DIGITAL SIGNAL BE IMMUNE TO NOISE?
RB - The MPEG-2 standard only sends the changes in the picture which have occurred since the previous frame, resulting in relatively fewer bits to transmit. The net effect is a reduction in exposure time to noise. The encoding process has other features to reduce susceptibility to noise, which have been proven to work well.
The audio is bundled right in with the video and is equally robust. Sound quality is very similar to CD audio and it has excellent signal-to-noise, better than wide-deviation FM . Unfortunately, it "hits the wall" badly if peak levels are too high. This can be prevented through the use of suitable limiters at the point of broadcast.
JP - HOW GOOD IS THE DTV SIGNAL?
RB - In general, the signal coverage performance of DTV can be equated to "FM broadcast on steroids". As most of us know, an FM signal is usually present or not present due to "capture effect". The DTV signal is like that only more so. There's a "cliff edge" effect out at the fringe areas where on one side of the street you get wonderful pictures, and the on other (farther) side, you would receive nothing at all!
Based on our experience, coverage will be very good here in Indianapolis. At 250 kilowatts effective radiated power (ERP), which is one-quarter of our licensed power level, we put a solid signal into Lafayette, Indiana, some 45 air miles away.
JP - WILL CABLE TV (CATV) SYSTEMS BE ABLE TO HANDLE THE DTV FORMAT?
RB - It seems that CATV will use a different scheme by recording the off-air feed from "8VSB coding" into "64QUAM coding". Another term for this could be digital translation/regeneration. In my opinion, it seems that the 8VSB signal could be passed by a CATV system but phase noise and instability in the CATV system "modules" would likely degrade the data. Regardless, of what DTV format is used, the CATV system will have to be rebuilt in order to be able to deliver DTV to subscribers.
So there it is. Does DTV have a grand and glorious future? Maybe--at least it's some kind of future, which is a good thing, as direct-broadcast satellites, videotapes and cable TV have been kicking broadcast TV around pretty hard in recent years. Our station is putting forth an effort to be the first serious source of good quality, high-definition TV for viewers and that's a good thing in itself.