Multipath and DTV

[Gregg Lengling]

Questions have been asked by some forum members who aren't familiar with multipath and it's effects...this will try to help you understand:

Multipath is the problem caused by a transmitted signal arriving at a receiverthrough multiple paths. These paths are reflections off buildings, airplanes, people moving around aroom, etc. In analog video, multipath manifests itself as ghosts in the image, shadows of objectsshifted across the screen. In really bad cases, the video breaks up. In digital video, which is eitherperfect or `off', severe multipath interference can lead to loss of reception.

When a roof antenna is used, there is typically a line-of-sight path between the antenna and thetransmitter. Furthermore, the roof antenna typically has directionality; it tends to receive signals fromthe direction in which it is pointed and rejects others. If the TV viewer is using a roof antenna, multipathis generally not a problem.Unfortunately, the problems caused by indoor reception are not so easily addressed. The TV viewermay use a simple `rabbit ears' antenna, or some other small, visually-attractive antenna that hasvirtually no directionality. In indoor applications, no directionality is required-it is impossible toknow from where the signal is coming.

In a typical path in an urban setting, the signal will reflect off some buildings and enter the homethrough a window. The primary path may then reflect off walls and the like before arriving at theantenna. This is obviously a big multipath problem.

WHY DIGITAL TV?
Before addressing the solutions to these problems, I think it important to step back and review why theU.S. (and the world) is headed down the digital path in the first place. There are two main thrustsbehind digital television. The first, and most important one, is more efficient use of the spectrum. Thesecond is providing consumers with a better television experience. `Television experience' can beexpanded to include other classes of broadcast digital services, such as data broadcasting, on-demand programming and interactive television.Spectrum is valuable. Until now, the television industry has enjoyed a prime, disproportionately largesegment of our nation's spectrum. More than half of this spectrum, ranging from 54 to 806 MHz, is leftunused in any given locale. The decision-makers in Washington realize that a significant portion of thespectrum dedicated to broadcast television is currently unused

THE INTERFERENCE PROBLEM
Would it surprise you to know that a primary reason for the inefficient use of spectrum is televisiontuner performance? In fact, the FCC, in granting licenses to broadcasters, specifically avoids, bymandate, the use of more than half of the available channels in a region. These channels, calledtaboo channels, consist of channels adjacent to an existing broadcaster and those at the so-called`image channel' of an existing broadcaster. Channels that are harmonically related are alsoconsidered taboo. These taboo channels can potentially cause interference, manifested as lines orpatterned noise on an analog TV set with a conventional television tuner. In digital TV, they can causeloss of reception. A diagram presenting the current analog television spectrum of Los Angeles isdepicted below. It illustrates how much of the spectrum is `protected' because of the taboo channels,a condition that is mirrored in major markets throughout the U.S.

In the UHF band, nearly three quarters of the spectrum is unused. Almost every channel has an imagechannel. If the allocation scheme also excludes use of adjacent channels and image channels, morethan half of the spectrum is unavailable for use.With higher-performance television tuners, those that offer the higher selectivity needed to reject thetaboo channels, the spectrum could be more efficiently utilized. This would lead to a richer set ofbroadcast media and/or a reclamation of spectrum for other uses.Recently, the new DTV broadcasters have been granted licenses to broadcast in the taboo channels.
DTV broadcasts within the NTSC spectrum are being permitted under the assumption that digitalsignals will not cause interference problems for existing analog channels. This view is based on thenature of a digital signal. A digital signal does not possess a strong picture carrier and it distributes itssignal energy evenly across the channel. It is believed that these characteristics will preventinterference. Many experts are doubtful of this. In fact, broadcasters, the FCC and TV manufacturersare beginning to realize that advances in tuner technology are required to prevent interference in thereception of both analog and digital signals.

SPECTRUM EFFICIENCY AND DEMODULATION
Spectrum efficiency also impacts the issues of 8-VSB and COFDM. 8-VSB has more bits per hertz ofbandwidth than COFDM, and consequently it uses spectrum more efficiently. According to theAustralian DTTB Report, COFDM needs an extra megahertz of bandwidth per channel (7 MHz forCOFDM versus 6 MHz for 8-VSB) to get 19.3 Mb/s of data, the amount required for HDTV. The ideabehind COFDM is to provide immunity to multipath. It does this by taking a bit stream and breaking itup into several concurrent bit streams. Additional complexity is incurred in the COFDM demodulatorby dividing the streams into several isolated (orthogonal) channels. The bits are coded into symbols,and because symbols are now placed in several parallel channels, the symbol rate can be low(1/256us), so low in fact that the time between symbols is very long, relative to the time difference ofarrival of the multipaths.This is not the case for 8-VSB. The symbol rate (1/93ns) is such that typical multipath delays are ameaningful proportion of the period between two symbols, resulting in intersymbol interference.However, 8-VSB is much simpler because it is only one channel.Proponents of 8-VSB point out that the intersymbol interference issue can be handled by sophisticatedequalizers. These equalizers sense the secondary path or paths, invert them and subtract them fromthe signal, thus removing the offending multipath(s). Some multipaths are harder to remove than otherssince they are caused by moving objects rather than stationary ones. These objects could beairplanes or, even, people walking around a room. This is the dynamic multipath problem, andrequires that the equalizer adapt to changing conditions. In the Sinclair tests, latest and greatestequalizer technology was not used, and as a result, the results are not alarming.Another important issue is the power level required by each demodulation scheme. The AustralianDTTB Report also found that an additional 4 dB of power was required for DVB-T (COFDM) to achievethe same coverage as ATSC (8-VSB). Considering both spectrum efficiency and power, 8-VSBappears to be a good choice, assuming that advanced equalizers will solve the multipath problem.

[FreQi]

A diagram presenting the current analog television spectrum of Los Angeles is depicted below

... ?

[Gregg Lengling]

I didn't put the diagram into the post...if you find it important I can email it directly to you.