It is a little early for ghost stories, but let’s hope single frequency networks don’t come to haunt broadcasters.
Ghosts plagued the reception of radio signals long before there were TV signals. At night radio signals reflected from the ionosphere return to earth sometimes thousands of miles from the station. At such distances, the ground wave is nonexistent, so the station wasn’t heard, but its ionospheric reflections were heard. You heard ghosts.
Early TV experimental broadcasts used AM radio transmitters around 1,500 kHz. Any night, their pictures could be received, but with a ghost or two below the image—not to its right side as with analog TV, but below because the radio wave traveled hundreds of miles to the ionosphere and back.
When commercial TV broadcasting commenced just before Worl War II, ghosts due to reflections from hills and manmade structures appeared to the right of the image. The only way to remove them was with a rooftop, directional antenna aimed to reduce ghosting.
We never see ghosts with DTV, but we do have echoes. You will never see a ghost on a DTV screen, but because of echoes of the DTV signal you may see and hear nothing at all if the ATSC receiver’s Adaptive Channel Equalizer (ACE) cannot cancel the echo or echoes present without lowering the signal-to-noise power ratio (SNR) to or below the threshold SNR of the ATSC System, 15.2 dB.
Echoes upset the flatness of the spectrum of all DTV signals, be they ATSC, with 8-VSB modulation or DVB-T or ISDB-T with COFDM modulation.
With COFDM modulation, there are thousands of carriers spaced a few kilohertz apart across the channel. Each carries a very low data rate as the Symbol Time is in milliseconds, not nanoseconds as is the case for our single-carrier 8-VSB modulation scheme.
Since COFDM receivers do not have an ACE, they get rid of echoes by shutting off the receiver for the first few tens of microseconds (the Guard Interval) of each Symbol Time. This costs them some reduction in data rate, but it is highly effective in killing ghosts. Any echo, which falls outside the Guard Interval acts as noise lowering the received SNR.
About 2005, excellent designs for the ACE of ATSC receivers were introduced. In the June 23 issue of TV Technology, spectrum plots of ATSC signals and DVB-T (with COFDM) afflicted by a single strong echo were published. You can still see these on www.tvtechnology.com under Digital TV.
Leading echoes, those which arrive before the stronger signal, are usually cancelled by a Finite Impulse Filter (FIR) in the ACE, and those echoes arriving after the stronger signal, lagging echoes, are always cancelled by an Infinite Impulse Response Filter.
This means while some receivers may behave in the same way for lagging echoes, they may react differently to leading echoes. What it also means is that different receivers behave differently as the design of these filters is proprietary and is not subject to performance specifications by the FCC.
In countries using COFDM, the administrating governmental entity for their transmitting facilities determines the width of the Guard Interval and all receivers behave in the same way. In mountainous terrain, larger Guard Intervals are chosen.
We have more than one hundred million DTV receiving devices whose echo cancelling is not standardized. It varies with the chipset used by the receiver manufacturer, and it varies model year to model year.
What we do know is the velocity of all electromagnetic radiation is 300 meters per microsecond or about 1000 feet per usec or 0.19 mile per usec. The echo delay is 5.28 microseconds per mile.
This means that, the ACE in the best 2005 ATSC receiver shown in Fig. 1 of my June 23 column performs well in locations with echo delays of less than two miles. However, its performance degrades with echos of 10 µsec or greater. For an echo at +/- 60 µsec, it can do nothing. Such echoes act like noise in the receiver. A convenient estimate of signal field strength vs. distance is 0.9 dB per km or 1.4 dB/mile.
INTERFERENCE BETWEEN TRANSMITTERS
Fig. 1 shows an SFN composed of seven transmitters, each of which radiates enough power to provide reception for say 10 miles around the tower. The circles around the transmitter sites correspond to the minimum usable ATSC field strength, which the FCC says, for a directional, rooftop antenna feeding one TV set is 41 dBuV/m. Field experience suggests that it may be as high as 50 dB µV/m.
Fig. 1: 41 dBμV/M present coverage perimeter 60-mile diameter The large outer circle represents the minimum field strength from a single transmitter, which provides the noise-limited coverage the station expects, per the FCC. I chose a 60-mile radius typical of a medium-sized market. Each small circle (10-mile radius) represents that same field strength, but from one of the seven smaller transmitters of this SFN.
The shaded areas of Fig. 1 indicate the lost coverage of this SFN, which is about 22 percent of the former coverage area. By increasing the Effective Radiated power of each Tx by 1.1 dB much of the former coverage area could be regained, as the coverage area is no greater than the present FCC coverage allocation. It is not a quite circular.
Some will still lose reception where the field strength is too weak at their home unless they install a well-designed low noise amplifier at their rooftop directional antenna. There are six echoes from the other transmitters, plus whatever reflections are present at that site.
Fig. 1 shows the echo path lengths in microseconds to an arbitrary receiving site. The nearest transmitter of an SFN does not always provide the strongest signal because the direct path from that Tx to some sites may be blocked either by terrain features or manmade structures. This is especially true for an SFN where the transmitter antennas will be on short towers, perhaps 250 feet compared to the average height of 1250 feet for present towers.
In Fig. 1, a bisector A–B is shown perpendicular to the baseline between T1 and T2. Along this bisector the paths from T1 and T2 are of equal length, so in theory, these signals are of equal power and equally delayed. This gives rise to the notion of a zero dB echo. Anywhere along this bisector, many modern ATSC receivers can be expected to work as they can handle one zero dB echo.
In practice, points where these signal powers are equal will be distributed around the bisector because the path losses from the transmitters are unequal. Moreover, the directional rooftop antenna should be pointed towards one of these transmitters and therefore away from the other. The FCC assumes the DTV receiving antenna provides 10 dB gain and a front-to-back ratio of 14 dB in the UHF band. This would shift the equal power line far from where it is in Fig. 1.
One mile from the bisector in Fig. 1, one echo arrives 10.3 microseconds before the other. If the early signal is weaker than the later signal, we have a leading echo of –10.6 µsec. If the early signal is stronger than the other, we have a lagging echo of +10.6 µsec.
The adaptive channel equalizers of some unknown number of ATSC receivers and/or down-converters have limited capabilities to reject a leading echo. These may fail within one mile from the bisector line. Next month, we will continue this analysis.
Charles Rhodes is a consultant in the field of television broadcast technologies and planning. He can be reached via e-mail atcwr@bootit.com.
