[opendtv] Seeing Ghosts on a Single Frequency Network

From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
Date: Tue, 18 Jan 2011 14:56:31 -0600
Another excellent article form Charles Rhodes. But I don't entirely agree with 
his conclusion:

"Let's hope the FCC doesn't mandate SFN topology for U.S. broadcasters."

My position has always been, if the FCC wants to allow certain difficult 
conditions to exist, for example towers that are scattered throughout a market 
area, or certainly SFNs in market areas, then the FCC must make it its 
responsibility to mandate receiver performance minima that are compatible with 
its difficult conditions.

We have seen over the years that a combination of matched filtering in the 
tuner, tracking tuners, dual conversion IF stages, and good equalizers, can 
result in very good performance. The "Table I" printed with the article does 
*not* support the notion that 8-VSB can't be used in SFNs. What it says is the 
opposite. What is says is that 4 of the 26 government approved receivers tested 
were in fact quite capable of operating in small area SFNs. So clearly, we're 
not looking for some sort of breakthrough discovery here. Even if the four good 
receivers are all built with the same chips, the evidence is that the problem 
is solvable and can be made available at decent price (else, the receiver would 
not have been one of the anointed ones).

Add to this a more clever use of the rapid symbol arrival times, to improve on 
dynamic echo equalization, the use of dual receive antennas, and the better 
combined use of the Reed-Solomon and Viterbi trellis codes, and the FCC could 
go *well* beyond what is the status quo today, EVEN WHILE retaining 8-VSB as is.

If things don't evolve this way, my belief is, it's all because there's no 
money in it for certain interests. Not because it can't be done.

Bert

---------------------------------
http://www.tvtechnology.com/article/112308

Seeing Ghosts on a Single Frequency Network
by Charles W. Rhodes, 01.18.2011.

Ghosts of DTV signals could easily be seen on analog TV screens, but with DTV, 
they are invisible on the TV screen. DTV ghosts, also known as echoes, can only 
be seen on the screen of a spectrum analyzer.

TO ILLUSTRATE

Fig. 1 shows a single ghost or echo delayed by 1.0 µs. The spectrum analyzer is 
tuned to channel 37 whose center frequency is 611.000 MHz. Channel 37 begins at 
608 MHz, ending at 614 MHz. The instrument's span is 10 MHz.

The interaction of the signal spectrum and that of its ghost produces broad 
peaks, one for each 1.0 MHz, and between these are deep and sharply defined 
minima. Ghosts, in the case of single frequency networks (SFN) are the weaker 
signalssreceived from the multiple transmitters of the SFN while the signal 
received with the greatest received power is the received signal.

In Fig. 1, I have shown one echo for clarity. Multiple echoes are much more 
complicated so let's leave them out of this story. The echo in Fig. 1 is only 
0.5 dB weaker than the signal.

Some of you have tried to see DTV ghosts and failed. Perhaps you tried a 10 µs 
single echo and got the spectrum like that in Fig. 2.

Where is that ghost? A 1.0 µs echo produces minima every 1.0 MHz, as shown in 
Fig. 1, so a 10 µs echo would produce a minima every 100 kHz. But you can't see 
them in Fig. 2.

If we reduce the instrument's span from 10 MHz to say 500 kHz, and make sure 
that its resolution bandwidth and video bandwidths are significantly reduced 
and set the sweep speed to 2 seconds (this is most important), you will see the 
interaction of a 10 µs echo and the signal as in Fig. 3.

This is how I measure echo delay in my laboratory. If you are careful, you can 
measure echo delays beyond +/-50 µs by this simple, but tricky, technique. Use 
Fig. 3 as your model for setting up your spectrum analyzer.

With my Rohde & Schwarz SFE DTV Signal Generator, many echoes are available, 
each quantified as to delay in microseconds; amplitude relative the signal 
power in dB; and its phase relative to the signal power in degrees.

Phase is specified for a signal frequency at the center of the screen. The 
importance of the phase relationship signal-to-echo is just as important as the 
signal-to-echo delay (in microseconds), but far less well understood. My 
spectrum analyzer can be set up for any phase (zero-360 degrees) at whatever 
frequency is at the center of the screen.

NOW FOR THE TEST

Table I: Test Results with 26 NTIA -approved DTV "converter boxes"

(Have to go to the URL for the article.)

Where is this ghost story leading to? My colleagues and I wanted to understand 
how the phase of an echo at the pilot carrier frequency of the ATSC signal 
might affect whether or not it can be received. It does adversely affect 
reception.

We set the center frequency to 611.000 MHz, the Span to 500 kHz, the resolution 
bandwidth to 1 kHz and the sweep time to 2 seconds to "capture the ghost." This 
capture is shown in Fig. 3.

What we found for a +10.0 µs echo 0.5 dB below the signal is that the pilot 
carrier power varied with the phase of this echo, down -25 dB for 180-degree 
phase relationship, or up +3 dB for 0 dB phasing. It is the difference between 
path lengths of the signal and its echo that determines the echo delay. A 10 µs 
delay is due to a path length difference of 10 km or 6 miles.

All this trouble was well worth it for we then tested how the phase of this 
static single echo affected the minimum usable ATSC signal power of 26 
NTIA-approved DTV converter boxes. Those results are shown in Table 1.

I've color coded these results. GREEN means the unit worked continuously. 
ORANGE means it works most of the time, and RED means it did not work at all. 
The vertical columns go from 180 degrees to 360 degrees in 30 degree increments.

A viewer receiving an out-of-phase strong echo on your channel is in deep 
trouble. But without a spectrum analyzer and what you now know, the cause of 
the reception failure of your signal remains a mystery. Viewers cannot be 
expected to understand such mysteries or what to do about them, except to 
subscribe to cable or DBS services.

Let's hope the FCC doesn't mandate SFN topology for U.S. broadcasters.

Charles Rhodes is a consultant in the field of television broadcast 
technologies and planning. He can be reached via e-mail at cwr@xxxxxxxxxxx
 
 
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