[opendtv] Re: DTT tuner design

  • From: "Albert Manfredi" <bert22306@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Wed, 04 Jul 2007 15:50:58 -0400

Al Limberg wrote:

The real problem is that broadcasters have looked to CE
manufacturers to spend the money to do R & D to
develop a system for OTA.  It is not a good investment
for CE manufacturers  Broadcasters would have done
better to hire a R & D lab to do the necessary work to
develop a viable system & protect their OTA business.

Took the words right out of my mouth. And the broadcasters already have an organization that could do this, and did for a short time: the NAB. Why did they drop the ball?

Dale Kelly wrote:

That, Frank, is pure BS....and spoken like a true CE advocate.

Wait, wait, Frank does make an interesting point. Just what did the FCC assume to create its DTT coverage contours, compared with other RF services? I was too lazy to ferret out the cell phone or DBS assumptions Frank wonders about, but I knew right where to go for the French TNT system.

The FCC assumptions are here:
http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet69/oet69.pdf

And the French CSA assumptions are here:
http://www.csa.fr/pdf/Rapport-GT2-Aspects_Radiofrequence_de_la_TNT.pdf

To begin, both assume a receive antenna height of 10 meters.

The bottom line, after all their assumptions are taken into consideration, is signal strength at the various frequencies. Here is the comparison:

Low VHF:
FCC criterion is 28 dBuV/m
CSA is N/A

High VHF:
FCC criterion is 36 dBuV/m
CSA criterion is 39 dBuV/m

UHF at 500 MHz (lower channels):
FCC criterion is 39.2 dBuV/m
CSA criterion is 44 dBuV/m

UHF at 800 MHz (beyond Ch 51, but the FCC gives a formula):
FCC criterion is 43.3 dBuV/m
CSA criterion is 48 dBuV/m

So after all the numbers are jiggled, and obviously the FCC and the CSA jiggle them in slightly different ways (except identical receive antenna height), the differences boil down to 3 to 4.8 dB strength advantage given to the French TNT criteria.

This is actually a good comparison, because the CSA uses 64-QAM, 2/3 FEC or 3/4 FEC as their nominal settings, which are reasonably close to the same spectral efficiency as 8T-VSB.

Also, the CSA adds 2-3 dB to the theoretical C/N margin of the COFDM mode, and in their example they used the ricean channel C/N margin, whereas the FCC sticks with 15 dB of C/N margin, which is the nominal required for a gaussian channel. So Frank is right. The FCC does APPEAR to be far more strict. But then again, the FCC considers downlead losses and other factors that perhaps the CSA didn't consider. So it's a mixed bag. The end result is all that really matters, though.

My takeaway is this: ATSC receivers need to implement good equalizers, which make use of echo energy, and they need to exploit the lower C/N margin advantage of a single carrier scheme, in order to compensate for the more demanding countour criteria compared with the French planning factors. If receivers do this, then the two systems should come very close.

My other takeaway is that the DVB-T2 program aims to do exactly what the FCC has done from the start. Which is to get 4 dB better than where they are today. So it's not all bad, guys.

Bert

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