[opendtv] Re: 20040630 Wolfsson's Wee-hour Wednesday Words (Mark's Monday Memo)

>      This next URL is a spreadsheet relating to success of
> demodulating
> certain captured signals.  Many were not:
> <http://www.atsc.org/standards/a_74_RF_capture_spreadsheet.xls>

Multiple sites showed failed reception. True.

However ...

If I read this correctly, this spreadsheet consists of a compilation
of 50 difficult sites in NYC and the Wash DC area. Of these 50 sites,
40 used only an indoor antennas. In DC, double bowties. In NYC, an
assortment of indoor antenna types. Outdoor antenna sites only in
the Wash DC area.

The signal captures in this spreadsheet were used as examples of
mostly difficult sites for the ATSC recommended receiver performance
guidelines document A/74, and the tables are in fact incorporated
within that document.

But most of all, the reception dates for these sites are all between
May and October of 2000 (May-July in DC, October in NYC). Which
means that the prototype receiver they're talking about is most
likely the 2nd generation NxtWave?

Going down through the plots, I noticed that several had no
associated reception report in the tables. The ones that were
mentioned in the tables could be predicted fairly easily, keeping
in mind that first gen receivers were capable only of echo
tolerances of about -0 to +15 usec. Pre-echoes of greater than
-15 dB strength or post-echoes beyond 15 usec resulted in failed
reception. Second gen receivers didn't do much better for pre-echo,
but had post-echo tolerance up to 44 usec or so. So almost all
reception results were explainable.

Some of these plots showed the echo swapping back and forth
between pre and post. The one which had a reception report
showed failed reception, as I'd expect.

There were some surprises, such as NYC 216/56/01. This was
essentially a Brazil E ensemble, but the three equal strength
signals are spread out 6 usec between echoes vs 1 usec in true
Brazil E. Which I'd interpret as being even more difficult than
Brazil E. And yet, a 2nd gen receiver got good reception.
Astounding.

The good news is that at least in theory, the 4th gen Lynx
and 5th gen Zenith should be able to handle all of the plots
shown in A/74.

A/74 describes these tables, derived from the spreadsheet, as
follows:

"In the first part of this section, a dataset of field ensembles
(DTV captured signals) is furnished as an example of the various
conditions that can be observed in the field. Most of the field
ensembles contain data captured at sites where reception was
difficult. The field ensembles are clearly not meant to represent
the statistics of overall reception conditions but rather to serve
as examples of difficulties that are commonly experienced in the
field. A few mild ensembles are included in the data so that
receiver design does not focus solely on new difficult conditions,
overlooking performance requirements shown to be necessary in the
past."

Bert
 
 
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