FreeLists / opendtv / [opendtv] Re: Demand for free DTV rising in Australia

["Dale Kelly" <dalekelly@xxxxxxxxxxx>]

Bert wrote:
> The problem is still alleviated, though. Because the analog stations are
> typically much more powerful than the digital stations that will replace
> them.

Unfortunately this is not as straight forward as it might seem. Adjacent
channels are not the only IM issue but has lessened due to the upgraded DTV
Mask filter requirements. However, if the D/U ratio is moderately high it is
still a problem and there is more than a 10db spread in DTV channel
assignments - many have 1000KW while others are 100KW or less.
Also, the potential for non licensed device interference is very high.

See the following remarks from a highly qualified independent RF engineer,
they are instructive and include the recent FCC receiver test report URL.
Also, the CRC recently did a similar test that reached many of the same
conclusions.
Dale

This 200+ page Report can be downloaded from the FCC website:

http://www.fcc.gov/oet/info/documents

Scroll down to the Reports section, click Reports. The Report is at the top
of the list that will appear.
This Report has been very professionally done. In my opinion, it supports my
own beliefs;



1). The ATSC doc. A/74 does not provide suitable and effective Protection
Ratios for DTV-DTV interference.


2). The receivers tested do not meet the ATSC Guideline specifications, some
fell far below.


3)  The receivers tests all have single conversion tuners whereas the
prototype tuner tested in 1995 was a double conversion tuner. The
interference the FCC has documented largely is due to the universal usage of
the cheaper single conversion topology.


4). Combinations of two Undesired signals that produce 3rd order
Intermodulation products falling in the Desired UHF  channel will be a very
significant source of unanticipated interference. In some cases this will be
the dominant source if interference.


5).  While the FCC engineer and I both thought the interference due to
receiver non-linearity would be most significant only where the Desired
signal is very strong, that is not't what these tests found. Too bad for
indoor reception after 2/17/08.


I have no idea what the FCC will decide to do about this unanticipated
interference.


Perhaps if this Report gets widespread distribution to the receiver/tuner
manufacturing community better tuners will be designed for this marketplace.


I personally think the ATSC should take action such as withdrawing A/74 and
convening a Working Group to come up with realistic Protection Ratios and
scientifically based laboratory test procedures.


As it stands now, millions of federally subsidized DTV downconverters spec"
d. per ATSC A/74 will be made and sold.


 
 
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