[opendtv] Re: Seeing Ghosts on a Single Frequency Network

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
  • Date: Wed, 19 Jan 2011 17:44:16 -0600

Dan Grimes wrote:

> Of course, the phase issue makes perfect sense.  But my guess
> is that the receiver antenna wouldn't have to move much to
> affect the relative phase between the direct signal and echo.

For DVB-T or ATSC, so-called "diversity antennas" are a good idea for mobility. 
Just for that reason. The deepest nulls tend to be localized.

> Of course, for those of us with fixed antennas on the roof,
> that adjustment isn't practical.  And my directional antenna
> is not exactly a match for an SFN.  However, I'm sure the SFN
> with ATSC is really intended for mobile applications, which
> then begs the question, how will mobile affect these off-phase
> echoes?  I'm sure it depends on the period of the cancelation
> which is dependent on the speed of the receiver.

Don't know about honest-to-goodness SFNs, but the CRC some years ago did test 
DOCRs with 5th gen ATSC receivers. They found that the effect of OCRs was to 
make the aiming of the receive antenna much less important. If two OCRs were 
within range, the receive antenna could be oriented any old way. 360 degrees. 
BUT, if SFNs are designed like Charles Rhodes illustrated previously, with low 
power towers spread as far apart as possible, so as to avoid overlapping 
coverage as much as possible, the situation would be quite different.

The problem Charles Rhodes points out, though, is that the vast majority of 
receivers could not match the good ones (or one, perhaps). As long as one can 
do so, we know we aren't dealing with "cold fusion." (John.)

Bert
 
 
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