Multipath question

[maxwell]

Archived from groups: alt.tv.tech.hdtv (More info?)

DTV has been designed to operate in the presence of signal reflections up to
certain amplitudes and delays relative to the main signal. I have seen
reports on the Web about indoor DTV tests where reflections were measured
both to lead and lag the main signal by up to tens of microseconds.

Does anyone know how reflections could lead (arrive before) the main signal?
Seems strange that the main signal (strongest signal) wouldn't be the most
direct path and therefore have the shortest propagation from the
transmitter.

Maxwell

 

[Guest]

Archived from groups: alt.tv.tech.hdtv (More info?)

It depends upon your definition of "main signal".

I used to have a situation where the signal reflected off a nearby hill
was stronger than the direct path signal which had to diffract over a
different hilltop. So you had a weak direct path signal, followed by a
stronger reflected signal. Which was the "main signal"?

Maxwell wrote:
> DTV has been designed to operate in the presence of signal reflections up to
> certain amplitudes and delays relative to the main signal. I have seen
> reports on the Web about indoor DTV tests where reflections were measured
> both to lead and lag the main signal by up to tens of microseconds.
>
> Does anyone know how reflections could lead (arrive before) the main signal?
> Seems strange that the main signal (strongest signal) wouldn't be the most
> direct path and therefore have the shortest propagation from the
> transmitter.
>
> Maxwell
>
>
>
>

 

[Guest]

Archived from groups: alt.tv.tech.hdtv (More info?)

"Maxwell" <nospam@nospammmmm.com> wrote in message
news:fqKRd.1536$DC6.53@newssvr14.news.prodigy.com...
> DTV has been designed to operate in the presence of signal reflections up
> to
> certain amplitudes and delays relative to the main signal. I have seen
> reports on the Web about indoor DTV tests where reflections were measured
> both to lead and lag the main signal by up to tens of microseconds.
>
> Does anyone know how reflections could lead (arrive before) the main
> signal?
> Seems strange that the main signal (strongest signal) wouldn't be the most
> direct path and therefore have the shortest propagation from the
> transmitter.

Sometimes the straight path signal is not the strongest and what appears to
be the main signal is actually just the best of the reflections.

 

[Guest]

Archived from groups: alt.tv.tech.hdtv (More info?)

"Maxwell" <nospam@nospammmmm.com> wrote in message
news:fqKRd.1536$DC6.53@newssvr14.news.prodigy.com...
> DTV has been designed to operate in the presence of signal reflections up
to
> certain amplitudes and delays relative to the main signal. I have seen
> reports on the Web about indoor DTV tests where reflections were measured
> both to lead and lag the main signal by up to tens of microseconds.
>
> Does anyone know how reflections could lead (arrive before) the main
signal?
> Seems strange that the main signal (strongest signal) wouldn't be the most
> direct path and therefore have the shortest propagation from the
> transmitter.
>
> Maxwell
>

One example is Cable TV where the strongest signal is behind the direct
signal from over the air. Another is a poor signal area (in a valley be it
natural or man made) where the best (signal strength) signal is a reflection
off a building.

Your logic is correct but when thinking about signal, you are looking for
best signal not direct in some cases.