[NLRS] Is there published data on tree foliage ...

Fri Jul 11 13:53:33 EDT 2014

If you have access to a spectrum analyzer, you can easily see the effects of multipath on a DTV signal.  The ideal display of the DTV channel is that of a "haystack", with sharp edges and a flattened top.  If severe multipath is present, the top of the haystack will have "divots" or gouges in it, the result of signal cancellation from the mixing of the desired signal and the reflections.  The equalizers in a DTV receiver can compensate for some fairly complex multipath, where the offending reflections are a much smaller amplitude than the desired signal, but if the desired signal and the reflection are fairly close in amplitude, the equalizer has trouble figuring out which signal is the desired one, and thus delivers nothing to the decoder section of the receiver.  The forward error correction that is built in to the DTV signal is pretty robust, as well, and helps keep the recovered picture intact during minor glitches in the recovered bitstream.

All in all, the ATSC signal is fairly robust, unless you are using a receiver that is physically in motion, are in an urban environment with lots of strong reflections present, or at the extreme edge of the station coverage area, where minor signal level variations result in a recovered bitstream that is on the hairy edge of the error correction.

73,

Phil   KF9US

>________________________________
> From: Chris Elmquist <chrise at pobox.com>
>To: Phil Hejtmanek <p_hejtmanek at yahoo.com> 
>Cc: "nlrs at mailman.qth.net" <nlrs at mailman.qth.net> 
>Sent: Friday, July 11, 2014 9:36 AM
>Subject: Re: [NLRS] Is there published data on tree foliage ...
> 
>
>
>
>
>How would a guy "visualize" multi-path on an ATSC signal?  It seems to
>me like you could only see it after detection right?   There's no way
>to see it in the RF domain with a spectrum analyzer for example.
>
>I'm dreaming of some kind of monitor scope that would show two spikes
>moving around closer and farther apart as the path delay changes--
>as the wind blows, as the leaves get wet, etc.
>
>Chris N0JCF
>
>On Friday (07/11/2014 at 07:01AM -0700), Phil Hejtmanek via NLRS wrote:
>> 
>> 
>> 
>> 
>> ATSC digital TV has a heck of a time with multipath, and today's receivers have a much more sophisticated active equalizer than the first designs, enabling the receiver to find the best of the signals to decode.  ATSC was never really designed with mobile receivers in mind, and there has been an ongoing effort in the broadcast community to evolve to another transmission standard which better addresses the multipath problem, like some variation on OFDM.
>> 
>> One of the big sources of confusion among consumers is the "S meter" that Jon mentioned.  It typically is not a signal strength measurement, but rather an indication of BER (bit error rate) of the station.  It is possible to have a more-than-adequate signal level present at the antenna terminal of the receiver and yet show a zero signal level, simply because the receiver is seeing multiple, time-displaced copies of the same signal (multipath), and can't discriminate between them.
>> 
>> ATSC also reacts badly to burst noise, causing the receiver to lose digital lock.  That's why sometimes your level indication can show almost maximum signal level, and then jump to zero, and then come back to normal in a few seconds.  
>> 
>> 
>> I can't tell you how many hours I have spent trying to explain to viewers why their TVs show "low signal" when they are close to the transmitter site.
>> 
>> Phil  KF9US
>> 
>> 
>> >________________________________
>> > From: Jon Platt via NLRS <nlrs at mailman.qth.net>
>> >To: nlrs at mailman.qth.net 
>> >Sent: Friday, July 11, 2014 8:09 AM
>> >Subject: Re: [NLRS] Is there published data on tree foliage ...
>> > 
>> >
>> >
>> >
>> >
>> >
>> >
>> >I just don't understand why I can't get enough signal sometimes. 
>> >
>> >
>> >I suspect multi-path.  I don't have a deep understanding of the new digital TV format but I suspect that the TV receiver might be having a hard time determining which time varied signal to sync with?   Although I live farther from the Shoreview I see it too.  
>> >
>> >Newer TV sets have a build in signal monitor (S-meter) that you can find in the TV's menu structure and when I watch mine I have a signal level of 97/100 or 98/100 and the meter is in the "green", then suddenly the picture drops and the signal reading goes to 0/100 before the TV "relocks" and the picture appears again.
>> >
>> >I think the new digital TV format is designed to be sent over a cable and not propagated via the ether?  Maybe a sharper antenna would help .... it may depend on where the strongest multipath reflectors are coming from?
>> >
>> >73, Jon
>> >W0ZQ  
>> >
>> >
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>-- 
>Chris Elmquist
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