[opendtv] Re: Diversity USB DVB-T stick
- From: Richard Hollandsworth <holl_ands@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sun, 25 Feb 2007 14:54:43 -0800 (PST)
Here's a (pricey) MRC for ATSC, intended for use in studio-to-tower up-links:
http://www.mrcbroadcast.com/products/diversity.htm
=================================
A cheap and dirty Switching Diversity Combiner simply picks one of the
antenna inputs and if it doesn't like it (high BER, low SNR, whatever),
it picks a different input--repeat ad nauseum....
(e.g. the SANYO FM combiner mentioned below).
An Equal Gain Combiner simply sums multiple antenna inputs and hopes
for the best....such as is used in Stacked Antennas....
Ideally with a Stripline Coupler (e.g. Lindsay) or poorly with Hybrid RF
Combiner (RF Splitter in reverse).
Hybrids are esp. problematic, cuz any phase/gain mismatch between
the two inputs is dissipated in the internal "balance" resistor.
A Maximal Ratio Combiner (MRC) sums the inputs from multiple antennas,
except that it varies the relative gain ("weights") each input based on an
estimate of relative SNR. Hence a weak & noisy input is eliminated.
If Adaptive Equalizers are employed, it is also possible to align
the phases so that they can be COHERENTLY combined--thereby
enabling a 3 dB processing gain under most conditions, rather than
relying on perfect matching for the antenna characteristics.....
which will almost never happen in an indoor or mobile environment.
===================================
You can find more info by searching for "Adaptive Antennas".
This MIMO article describes MRC vice alternative combiner techniques:
http://www.wirelessnetdesignline.com/showArticle.jhtml?articleID=52600922
This article describes alternative diversity techniques (e.g. frequency, time,
space):
http://www.comlab.hut.fi/opetus/333/2004_2005_slides/Diversity_text.pdf
But it doesn't really explain where the weighting factors come from--
cuz that's where the proprietary algorithms begin to work their magic....or
not....
==================================
The fol. also includes a simplified description of alternative antenna diversity
combining schemes, lists Mobile DVB-T refs using Diversity Antennas
and lists various ATSC Diversity Antenna Combiner/Receiver
tests that never made it into production (thus far):
http://hdtv.forsandiego.com/messages/460/5569.html#POST27838
Note availability of Diversity Combining chips for FM radio, including MRC
and Sanyo Switching Diversity Combiner that works
with a single tuner receiver.
I was grumbling re not finding replacement car radios with FM Dual Diversity
any more....and apparently no Dual Diversity HD Radio either....
FYI: Samsung rep said they use Maximal Ratio Combiner (MRC)
in their Dual Diversity A-VSB Prototype:
http://www.avsforum.com/avs-vb/showthread.php?p=7647247&highlight=maximal#post7647247
http://www.tvtechnology.com/pages/s.0001/t.1097.html
http://www.tvtechnology.com/pages/s.0115/t.715.html
http://www.twice.com/article/CA6404334.html?nid=2402
Anyone see any detailed Buffalo or CRC test results (incl w and w/o
diversity)????
=============================
This is earlier discussion re Diversity (some DVB-T refs are dupes):
http://www.avsforum.com/avs-vb/showthread.php?p=5842783&highlight=dibcom#post5842783
If you read up a couple posts and down several more, you'll see my
experience with an ATSC "Smart Antenna" (doh!!! did it again!!!)
And indeed the DTA-5000 also isn't very smart cuz it doesn't used MRC
and doesn't support attachment of external antennas.
holl_ands
======================================
Tom Barry <trbarry@xxxxxxxxxxx> wrote: How does a maximal ratio diversity
combiner work? Is it something a
consumer can pick up at Radio Shack? Would I want to?
- Tom
Bob Miller wrote:
> On 2/25/07, Richard Hollandsworth wrote:
>
>> Why would anyone be "annoyed" by the "need" to handle two antennas????
>>
>> First of all, a maximal ratio diversity combiner, will automatically
>> "zero-out" the noise contribution from an unused antenna input
>> if the user only wanted to "bother" with one antenna.
>>
>> Secondly, if both antennas are used, up to 3 dB of increased sensitivity
>> (in a uniform signal field) can be realized (as alluded to in the specs).
>>
>> Thirdly, reception (esp. indoors) has sweet spots is plagued with way
>> too many bad spots---the location of which will vary for different
>> channels.
>> A dual (space) diversity antenna system significantly improves the
>> chance of finding a sweet spot for a particular channel.
>>
>> With the two included short whip antennas, the "optimum spacing"
>> may be as far apart as the short cords permit....but it's gonna depend
>> on where those sweet spots are....
>>
>> BTW: First thing to do is replace those whip antennas with a pair of
>> real indoor antennas (e.g. Silver Sensor). Although in some locations,
>> a pair of monster Yagis may need to be mounted outdoors....
>>
>> Although stacked antennas always seem to be the first thought---
>> here in the U.S. far more people would be ecstatic to be able
>> to point each antenna in a DIFFERENT direction.
>>
>> Drat!!! Can only dream of Diversity Antenna for ATSC....must be "too
>> hard".....
>>
>> holl_ands
>> [One of the rare owners of an actual ATSC "Smart Antenna".]
>>
> It is painful to see ATSC and Smart so close together.
>
> Bob Miller
>
>
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--
Tom Barry trbarry@xxxxxxxxxxx
Find my resume and video filters at www.trbarry.com
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