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Diversity reception of video

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#1 Wolf Seeberg

Wolf Seeberg

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Posted 23 January 2008 - 12:41 AM


Here's an explanation from Coherent Communication manual:

How Diversity works: Imagine dropping a pebble into a still pond. If
the pebble is far enough away from the shore, the ripples continue
away from the point of impact undisturbed. But what happens if we
drop the same pebble near the shore? On the side facing away from the
shore, the wavelets will be as before, undisturbed. If we look at the
side facing the shore, what do we see?

If the shore is rocky, we may see that the waves are reflected back
from the rocks into the incoming waves. If we look closely, we will
see that the reflected waves combine with these incoming waves.
Depending on their position relative to each other, they may combine
and add to make a larger wave, or if the top of one wave and the
bottom of another meet, they will even out leaving no sign of a wave
at all.

This is what happens to radio waves as well. We have few reception
problems when the Transmitter and Receiver are situated over flat land
or water and the radio waves are undisturbed.

But in an urban environment, the radio waves are reflected from
buildings, cars, light posts and metal objects in general, and may be
absorbed by trees, shrubs and people. Inside a building, high
frequency radio waves will be reflected by filing cabinets, metal
studs in walls, lighting fixtures, metal desks, etc.

This is why when we try to use a cellular phone inside a building we
may experience good reception only a foot or two from a problem area
as the waves may combine and cancel in one area and add in another.
Now if we get tricky, we can place three antennas, several radio
wavelengths apart, and look at the signal coming from each one.
Statistical studies show that almost 100% of the time, one of the
three signals will be good as long as they are within range.

But we can't just combine the three signals and hope for the best,
because they will have a random relationship with each other and we
will have waves adding and canceling, just as when the wake from two
passing boats cross. This is called antenna diversity, and it just
doesn't work. Instead, our diversity receivers look at the output of
all three antennas and very quickly sample the signal strength of each
one and then look at the quality of the Video as well. An internal
microcomputer decides which antenna has the best signal, and switches
to it, giving you the best possible picture and sound. This happens so
quickly that you will not be aware of the bad image from the bad
antenna. The receiver switches 60 times per second while there is no
picture being written to the monitor. Often there are lights on the
front panel that show which antenna has been selected.

What can I do to improve reception when my receiver antennas have to
be close together?

Larry Fisher says:
You can tilt the antennas so that they are at 90 degree angles to one
another. That is to say, bend one 45 degrees to the left and the other
45 degrees to the right. The tilted antennas are a reasonable way to
operate and the best way if the antennas are fairly close together
since they couple together much less than if they are both pointed in
the same direction (parallel).

The antenna diversity used in our receivers does not select one
antenna or the other; it sums the two antennas together and corrects
the phase of one antenna so that the antenna signals do not cancel
each other out as they might do if they were 180 degrees out of phase.
So it does not make too much difference which way the antennas point
since the receiver will correct the phase.

Additionally, in any usual environment, the signals coming to the
receiver from the transmitter are not in any well defined phase
relationship or direction. The signals are reflected from cars, the
ground, metal studs, wire in walls, camera equipment and even people,
so that the signal that gets to the receiver is pretty well scrambled
and impossible to predict. The problem with reception occurs when all
the signals from all the reflectors get to the antenna and cancel out.
If you use two antennas, then the signals probably will not cancel out
at both antennas simultaneously. There is a new problem, though, if
you simply add the two signals together. When the signals at each
antenna are equal and exactly out of phase they cancel out at the
receiver. The phase diversity system that we use on our small
receivers detects this condition and simply inverts the phase of one
of the antennas. Now the antennas add the signals together for a 3 dB
pickup in power. For a good explanation of this, that is more
comprehensive than what I can do here, go to this link to our web site.
Dropouts and Noise-ups 1 http://www.lectroson...ice/dropout.htm
It is part of our wireless guide. In fact you might want to down load
the entire wireless guide because it is pretty good and pretty neutral
in its treatment of wireless microphones (and everything that applies
to wireless microphones applies to UHF video). [notes in brackets like
this by the editor]

Wireless Drop-outs and Noise-ups [and flickers]
You have everything setup and connected. Your frequency coordination
has been done and you're performing a sound [video] check from the
stage. As the talent [camera] moves across the stage with the wireless
mic [video transmitter] in operation you hear a "phffft" or maybe a
"swisshhh" [or see a flicker] from the wireless system. You have a
drop-out problem. No, not the high school variety, but rather a type
of RF signal degradation that causes the desired signal to drop way
down in strength to the point where the noise floor rises up and is
heard [ or the video goes away comes back and takes a few frames to
sync up again].

A wireless transmitter sends out a radio signal in all directions.
This signal will often bounce off nearby walls, ceilings, etc. and a
strong reflection can arrive at the receiver antenna along with the
direct signal. If the direct and reflected signals are out of phase
with each other at the receiver antenna, a cancellation may occur,
resulting in a drop-out [a flicker]. A drop-out sounds like either
audible noise (hiss, swish, pop, etc [looks like a loss of video
sync]), or in severe cases, may result in a complete loss of the RF
carrier and the sound [video] when the transmitter is positioned in
certain locations in the room.
A VHF drop-out normally sounds like hiss or a swishing sound. UHF
dropouts are more brief due to the shorter wavelength, sometimes
sounding like a click or pop [ in video the pix goes away for a
shorter time with UHF]. Moving the transmitter even a few inches will
usually change the sound of the drop out, or even eliminate it [video
will reappear suddenly]. A drop-out situation can become either better
or worse as a crowd fills and/or leaves the room, or when the
transmitter or receiver is operated in a different location.

In non-diversity systems (one antenna) the best way to get rid of
dropouts is to reposition the receiver antenna so that it is as close
as possible to the transmitter. Be sure the transmitter antenna has a
line of site shot to the receiver antenna. Many times, simply moving
the receiver or transmitter antenna a few inches [up or down] or even
three to four feet will cure the problem.

Diversity systems (two antennas) were developed to attack this
particular problem. If one antenna is receiving two signals which a re
out of phase with each other causing a dropout, chances are good that
the other antenna on the diversity receiver is receiving a good solid
signal. There are several different types of diversity receiver
designs used by various manufacturers. Some of them switch antenna
phase, others alternate between two different receivers, and others
use non-switching, panning circuitry to blend the audio [video]
outputs of two receivers. Circuits in the receiver will automatically
decide which antenna is receiving the best signal and either switch to
that antenna, or pan toward the stronger signal. If conditions cause
the multi-path problem to move to the other antenna, the receiver will
automatically switch over maintaining a good audio signal at its output.
For more information on Diversity systems and dropouts see the
Diversity Reception http://www.lectroson...m/wg/wg2000.htm
section of our Wireless Guide.

Even diversity receivers can be susceptible to drop-outs. Be sure to
position the antennas at least three or four feet apart and so that
they are not within 3 or 4 feet of large metal surfaces. If this is
not possible, try to position the antennas so that they are as far
away from metal surfaces as is practical. It is also good to position
the receiver so that there is a direct line of sight between the
transmitter and the receiver antenna. In situations where the
operating range is less than about 100 feet, the antenna positioning
is usually less critical, since the overall RF signal level is
generally higher. The antennas can also be configured with one whip
mounted directly onto the receiver, and the other one mounted remotely
[best use 2 or more flat panel antennas]. Lectrosonics builds VHF and
UHF receivers with a sophisticated diversity design ???

In the event that you do encounter a drop-out problem, first try
moving one antenna to a new location at least 3 or 4 feet from where
it was. This may alleviate the drop-out problem on that antenna. If
drop-outs are still a problem, try moving the antenna to an entirely
different location in the room or moving one or both antennas closer
to the transmitter location. By observing the OPTI-BLEND LEDs [only
the DX400 has signal strength LEDs] on the front panel of a
Lectrosonics ratio diversity receiver, you can determine which antenna
is suffering weak signals.

The above Paragraphs are written by Larry Fisher from Lectrosonics, a
RF genius and a nice guy. larryf@...

for best antenna use our flat panels:

and for more knowledge read:

enough for now wolf
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