Diversity reception of video

[Wolf Seeberg]

THE ADVANTAGES OF DIVERSITY RECEPTION

 

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.lectrosonics.com/service/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 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].

 

WHAT CAUSES DROP-OUTS [flickers]?

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 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.

 

 

WHAT CAN I DO TO GET RID OF DROP-OUTS?

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

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.lectrosonics.com/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:

http://wolfvid.com/datasheets/Antenna_UHF_flat_panel.pdf

 

and for more knowledge read:

http://wolfvid.com/datasheets/!OPERATO...OOK_vidasst.pdf

 

enough for now wolf