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Review
In the last issue we went through
some of the problems that wireless microphones can experience. now
here are some solutions.
Solutions
- 1. Use a high gain receiving
antenna at the mix position: High gain antennas can improve the
signal-to-noise ratio, and may thus reduce fades and dropouts if they
are due to weak. Signal cancellations will not be aided. high gain
receiving antennas are generally also a bad idea because: (a) the
transmitter is constantly moving around with the performer so the
antenna would have to be continuously re-aimed, and (b) much of the
received radio signal is actually caught on the bounce from walls,
props, etc., so even if one stood offstage and aimed a beam antenna
at the performer, it could be aiming at the wrong target.
2. Place the receiving antenna(s)
and receivers near the mic(s) and run audio signals back to the mix
position: With wireless mics, an alternative is to place the
receiving antenna(s) on or above the stage, run a moderate length of
antenna cable to a nearby wireless mic receiver, and then run a
standard audio cable between the receiver's audio output and the
mixer's input. Most receivers provide line level outputs that are
ideal in this situation. This keeps the mic transmitting antenna(s)
and the receiving antenna(s) reasonably close, which optimizes the RF
S/N ratio.
3. Diversity reception: In some
wireless microphone installations, it may be impossible to locate a
single antenna to eliminate multipath dropout or signal fading. The
technique that has been adapted for wireless microphones to minimize
multipath dropouts is called diversity reception. This is the
application of two or more receiving antennas to receive signals that
have been diverted into more than one path (multipath). The idea, in
general, is that if the signal is weak at one antenna, it will
probably be stronger at the other, at any given instant. Diversity
reception enhances the performance of a wireless mic system. It is
usually effective, although nothing will guarantee a total absence of
dead spots. There are a number of different ways to accomplish
diversity reception, and each manufacturer of wireless microphones
tends to favor one approach or another. The conditions required to
achieve this reception are:
-
A single transmitter source
- Uncorrelated, statistically
independent signals
- Multiple receiving antenna systems.
This success of any diversity
reception system depends on the degree to which the independently
received signals are uncorrelated. If a diversity reception system
cannot produce uncorrelated, statistically independent signals, then
diversity reception does not exist.
Implementation of a diversity
reception system can be accomplished in several ways, but all system
implementations have the need to combine the received, independent
signals in some method. The major drawback with any multiple
reception diversity system is cost. Combining techniques are chosen
based on cost and the degree of improvement required. The less
predictable or less closely related the signals, the more significant
the benefits of the diversity system.
There are various techniques of
diversity reception based on the exact method for processing and
extracting the transmitted signals. Space diversity is the technique
most commonly used for wireless mics. Space diversity can be
implemented in many different ways, but the three basic requirements
of diversity reception mentioned earlier must be satisfied. Two or
more receiving antennas are required and must be at least one half
wavelength apart (typically three feet). The amount of separation
determines the degree of the uncorrelated signals. Polarization
diversity is a method of space diversity in which the antennas on the
receiving system are placed at angles to each other in order to
capture the uncorrelated, independent signal. Each antenna provides
an independent path that is selected or combined to produce the
desired signal improvement. These selecting and combining methods of
processing the independent signals are shown below:
In space diversity the incoming
signal with the best signal-to-noise ratio is selected from the two
or more antennas used. The signal selection can be accomplished
either prior to or after audio detection.
Another method of signal
improvement is that of combining the incoming independent signals.
The two methods of doing this are called maximal ratio combining and
equal gain combining. In maximal ratio combining, independent signals
are combined in order to derive the maximum signal voltage/noise
power ratios from each of them. A modification of this approach is
equal gain combining in which all incoming signals are set to an
average constant value.
Clearly, the maximal ratio
combining method offers the best possibility for improvement over a
non-diversity system, although it is the most difficult to implement.
Wireless mics typically use selection or equal gain combining
diversity. The choice is based on greatest reduction of the
probability of dropouts. Any of the selection/combining techniques
can be implemented in the pre-detection or post-detection stage of
the receiver. |
