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Wireless Problems


Transmission Loss

There is a calculated transmission loss between transmitter and receiver through use of an isotropic antenna. Less transmitter power is required for an equivalent signal strength at the receiver as frequency is lowered. One problem with wireless microphones is the difficulty in designing antennas that are small but efficient in the VHF low-band area. However, for the VHF high-band, small and efficient antennas are practical.

Interference from other radio services is the major problem at both VHF and UHF. The only clear channels available are the unused TV channels in a given location and the "B" channels. For touring groups the TV channels become a problem, as a clear TV channel in one city may not be clear in another. Therefore, the "B" channels are recommended for this purpose. [The "B" channels are specific frequency bands designated by the FCC for wireless mics.]

Dropout

Loss of reception at the receiver of a wireless mic due to radio wave cancellation called multipath reflection is usually referred to as dropout. This problem has several possible sources. Dropout characteristics are different between VHF and UHF frequency bands. The dropout zones are much shorter at UHF where rapid flutter is often heard.

Loss of reception may also be caused by a transmitter being too far away from the receiver. This may be corrected by relocating either the transmitter and receiver antennas closer to each other.

The power of a signal received by an antenna is a critical factor in causing dropout. When examining practical solutions and limitations in alleviating dropout, it is important to consider that not all of the power transmitted will reach the receiver. A wireless mic transmitter radiates power in many directions simultaneously, depending on the specific mechanical configuration of the antenna system. This makes the transmission vulnerable to many types of interference.

System performance is degraded by path losses due to interfering objects between the transmitter and receiver, such as other equipment or people, as well as by the position of the transmitter antenna and interfering signals due to multipath reflections.

Several paths can occur when the environment in which the wireless microphone is operating contains objects such as cameras, lighting equipment, or stage props made of metal or other materials that reflect radio signals. Due to phase differential of the arriving signal, the resultant signal can be enhanced or totally cancelled, causing multipath dropouts. These path losses affect the total power received at the antenna. Multipath cancellation is the most common cause of dropout.

Solutions

*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.

*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.

*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.

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