The receiver is a crucial component of wireless microphone systems, as it is used to pick the desired signal and transfer its electrical information into an audio signal that can be amplified and used for a loudspeaker system.
Understanding basic receiver design, audio processing, squelch, and diversity operation can help ensure optimum performance of the system.
Virtually all modern receivers feature superheterodyne architecture, in which the desired carrier is filtered out from the multitude of signals picked up by the antenna, then amplified and mixed with a local oscillator frequency to generate the difference: “intermediate frequency.”
This “IF” undergoes more controlled discrimination and amplification before the signal is demodulated and processed to restore the output with all the characteristics and qualities of the original (Figure 1).
The audio signal processing of a receiver is the mirror opposite of the transmitter. Processing done in the transmitters often include pre-emphasis (boosting high audio frequencies) as well as compression. These are reversed in the receiver by the de-emphasis and the expander circuit.
An inherent RF noise floor exists in the air. The squelch setting should be set above this noise level. This acts as a noise gate that mutes the audio output if the wanted RF signal falls below a threshold level, preventing a blast of white noise through the FA if the RF signal is completely lost.
Figure 1: Block diagram of a receiver. (click to enlarge)
If the squelch setting is too low, the receiver might pick the noise floor and this noise can be heard. If the squelch setting is too high the range of the wireless microphone is reduced.
The Pilot Tone Squelch gives additional security, as the audio signal at the receiver is muted as long as there is no Pilot Tone. The Pilot Tone is a very high frequency which the human ear cannot detect (very often 32 kHz). If the receiver detects a Pilot Tone, the audio signal is un-muted.
WHY DIVERSITY IS A MEANINGFUL APPROACH
The Radio Frequency (RF) signal between transmitter and receiver follows multiple paths. Signals reflected from boundaries and objects will arrive slightly later at the receiving point than the direct signal (Figure 2).
Figure 2: Multi path signal propagation. (click to enlarge)
Because of the different amount of time the RF signal needs to travel from the transmitter directly to the receiver antenna and the reflected signal from a wall, both signals have a different phase at the antenna.
Both signals the reflected signal and the direct signal - are added at the antenna.
In the worst case scenario, the two signals are out of phase and the result is that no RF signal will arrive at the antenna (Figure 3).
This extreme example will cause a RF drop out which will cause an audio disturbance at the output of the receiver. The probability that this will happen at two different positions at two different antennas is very small.
Figure 3: Two RF signals are out of phase and result in a smaller RF signal at the antenna. (click to enlarge)
The two graphs illustrate how the signal strength from the same transmitter varies over time at two different pickup locations. If both signals are compared at any time and the stronger RF signal is picked, the sound quality is improved significantly.
There are different kinds of diversity concepts available. Antenna Switching Diversity uses two antennas and a single receiving circuit. If the level at one antenna falls below a certain threshold it switches to the other antenna.
This is an economical architecture but it leaves the chance that the second antenna could be experiencing an even lower signal then the one that falls below the threshold level.
Another approach is the switching of the audio signal of two independent receiver units where each receiver unit is connected to its own antenna. This is known as true diversity, and it can improve the effective RF receiving level by about 30 dB. Depending on the diversity concept, an active switching between the two antennas is a desired result.
The minimum distance between the two diversity antennas is very often an issue of debate. A minimum of 1/4 of a wavelength of the frequency wave seems to be a good approach.
Depending on the frequency, five to six inches is the minimum distance. In general, a greater distance is preferred. By using remote antennas, the positions at the sides of the stage are very effective. (Go here for more about antennas.)
Volker Schmitt is a senior engineer for Sennheiser US, and Joe Ciaudelli also works with Sennheiser US and has a history of providing frequency coordination for large multi-channel wireless microphone systems used on Broadway and by broadcast networks.