SR/Live Sun, November 23, 2008

Sound Reinforcement/Live Sound | Features |

Receivers - Understanding receiver design, processing, squelch and diversity

Summary

  • Understanding receiver design, processing, squelch and diversity
    By Joe Ciaudelli & Volker Schmitt

    This article is part of a series of wireless educational pieces from Sennheiser US.

Figure 1- Block diagram of a receiver.

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 speaker 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 (See 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. This prevents a blast of white noise through the PA if the RF signal is completely lost. 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.


Volker Schmitt is a Senior Engineer for Sennheiser US. Joe Ciaudelli also works for Sennheiser US and has a history of providing frequency coordination for large multi-channel wireless microphone systems used on Broadway and by broadcast networks.


Figure 2 - Multi path signal propagation.

Figure 3 - Two RF signals are out of phase and result in a smaller RF signal at the antenna.