7. Key Terms
On this page you can find frequently asked Key Terms:
Very High Frequency - covers the radio frequency spectrum from 30M Hz to 300 MHz and is used mostly for TV broadcasting and FM radio.
Ultra High Frequency - covers the radio frequency spectrum from 300 Mhz to 3 GHz and is used mostly for TV and mobile phone services.
All radio systems are susceptible to interference, and in extreme cases the signal can cut out altogether. To avoid the likelihood of this, diversity receivers use two aerials spaced a distance apart. The signal received on each aerial is compared by the receiver, and the system automatically uses whichever is the stronger. As its unlikely that both aerials will ever experience drop out simultaneously, the resultant signal is very stable. (Diversity receivers do not require diversity transmitters the aerials are merely picking up two different versions of the same signal due to their physical separation in space.) Systems with only one aerial are known as non diversity - these should only be used when your budget is tight and the operating range of the system will be short.
This refers to problems of interaction with other electronic equipment. There are three common types of interference - electrical interference, radio frequency interference or RFI, and intermodulation (see below). Electrical interference can be caused by any electrical device, for instance computers, lighting and domestic products such as hi-fis and white goods. RFI is caused by radio, TV and other equipment that generates radio frequency energy in its normal operation. Well designed radio mic systems will avoid all types interference, but the cheapest may not!
This is a type of distortion where two or more radio frequencies interact, resulting in new frequencies not present in the original signal. This can happen when a receiver picks up two dissimilar frequencies that interact with its own electronics to produce sum and difference frequencies, manifested in a whistling noise. Its often also called Third Order Intermodulation due to the third order harmonic that is created - this can occur when two or more wireless systems are used together as they interact to create additional frequencies. The more wireless systems you use simultaneously, the more intermodulation problems are likely to occur. Some transmitters have frequency coordination software that attempts to eliminate the harmonics, and many manufacturers provide tables of frequencies that can be used safely together without intermodulation.
This is a form of circuitry not unlike a noise gate that is often built into wireless receivers. It mutes the audio output when no RF signal is present, or only a very low one - this cuts out any extraneous white noise that would otherwise be heard when insufficient signal is received from the microphone, or when the mic is switched off. Most receivers have an adjustable squelch threshold.