7. A short radio glossary

The frequency band between 30 MHz and 300 MHz with wavelengths of about 10 to 1 metres. Most of you will be familiar with FM radio stations, most of which broadcast in this range, between 87.5 MHz and 108 MHz. The VHF range relevant for radio mics is between 174 and 230 MHz.

Also known as the decimetre band, this is the frequence range between 300 MHz and 3,000 MHz (3 GHz) with wavelengths of between 10 and 1 decimetres (1 m to 10 cm). When we speak of UHF, we usually mean the range relevant to us, i.e. between 470 and 865 MHz. Strictly speaking, however, the 1.8 GHz, 1.9 GHz and 2.4 GHz ranges are also part of the UHF band.

Secured reception using two antennae. Reduces the risk of signal dropouts caused by intereferences weakening the signal. The two main types are Antenna Diversity, where reception switches between two antennae and the superior True Diversity, which involves two separate receivers. The latter does not simply switch between the antennae whenever the signal grows weaker but first compares whether the other antennae even has better reception. In practice, however, the difference is not very significant, especially compared to Non Diversity receivers.

The opposite of true diversity - the receiver features a single reception unit only.

Amplification or attenuation through overlap of various signals in varying phases. Caused by reflections and detours within a room, it can lead to cancellation of the signal. This danger is countered by using diversity receivers, whose two antennae offer greater reception security, or by using directional mics to eliminate interference intruding from the sides.

An effect arising from using two or more transmitters. So-called intermodulation products are generated, which appear as interference in addition to the transmitters' carrier signals in the frequency band. The 3rd-order intermodulation products arising in immediate proximity to the chosen carrier frequencies are particularly disruptive. Thus, carrier frequencies should be chosen to eliminate collision with intermodulation products. Manufacturers offer frequency banks calculated for this purpose and listing frequencies compatible with each other.

An output suppressor. It is located on the receiver of analogue radio systems and can usually be adjusted using rotary pots or the receiver menu controls. The squelch has the task of opening the signal path if a sufficiently strong signal is received, and to close the path whenever the signal falls below a certain threshold value, for example if the transmitter moves too far away from the receiver. If properly set, the output suppressor is activated the instant the transmitter signal becomes weaker than any existing interference signal. A squelch set too low may admit interference signals as audible white noise, while a squelch set too high goes to mute unnecessarily early.

re-converts the high-frequency radio signal into a low-frequency signal as faithfully as possible.

Modulates the vibrations generated by the mic capsule into a high-frequency signal which can be received by a receiver. There are handheld transmitters, which contain the mic capsule and the transmitter in a single unit, and body packs, which are connected to the mic via cable and placed on the body or in a pocket as discreetly as possible.

Portmanteau based on the words compressor and expander. Dynamics processing in transmitter and receiver which is necessary in analogue radio mics due to the limited frequency swing.Without compander, the dynamics transmitted would not be sufficient for application in music. However, compander processing may lead to loss in sound quality; the compander's quality is decisive. Digital radio systems can do without this pressing step, which is one of their advantages.

Additional radio signal to send information between transmitter and receiver. It may serve, for example, to convey the mute or battery status, but also identify the associated transmitter. Whenever this is out of range or switched off, the receiver can automatically switch to mute and thus won't even be tempted to any unwanted signals not intended for it to the speakers

Also known as the switching bandwidth, this is the range between two carrier frequencies in MHz or GHz within which a radio system cand transmit and receive. Manufacturers often offer their systems in various frequency bands, e.g. 518-542 MHz or 823-832 MHz. This offers the user greater flexibility and is also informed by the legal regulations, depending on who is authorised to use frequences in what location and to what purpose. Furthermore, there are technical reasons for radio systems' inability to work on all frequency from the lowest VHF to the highest GHZ frequencies. Thus, a radio system with swwitchable frequencies will always only work within a limited frequency band. While manufacturers indicate the band their products can operate in by using wildly imaginative codes in their product names, it all boils down to the MHz number.

List of frequency in a grid showing compatibility. The manufacturer sugggests one or more frequency banks to the user within which they can operate several transmitters and receivers simultaneously without interference. The frequencies listed in the grid are calculated in a manner to avoid interefering intermodulation products as much as possible. If several groups are available, using systems from different manufacturers simultaneously may be easier.

Sum of the variation in frequency caused by a carrier signal's frequency modulation. The frequency swing is limited to 100 kHz (± 50 kHz) at most, and it varies according to the amplitude of the modulation signal. Lower-volume audio signals generate a narrower frequency swing than higher-volume ones. In frequency modulation, the frequency of the modulation signal determines how often the frequency swing is traversed. For example, with a 1 kHz modulation frequency the frequency swing is traversed 1,000 times per second.

The only principle applied in analogue radio mics to modulate the low-frequency audio signal onto a high-frequency carrier signal. In frequency modulation, the modulation signal alters the carrier signal's frequency. The amount of the alteration is referred to as the frequency swing. In digital radio systems, Frequency Shift Keying (FSK) works along similar lines. Besides changing the frequency, however, some digital radio systems also employ modulation processes which change the carrier system's amplitude (ASK = Amplitude Shift Keying) or phase (PSK = Phase Shift Keying).

"Channel" refers to the set carrier frequency in MHz or GHz. Some entry-level radio mics feature only a single frequency (fixed frequency), while most feature switchable channels. In more affordable systems, often only one channel number is shown, while more sophisticated systems also show the corresponding carrier frequency in their display. Higher-quality radio systems are also organised by grupos and channels in a grid calcualted in such a way to allow interference-free multi-channel operation.

The available VHF and UHF range is divided into so-called TV channel frequencies, which are frequency blocks 7 Mhz wide in the VHF range and 8 MHz wide in UHF. Historically, they originate in the time of analogue terrestrial television. They used to carry the sound an image information for one TV channel each. Terrestrial television of this analogue type no longer exists, but the division into these blocks remains. In VHF, we use TV channel frequencioes 5-12, and TV channel frequencies 21-69 in UHF. While radio mics generally comprise several TV channel frequencies, the 7 or 8 MHz grid usually still holds.