2.4 GHz - a band of frequencies clustered around 2.4 GHz has been designated, along with a handful of others, as the Industrial, Scientific and Medical radio bands (ISM). This band allows users to operate with no license requirements. Manufacturers of wireless communication devices (intercoms) have been using 2.4 GHz band for transmitting and receiving voice communications. In the US, the 2.4 GHz ISM band is 2400 - 2483.5 MHz.

Antennas - generally deal in the transmission and reception of radio waves and are a necessary part of all radio equipment. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless LAN, cell phones, radar and spacecraft communication. A common antenna is a vertical rod a quarter of a wavelength long. Such antennas are simple in construction, usually inexpensive and both radiate in and receive from all horizontal directions (Omni-Directional). In air, radio signals travel very quickly and with a very low transmission loss. The signals are absorbed when moving through more conductive materials, such as concrete walls or rock. When encountering a surface, the waves are partially reflected and partially transmitted through.

Audio Frequency Response - the audio range over which an audio component can effectively produce a useable and fairly uniform, undistorted audio output signal. When referenced to intercom frequency response is most critical and most used in relation to the headphone or earpieces of the headset and the range that the base station is capable of reproducing.

Beltpack - portable headset user station. This station is designed to be worn on a user's belt with the idea of semi-portability. It can be either a single, 2, 4, or 6 intercom channels. It requires a headset or handset.

Call Signaling - This feature is included with the majority of Clear-Com products. It can be an audible and/or visual alert on a user station (a lamp or LED) used to attract the attention of an operator signifying that someone at another station wants to initiate a conversation. The call signaling feature is used for two different purposes: 1) to primarily get a user's attention of incoming communication, 2) to indicate a cue. Typically, a light on means standby, a light off means apply.

Channels - a "channel" is one individual circuit o f communication. An example would be a partyline channel for spotlight operators. It is possible for a user station (beltpack) to select between several channels available in a system with a channel selector on the user station. This allows for multiple conversations or information flows to occur independently as needed. An example would be a remote stage manager with carpenters on Channel A and the rest of the production on Channel B.

Co-Channel Interference - is crosstalk from two different radio transmitters using the same frequency.

Digital Enhanced Cordless Telecommunications (DECT) - a digital communication standard, which is primarily used for creating cordless phone systems. The DECT standard fully specifies a means for a portable unit to access a fixed telecoms network via radio.

DMX512 - a standard for digital communication networks that are commonly used to control stage lighting and effects. Recently, wireless DMX512 adapters have become popular. Such networks typically employ a wireless transmitter at the controller, with strategically placed receivers near the fixtures to convert the wireless signal back to conventional DMX512 wired network signals.

Duplex - refers to bi-directional communications. "Full" duplex describes bi-directional communications all the time. Regular communications between individuals conversing face-to-face is full-duplex. In other words, you can talk and listen simultaneously. Full-duplex communication allows simultaneous two-way conversations - one person can interrupt another. "Half" duplex allows two-way conversations, one way at a time - one person can NOT interrupt another. Half-duplex is where a user can only talk (transmit) or listen (receive) on their device at any moment and for wireless systems only one user can be transmitting on a frequency at any moment.

Four-Wire - A communications system where the path is different for talk and listen. In intercom channels, there are four wires (two paths). Four-wire systems can be four-wire balanced or four-wire unbalanced. Four-wire audio is more of less defined as a pair of conductors carrying an input/receive signal and a second pair carrying the output/send signal. The four-wire circuit gets its name from the fact that a balanced pair of conductors was used in each of two directions for full-duplex operation.

Frequency Hopping Spread Spectrum (FHSS) - a technique that uses a narrow band signal, approximately 1.5MHz wide and changes frequencies (or hops) 200 times a second or every 5ms. using a narrow band signal allows concentrated RF power into a smaller area of the spectrum. This allows systems to "burn through" RF noise and interference that could otherwise be a significant problem. Additionally, changing frequencies very rapidly helps make it less susceptible to single point external interference sources, intermodulation and multipath fading.

Headset - portable intercom connection from a beltpack to one or both ears via headphones with integrated microphone on a boom arm. Typically connects to a beltpack.

Headset Microphone Type - there are two types of microphones: dynamic and electret. Dynamic microphones convert sound pressure waves to electrical signals by means of a coil attached to a diaphragm moving in a magnetic field. Electret microphones use a capacitor as the sound pressure sensing element. Electret microphones are a special case of condenser microphones in that they are permanently polarized and require no special polarizing voltage. Electret microphone outputs are high impedance.

Heating, Ventilating and Air Conditioning (HVAC) - refers to the technology of environmental indoor or automotive comfort.

IFB - and acronym for Interrupted Fold-Back. It is a communication circuit or system that interfaces with the intercom system. It includes hardware listening devices, usually earpieces for the talent. The talent listens to the program all the time and is "interrupted" by the director (typically) with cues or instruction.

ISO - a temporary private discussion amongst two parties. A momentary private conversation with someone who may be talking and listening to a number of other people.

Null - a hybrid's ability to isolate the transmit signal from the receive signal in the 2-wire-to-4-wire interface is critical. The quality of this isolation is technically known as return-loss. A side tone nulling control fine tunes the circuitry to best match the devices to the acoustic conditions near the intercom, as well as to the electronic conditions on the intercom line. They should be set at the time of system installation and adjusted as is comfortable for the user. This hybrid circuit connects the four-wire audio to the single wire in such a way as to variably restrict the user's reception of his own voice on the intercom line, which is often referred to as side tone.

Partyline (PL) - the name PL (partyline) came from the original telephone systems where everyone shared the same line and could hear all conversations at once. It is often called 2-wire or TW. A partyline system allows a group of people to intercommunicate. A partyline is typically used when several users, such as beltpacks or camera intercoms, are engaged in a specific common activity and they need to talk and/or listen to each other all the time.

Personal Communications Service (PCS) - the name for the 1900 MHz radio band used for digital mobile phone services in Canada, Mexico and the United States. The PCS band (1850 - 1900 MHz) is divided into six frequency blocks (A through F). Each block is between 10 MHz and 30 MHz bandwidth.

Power Supply - the source of electrical power (power outlet). In North America, this source is generally 120 volts AC, 60 hertz. In Japan, the source is generally 100 volts, 50 or 60 hertz. In the United Kingdom, the source is 240 volts, 50 hertz. In Europe, the source is generally 220 volts, 50 hertz. In addition, some equipment is operable off of DC sources, such as batteries.

Private Branch Exchange (PBX) - a telephone exchange that serves a particular business or office, as opposed to one that a common carrier or telephone company operates for many businesses or for the general public. PBX's make connections among the internal telephones of a private organization - usually a business - and also connects them to the public switched telephone network (PSTN) via trunk lines.

Program - an audio source that is fed into the intercom channels.

Rack Unit(s) (RU) - a standard unit of measure used in electronic equipment racks. 1RU = 1.75"H x 19"W (44mm H x 483mm W).

Radio Frequency (RF) - rate of oscillation in the range of about 30 KHz to 300 MHz, which corresponds to the frequency of electrical signals normally used to produce and detect radio waves. In order to receive radio signals, an antenna must be used. However, since the antenna will pick up thousands of radio signals at a time, a radio tuner is necessary to tune in to a particular frequency (or frequency range).

Radio Frequency Spectrum - refers to the part of the electromagnetic spectrum corresponding to radio frequencies - that is, frequencies lower than around 300 GHz. Different parts of the radio spectrum are used for different radio transmission technologies and applications.

Rating - in audio electronics, a rating of .05 to 2 amp at 24 volts AC/DC maximum is common. A "phoenix" type connector plug is also common and it plugs into the relay contact port on the rear of the base station for wiring to external devices. A use for a relay is sometimes associated with turning on a light for attention, such as an on-air light.

Relay - an electrically operated switch. Commonly, these relays are normally open (NO) contacts, which mean they connect the circuit when the relay is activated. The circuit is disconnected when the relay is inactive. It is also called a Form A contact of "make" contact. in audio electronics, these relays are of the dry contact type. Dry contact refers to a contact of a relay which does not make or break a current - they simply turn something on or off.

RF Site Survey - the ultimate goal of an RF Site Survey is to supply enough information to determine the number and placement of antennas that provides adequate coverage throughout the facility. Most importantly, it allows a look at the increasing allocation to unlicensed spectrum use which brings concerns for interference to existing systems. Thus the assessment of the current status and forecasting the future of the spectral environment becomes important.

Side Tone - This is your own voice heard in your earphone as you are speaking.

Spread Spectrum - modulation techniques have become more common in recent years. Spread spectrum enables a signal to be transmitted across a frequency band that is much wider than the minimum bandwidth required by the information signal. The transmitter "spreads" the energy, originally concentrated in narrowband, across a number of frequency band channels on a wider electromagnetic spectrum. Benefits include improved privacy, decreased narrowband interference and increased signal capacity.

Stage Announce (SA) - typically a voice page made over a loudspeaker. In wired and wireless intercom, when an SA control is pressed, either at a base station or an assignable beltpack, the user's audio is routed to the stage announce connector on the back of the base station. This is usually an analog line level audio output. The user also loses their headset side tone as an indication that stage announce is activated. The other wireless beltpacks and wired users do not hear the user's audio.

Time-Division Multiplex (TDM) - a technique of putting multiple data streams in a signal by separating the signal into many segments, each having a very short duration. Each individual stream is reassembled at the receiving end based on timing.

Time Domain Multiple Access (TDMA) - in older analog intercom systems, each beltpack, as well as the base station, had their own individual frequencies. This was easy to engineer, but very spectrally inefficient and very susceptible to interference and multipath fading. By using TDMA, the base station and all of its associated beltpacks operate on the same frequency at any given time. This is called multiple remote access. All of the devices on the RF link share, or multiplex, the frequency. First, the base station "says" what it has to say to each beltpack, while all of the beltpacks "listen". Then, each beltpack in turn "talks" back to the base station with its own information. Once all of this occurs, the whole system changes frequencies and it happens all over again.

Transceiver - an antenna that is both a transmit and a receive antenna.

TW Radios - an acronym for Two-Way Radios or otherwise called Walkie-Talkies.

Two-Wire - a communications system where the path is the same for both talk and listen. in intercom channels, there are two wires (one path). Two-wire systems can be two-wire balanced or two-wire unbalanced.

Wi-Fi - a trademark of the Wi-Fi Alliance. It is not a technical term. However, the Alliance has generally enforced its use to describe only a narrow range of connectivity technologies including wireless local area network (WLAN) based on the IEEE 802.11 standards.

Wired Comms - an existing wired intercom of either PL type or a more comprehensive Matrix type.