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AC (60 cycle) hum » In “household” electricity, the current flow reverses direction alternately as a function of time. In the United States, current changes direction at a rate of 60 times per second (60Hz). This oscillation produces an audible tone that is measured in Hertz. The sound often has harmonic content at multiples of 60Hz, with the most objectionable being at 120Hz and 180Hz. (see ground loop)
Active » In electronics, active has two definitions. One indicates that an active device is a source of electric power, such as a battery or generator. The other definition refers to components that can increase gain or the power of a signal by drawing on a power source. Commonly, when speaking of devices such as PA speakers or studio monitors, the word “active” means powered, whereas passive means un-powered.
Balanced/Unbalanced » A method of wiring audio cables that serves to eliminate noise by virtue of common-mode rejection. The balanced cable consists of two twisted wires and a shield. One wire is considered positive with respect to ground, the other negative, and the shield, which carries no signal, is an outer wrapping of conductive material that is grounded at both ends. The signal is transmitted over one wire and received back on the other, while the shield rejects noise from electrostatic fields and magnetic interference. In a properly balanced cable, impedance is equal on both lines relative to ground.
This guarantees that noise is picked up equally on both wires. The balanced input of a preamp for example, only amplifies the difference between the lines, thus eliminating the noise that is common to both. In an unbalanced cable, there is only one wire that carries the positive signal, while the negative signal is carried on the shield. Any noise induced into the cable will make its way into the audio signal.
Clipping » When the output voltage of an amplifier exceeds the limit of the power supply, the top of the output waveform is flattened. On the oscilloscope, the waveform looks as though its top is cut or clipped. A clipped waveform produces extreme harmonic distortion comprising unpleasant-sounding, odd-ordered harmonics.
Common-Mode Rejection Ratio (CMRR) » In long cable runs, noise becomes an issue. Cables can pick up radio frequencies (RF) and other interference, which is why balanced cables are used for their noise-canceling ability. CMRR defines to what degree signals are canceled at the input of a balanced system.
Frequency Range » As it says, the actual range or span of frequencies from low to high that a unit can pass or reproduce.
DC (Direct Current) » Functionally the opposite of alternating current (AC), DC does not change directions. While most components within an electronic device are DC powered, DC is not very healthy for audio signals, particularly those traveling to speakers, since DC produces no sound and uses a great deal more power.
Dynamic Range » In an audio unit such as a preamp, dynamic range is a measurement taken from the ratio of the loudest undistorted signal (output voltage) to that of the quietest signal (noise floor) expressed in decibels (dB).
Electromagnetic Induction (or Inductive Coupling) » Refers to the transfer of a signal from one component to another through a shared magnetic field. A change in current flow through one induces current flow in the other. These components can be contained in a single unit, such as the primary and secondary sides of a transformer.
Equalization (Equalizer) » A class of electronic filters in acoustic systems that either amplify (boost) or attenuate (cut, reduce) selected frequencies (frequency-dependent amplification). Equalization is the act of modifying the frequency response of a signal.
Floating » A term used to describe a device or signal that is not grounded (“referenced to ground” as the electro-geeks like to say) or referenced to any other output. A microphone would be an example of a floating device since it has no reference to ground. Typically, floating outputs are fully isolated.
FOH (Front-of-House) » The mixing position for live shows, which is usually opposite the stage in a concert hall, club, auditorium, or theater, etc. Since some establishments have a sound booth setup that is either in the back or on the side, the term FOH generally refers to the location of the main mixing console.
Frequency » The number of complete cycles of a waveform in a given amount of time. Frequency is expressed in Hertz (Hz). One Hertz equals one cycle per second. As the number of cycles per second increases, so does pitch.
Gain » The amount of amplification (voltage, current, or power) of an audio signal, usually expressed in decibels (dB).
Ground » In an electronic circuit, ground is the common reference point or return path. Ground is also the point of zero voltage potential. In electronics, variance in potential causes electrons to flow from the higher potential to the lower. Grounding is a safety feature in terms of preventing electrical shock, as well as a means of preventing noise in interconnected audio systems.
Ground Lift » This is a means of removing AC hum caused by the presence of a ground loop in an interconnected audio system (see ground loop). The ground lift can take the form of a switch, in-line adapter, or transformer, which disconnects (lifts) one or more grounds in the system. Under no circumstances should you remove the ground pin of an AC line adapter, unless you want to become part of the ground loop, which can result in current passing from one of your hands to the other via your heart.
Ground Loop » This is the most common source of noise in electronic systems. When interconnecting electronic equipment, each component has it’s own internal ground, usually known as the audio signal ground. Connecting two (or more) devices can tie their signal grounds together via the cables. When the units are also tied together in another place, either by a line cord or the metal rails of a rack, current can flow in a closed loop from one unit’s ground to the other’s and back again. Noise or hum (see AC/60-cycle hum) occurs when this current flows through the unit’s audio signal.
Harmonics » A series of musical tones whose frequency is the multiple of a fundamental tone. Also referred to as overtones and partials.
High-Pass Filter » An electronic circuit that allows all frequencies above a fixed frequency (not zero) up to infinite frequency. High pass filters are useful for eliminating low-frequency rumble.
Impedance » A combination of factors that restrict current flow in an AC electrical circuit. Similar in concept to resistance, impedance has two components: resistance and reactance. Reactance also has two components, a “real” part (resistive) and an “imaginary” part, which is resistance induced by phase shift (e.g., the time it takes for other components such as inductors and capacitors to charge and discharge).
Limiter » Basically like a compressor, except that it does not allow the signal to go above the setting of the threshold. For example, if the limiter’s threshold (level at which the limiter will act on the signal) is set to –20dB, once the signal level reaches –20dB and beyond, the output level remains at –20dB. Limiters are used to prevent signals from clipping or overloading speakers, recorders, and power amplifiers, etc.
Loading (Load) » In electronics, a load consumes energy to do some form of work. Any component or device that is connected to a source and draws current from it is considered a load. Loading is an effect that is a function of the load’s impedance. For example, if a load has high impedance (resistant to current flow), it will draw a small amount of current from the source, thus the load is small (light loading). Conversely, a small amount of load impedance will draw higher load current from the source (heavy loading).
Mono » Short for monophonic or monaural. A system that takes one or more sound sources and either records, reproduces, or transmits them via a single channel.
Pad (or Attenuator Pad) » A passive network comprising resistors connected in series that reduces (attenuates) the voltage (or power) level of an input signal. Pads can be used to reduce the level of a unit operating at +4dBu so that it can be connected to a unit operating at -10dBu without overloading it. Or, if a signal from a high-output microphone in front of a loud source causes a mic preamp to clip (even with the gain turned all the way down), a pad will allow the mic preamp to handle the signal without distortion.
Passive » In common usage, passive simply means that the unit does not require power in order to operate or, in the case of a monitor speaker, relies on an external power source to operate. On the component level, a passive device is one that does not add power to the signal it’s acting on; rather it draws its operating energy from the signal passing through it. Examples would be resistors, inductors, and capacitors.
Phantom Power » In conventional DC-polarized condenser mics, phantom power supplies the voltage required to polarize the mic’s transducer element (capsule), using the same two lines as the balanced audio path. It is called "phantom" powering because the supply voltage is effectively invisible to any balanced microphone. The Neumann Microphone Company developed phantom power in 1966.
Polarity » What we call positive and negative electrical potential with respect to a referenced potential. Polarity is often confused with phase reversal. Phase is a function of time; polarity is not. Often in audio engineering we say a signal is out of phase, when in reality we mean its polarity is inverted.
Phase » A particular value of time for any periodic or cyclic function, and refers to what fraction of the cycle the signal has advanced to.
Phase shift » An angular relationship between two waves, whereby the fraction of a complete cycle is measured from a specified reference point and expressed as an angle.
Preamplifier » The first amplifier in the chain, the preamplifier takes a low-level signal, possibly from a guitar pickup, mic, or turntable and amplifies it. Technically speaking, the preamp provides significant voltage gain and small current gain, which makes a preamp good for recording applications. A power amplifier must follow in order for current to be amplified enough to power loudspeakers.
Saturation » The state when a material cannot absorb a stronger magnetic field, such that by introducing a greater magnetization force, no significant change on a magnetic field’s ability to exert a force on a moving charge occurs. In recording, tape saturation is the point at which magnetic tape has reached its maximum ability to accept signals, beyond which distortion occurs. As much as tape saturation is a thing to be avoided, it can be used as a creative tool. When program material is recorded at levels somewhat above 0dB, distortion is being added gradually, which has a pleasing effect. The upper frequencies become gradually compressed as tape is saturated, which also is more natural and pleasing to our ears.
Single-coil » This refers to the design of the magnetic pickups found on musical instruments such as electric guitar and bass. As the name implies, it employs one coil of wire wrapped around a magnet, whose job it is to convert string vibrations into an electrical signal by virtue of the string’s interaction with the magnetic field created by the magnet and coil. Single-coil pickups are passive, therefore they do not produce a high output and are more susceptible to noise.
Transformer » A passive component comprising two or more magnetically coupled coils or windings (usually wound around magnets) that use electromagnetic induction to increase or decrease voltage and current. Each coil or inductor shares a magnetic path. The more closely they are coupled magnetically, the more efficient they become. (see electromagnetic induction)
XLR » The original model number for 3-pin circular connectors invented by Canon, which has now become a generic term. XLR connectors are used to make balanced cables, such as those used for connecting microphones to preamps.