Pro Audio Glossary of Terms
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A/D Converter - The conversion of analog signals into digital signals. The analog signal is sampled every few milliseconds and its level is quantized into a digital word. The larger the digital word, the more accurate the representation of the analog value. See Quantization 1 and Sampling frequency The 02R uses 20-bit linear analog-to-digital converters for most of its analog inputs.
Aliasing - A form of
distortion which can occur during the conversion of analog signals into digital.
If the input signal is more than one half the sampling rate, only portions of
the signal will be present when the system samples the waveform. A false image
of the waveform based on the components that were actually sampled, is created.
Amplifier - A device used to obtain power or voltage gain in an audio circuit. In Sound Reinforcement, we are generally referring to power amplifiers which convert the low level program audio from our mixing console or other source and boost it sufficiently to drive the loudspeakers. In the case of audio power amplifiers, they are usually referred to in Watts (a unit of power) into a specified load impedance, usually 2, 4 or 8 ohms. It is important to point out that not all manufacturers rate their amplifiers in the same way or at the same impedance. An amplifier that is rated at 400 watts per channel at 4 ohms, may not actually put out as much power as a competing amplifier that is rated as 250 watts per channel at 8 ohms. It is important to read and understand the specifications very carefully.
Attenuation - Reduction of a signal. Usually referred to in decibels.
Automation System - A computerized system on many consoles that memorizes fader positions, switches individual channels on or off adjusts the equalization or pan positions, and changes the auxiliary sends - usually based on timecode or Midi pre-sets. It can recall scene' memories and programs from the equalizer, effects, dynamics, and channel libraries, also determined by the recorded timecode. You can record an entire mixing session, and then edit the individual channel settings in multiple passes. You can punch-in to fine tune a specific parameter or you can use one of the event editors to adjust the timecode position of channel settings or scene and library changes.
Auxiliary Send - Essentially separate mix busses, the auxiliary sends are used to feed signals from the input or tape return channels of a mixer to external effects processors, amplifiers, or multitrack recording equipment. They can also be used for stage monitor applications. Some Aux sends are pre-fader. These work independently of the channel's master fader. These are generally used for monitoring applications. Some are Post Fader, which means the send follows the gain setting of the channels master fader. These are usually used for effects sends. Many mixers are switchable pre or post fader. Some models also give the option of pre or post equalization, which as the name implies means the signal can can be sent either with or without the channel's equalization settings.
Balanced - The term "balanced" refers to the relationship between the signals in the conductors of your connecting cables. Balancing an audio signal requires two wires of identical length. They are usually twisted together and covered by a third conductor in the form of an overall shield, which covers the other two wires. The balanced connection responds to the difference in voltage between the pair of wires. Any noise that is induced by an outside source (hum, radio signals, etc.) will be induced evenly into both conductors. The balanced receiving device at the end of the cable will ignore these identical signals and respond only to the difference information between the two wires. This allows long cables to carry audio with few problems. The most common connector for a balanced signal is the three pin XLR (or Cannon) Connector. You'll find it everywhere from the back of a microphone to the input of a power amplifier. Unfortunately, the connector type does not always determine the signal format. The device itself may or may not be balanced, despite the connector. It is important to find out if your equipment is really balanced.
In most situations, balanced audio lines are always preferable to unbalanced audio lines, however there times when practicality dictates the use of unbalanced cables which usually are terminated in RCA (phono) plugs or 1/4" phone plugs.
Bit - A single unit of digital data. It can have a value of 1 or 0.
Bulk Dump - A MIDI function that allow the transfer of system specific data, such as sample files or mixer settings, between MIDI capable devices. The data are transmitted as MIDI System Exclusive messages.
Buss - A circuit that connects the input or tape and effect return signals with the output connectors.
Clipping - The point in gain at which an audio device no longer reproduces any more voltage or power, resulting in massive audio distortion. This is caused by overdriving the device Many devices have "Clip" indicators, which are usually red LED's labeled as such, or sometimes it's the top LED on a VU meter. They should not be lighting up under normal circumstances. The cure for clipping is simple: Turn down the volume. If the sound system is not loud enough at the point before amplifier clipping, you simply do not have a large enough amplifier and may need a much larger loudspeaker system. Besides being very unpleasant to listen to, clipping can lead to loudspeaker or driver failure, by literally tearing the loudspeaker apart or dramatically increase the heating of the loudspeaker voice coil. A good practice is to insert a compressor / limiter before the amplifier to prevent the amplifier from being overdriven.
Compander - A compander is a compressor/expander - a combination of signal compression and expansion. The compander attenuates the input signal above the threshold as well as the level below the width. For very dynamic material, this program allows you to retain the dynamic range without having to be concerned with excessive output signal levels and clipping. See Dynamics Processor.
Compressor - A compressor provides a form of automatic level control. It attenuates high levels, thus effectively reducing the dynamic range, making it much easier to control signals and set appropriate fader levels. Reducing the dynamic range also means that recording levels can be set higher, therefore improving the signal-to-noise performance. Limiting is an extreme form of compression, where the output signal is sharply attenuated so that it cannot exceed a particular level. See Dynamics Processor.
Confidence Monitoring - Listening directly from a recording medium to ensure the program material is being recorded correctly, Most analog recorders have a playback head trailing the record head, allowing you to hear the material directly after it has been recorded. Professional DAT recorders usually have four heads for confidence monitoring, as do a number of the modular digital multitrack recorders. Hard disk recorders offer their own form of confidence monitoring.
D/A Converter - The conversion of a digital data stream into analog signals. The digital word is buffered and then converted into an analog signal. After conversion, the analog signal is usually processed through a smoothing filter which removes the step transitions between the digital words. The 02R uses 20-bit linear digital-to-analog converters for most of its analog outputs.
DAT Cassette - Metal chrome cassette tape (thickness 13 (m, width 8.8mm).
Delay - A delay can be used as a signal processing effect, similar to an echo. It is also very convenient to use a digital delay to slow down the signal to remote fill speakers in a sound reinforcement system. Sound travels at approximately 1180 feet per second, while electricity travels at the speed of light (186,000 miles per second). Remote loudspeakers can be time delayed in large sound systems to compensate for this unavoidable law of physics.
Dither - In digital recording and signal processing, dithering is a mathematical process where a random noise is added to the least significant bit of a digital word. With very low level signals, the quantization error becomes correlated to the signal level. This creates a measurable amount of distortion. By adding dither, the correlation between the signal level and the quantization error is canceled, allowing the digital system to encode amplitudes smaller than the least significant bit. If you change the word size as a signal passes from one digital system to another, being able to add dither allows you to maintain a high quality signal.
Digital Signal Processor (DSP) - A specialized circuit, usually a chip, that is designed to manipulate large quantities of data in real-time.
Ducking - Ducking is used to automatically reduce signal levels when the level of a source signal exceeds a specified threshold. It is used for voice-over applications where, for example, level of background music is automatically reduced, allowing an announcer to be heard clearly. See Dynamics Processor.
Dynamic Range - The difference between the loudest (maximum output level) and quietest (residual noise floor) sounds produced in an audio system. The dynamic range in a digital system is determined by the data resolution, about 6 dB per digital bit. A 20-bit system has a theoretical dynamic range of 120 dB. The 02R has a specified dynamic range of 105 dB.
Dynamics Processor - Dynamics processors are used to correct or control signal levels. These processors allow you to compress, expand, compress-expand (compand), gate, or duck the signals passing through the mixer. See Compander, Compressor, Ducking, Expander, and Gate.
Equalizer - "Tone" controls. This can be as simple as one rotary knob to a very complex 1/6th octave equalization device. Most Mixers have three or four bands of equalization. (Low, Mid High, or Low, Low-Mid. High Mid and High). Better consoles allow the two mid bands to "sweep." The operator can adjust the actual frequency of the equalization point. Better still is Parametric equalization, where the operator can adjust gain, frequency and with of the band which is to be altered.
Expander - An expander is another form of automatic level control. By attenuating the signal below the threshold, the expander reduces low-level noise or expands the dynamic range of the recorded material.
Frequency - The center frequency of an equalizer band. See Equalizer. Human Hearing is about 20 Hz (Cycles per second) to about 16,KHz (thousand cycles per second). Some people can hear higher while others, especially those who have had prolonged exposure to loud noises, hear less. It is generally accepted that even though there some frequencies in the spectrum we can't consciously hear, we are still influenced by them and they contribute greatly to the overall perceived quality of the program material.
Gain - The amplification of a signal, usually measured in db (decibels). No gain (0 db) is called "Unity Gain"
Gain and 20dB (pad) - The analog input pre-amplifier controls. These controls are used to optimize the signal from the input connectors. The pad switch is used to reduce the channel gain for line level signals.
Gate - A gate or noise gate is an audio switch used to mute signals below a set threshold level. It can be used to suppress background noise and hiss from valve (tube) amps, effects pedals, and microphones. See Dynamics Processor.
Ground Loop - occurs when the grounds of two units are tied together in more than one place.
Metering - A method of visually determining audio presence. Most mixers use either LED bar graph meters or conventional mechanical "VU" meters. Generally he mono and stereo input channels, the tape and effect returns, and the auxiliary sends and buses are all metered in some fashion. Sometimes there are individual meters for each function. Sometimes a few meters do multi functions by means of a switch or switches.
"0" VU is where the red scale of most meters starts. That is usually a signal of +4 dbm or 0.775 volts into a 600 ohm load. What it means to the operator is you are running out of gain before the device will go into noticeable distortion (clipping). Usually you have 12-18db of headroom when the meter "hits the red." Generally speaking, the more expensive the device, the more headroom.
MIDI Implementation - MIDI is an acronym for Musical Instrument Digital Interface. It is an international standard that allows electronic musical instruments to communicate with each other.
Nyquist Sampling Theorem - This theorem defines the process of sampling audio with a digital system. Amongst other things, it states that the sampling frequency of a digital audio system must be at least twice that of the highest audio frequency, otherwise aliasing will occur. The Nyquist theorem was developed at Bell Labs by C. Shannon and H. Nyquist. See Aliasing.
Over-sampling - In a digital device, the input analog signal is sampled at a much higher rate than the normal sampling rate. Using the high sample rate, the digital data may be processed with a very steep slope digital filter. As the filter is in the digital domain, unpleasant side-effects, such as phase effects are eliminated.
PCM - Pulse Code Modulation. A scheme for encoding audio data as a series of pulses. Each pulse defines a transition from binary one to binary zero.
Peaking - An equalizer circuit that is used to cut and boost a signal, centered about a specific frequency, Using the bandwidth (Q) parameter, you can widen or narrow the effect of the circuit.
Phase - Phase is the frequency coherence of a signal. If two signals are out of phase, the trough of the first waveform corresponds with the peak of the second, resulting in cancellation. Many mixers can invert the phase of the input signals, which allows you to compensate for incorrectly wired conductors and out of phase devices.
Program Change - A MIDI message that is used to recall programs. On many mixers and effects devices with MIDI control they recall scene memories. These MIDI devices can be chained together so one button on the console can change many parameters simultaneously.
Q (bandwidth) - The bandwidth of an equalizer band. For high values the bandwidth is narrow. For low values, it is wide. See Equalizer.
Quantization - In a digital device, the encoding process when the analog input is approximated (quantized) to the nearest binary value available. These approximations are not an exact duplication of the analog waveform and are therefore contain quantization errors (noise). However, this noise is reduced by over-sampling.
RAM (Random Access Memory) - A memory chip that stores data that can be edited and changed. It requires a continuous electrical charge. Most digital devices are really small (or sometimes not so small) computers whose scenes memories and other libraries are stored in RAM. An internal backup battery provides the continuous charge. Contrast with ROM.
ROM (Read Only Memory) - A memory chip that stores data that cannot be edited. It does not require a back up battery. Contrast with RAM.
Routing - The process of assigning input or tape and effect returns to the output buses or the stereo bus.
Sampling Frequency - In a digital device it's the rate at which measurements of an audio signal are taken during A/D and D/A conversion. Once in the digital domain, the data usually remains at the same sampling frequency. The measure is samples per second.
Scene Memory - Many consoles use these used to store a snapshot of virtually all the digital parameters of the mixer. Settings that are not stored consist mainly of monitor controls, analog controls, and switches. Life is much easier when these memories can be named for easy identification.
Shelving - An equalizer circuit that is used to cut and boost a signal above or below a specified frequency High and low band equalizers are usually shelving type. In better quality mixers the equalizer can be configured as shelving or peaking.
Signal to Noise ratio (S/N) - The difference between the nominal signal level and the residual noise floor, usually expressed in decibels.
Sound - Sound is vibration. In order for something to produce a sound it has to vibrate. These vibrations disturb the air, causing it to move back and forth as well, referred to sound waves. If unobstructed, these waves spread outward from the vibrating object like ripples on a pond. When these waves reach your eardrum, they cause it to vibrate, which in turn stimulates a nerve that tells your brain that you hear something.
System Exclusive - A MIDI message that is used to transmit data between MIDI devices that is exclusive to those devices. Bulk dump data can be sent from any device to a MIDI data filer.
Talkback System - A system that allows the engineer in the control room to talk to the musicians in the studio or stage.
Timecode - Timecode is a
signal that contains a chronological record of the absolute time in a recording.
It is used for synchronizing different recorders. It can be used for electronic
editing. Timecode was initially invented for the motion picture business, as a
method of synchronizing the pictures recorded in the frames of a camera to the
sound recorded on tape recorder.
In audio, most common is SMPTE timecode, or MTC (MIDI timecode),
but there are several other versions.
White Noise - A random noise that contains an equal amount of energy per frequency band. That is, 100-200, 800-900, and 3000-3100. Pink noise has an equal amount of energy per octave. The bands I O0-200, 800-1600, and 3000-6000 all contain the same amount of energy.
Word - One sample of audio data.
Wordclock - Wordclock is a
sync pulse which allows devices to determine where the start of each digital
word is. When multiple digital devices are connected together, it is vital that
each device knows where a digital word starts and stops. Otherwise dropout or
distortion may result. Although most digital interconnect protocols are
self-clocking, it is more reliable to use a dedicated line for your wordclock
signal. This is especially important in a multitrack environment where up to
eight channels of digital data may be multiplexed on one cable.