Lee Engineering - Sound, Light and Video Solutions

AES/EBU
interface standard serial format for professional two channel digital audio signals using balanced line drivers. Usually uses a twisted-pair cable of 110? standard impedance with 3-pin XLR connectors. The EBU stipulates that isolating transformers should be used at each end of the connection.

AFL
After Fade Listen, a signal taken after the main channel fader on a mixer.

aliasing
Unwanted frequencies generated when sampling a signal with frequency more than half the sampling rate.

ANSI
American National Standards Institute

Balance
Used to change the relative loudness between the left and right channels in a stereo system. One channel is made louder by reducing the level of the other. cf. PAN (q.v.). Total stereo output power is kept constant with a 3dB increase in level when the control is centred.

balanced
A method for interconnecting electrical signals using a shielded twisted-pair cable. The signal is transmitted along two wires, with one copy of the signal inverted. The system relies on noise being induced equally (and in the same phase or polarity) on each wire. Balanced designs have equal impedance from each line relative to ground, guaranteeing equal noise susceptibility. Since a balanced input stage amplifies only the difference between the lines, it rejects everything else - the noise.

bandpass filter
A filter that has a finite passband. The bandpass frequencies are measured at the -3 dB points.

bandwidth
The numerical difference between the upper and lower -3 dB points of a band of audio frequencies. Used to calculate the Q (q.v.) of a filter.

Bass reflex
A speaker enclosure in which the speaker's rear sound wave is projected through a specially designed port (tube) in the baffle to reinforce the low frequency range. Below the tuned frequency bass response drops more dramatically than if no port were used.

baud rate
The transmission speed of a modem.

Bessel
Crossover with low-pass filters of linear phase response and a monotonically decreasing passband amplitude response (it starts rolling off at DC and continues throughout the passband). Linear phase response (phase shift vs. frequency) results in constant time-delay (all frequencies within the passband are delayed equally). Consequently there is no overshoot or ringing resulting from a sudden transition between signal levels. The drawback is a slower frequency roll-off rate.

Bi-amp
By separating the audio spectrum into two bands, i.e., high frequencies and low frequencies by means of an electronic crossover and using a separate amplifier for each frequency range, more efficient use is made of both amplifier and loudspeaker resulting in increased headroom and dynamic range, and lower IMD (q.v.)

bit stream
A binary digital signal not arranged in groups.

BNC
BayoNet Connector for coaxial cable

Bridging
Operating a stereo amplifier in mono using the bridge mode switch, which makes Channel A output the positive drive and Channel B output the negative (like push-pull). The power output of the amplifier is almost twice that of single channel operation, although internal power rails can often limit the output. Note that an amplifier's minimum load impedance is doubled when bridged.

bus
an electrical path used for transmitting (audio) signals or power.

Butterworth
A type of crossover circuit utilising low-pass filter design characterised by having a flat amplitude response, i.e., no amplitude ripple in the passband.

Cardioid
A type of microphone having a heart shape pickup pattern that picks up sound better from the front (on axis) than back (off axis).

CCIR
International Radio Consultative Committee

CD-R
Compact Disc-Recordable

Chrominance
The colour portion of the video signal - includes hue and saturation information but not brightness (see luminance).

CMRR
Common Mode Rejection Ratio. A measure of the ability of an input stage to reject signal introduced equally onto both wires of a balanced (q.v.) input.

Comb Filter
When two combining sound waves have different amplitudes, phases, and frequencies, the resultant soundwave develops nulls - spaces where the energy has cancelled. So called because the overall frequency response resembles a comb due to the areas of cancellation causing large drops in level.

Composite video
A video signal with luminance, chrominance, and synchronisation data on a single (yellow) cable.

Compressor
A signal processing device used to reduce the dynamic range of the signal passing through it.

Condenser microphone
A microphone design where a thin diaphragm is placed in front of a metal disc. A capacitor (or condenser) is created whose capacitance varies as a function of sound pressure. If the capacitor is first given an electrical charge then change in capacitance, since the charge is fixed, causes the voltage across it to vary proportionally to the sound pressure (Coulomb's Law). Condenser microphones require external voltage (usually phantom power (q.v.) ) to operate.

constant-Q equaliser
Applied to graphic equalisers when the bandwidth (& hence Q) of each filter remains constant for all boost/cut levels.

Continuous (RMS) Power
The amount of power an amplifier can deliver when amplifying a constant steady tone. It is usually quoted at a specific frequency (1 kHz) and distortion (e.g. <0.1%).

Crossover
An electrical circuit comprising high-pass, low-pass and bandpass filters used to divide the audio frequency spectrum into separate outputs. Crossover circuits are described by their filter type (e.g. Butterworth, Bessel and Linkwitz-Riley), and by the sharpness of their roll-off (dB / Oct)

Crosstalk
Interference between two signals, usually stereo L & R pairs. Tends to increase at HF.

cue
used interchangeably with solo (q.v.) or PFL (q.v.) as found on recording consoles.

DAC
digital to analogue converter

Damping factor
The ratio of the speaker impedance to the amplifier's internal output impedance. Damping factor is a measure of how well an amplifier can actually control the movement of a speaker cone or diaphragm. A high damping factor, as found on Sony amplifiers means, more control.

dBu A logarithmic voltage reference point equal to 0.775 Vrms

dBV A logarithmic voltage reference point equal to 1.0 Vrms.

dBFS
A reference level equal to "Full Scale." Used in specifying A/D and D/A audio data converters. Full scale refers to the maximum voltage level possible before "digital clipping," or digital overload of the data converter

de-esser
A special type of audio signal compressor that operates only at high frequencies (>3 kHz), used to reduce the effect of vocal sibilant sounds.

delay
A signal processing device used to delay its output signals by a controllable amount. Used to correct for loudspeaker drivers that are mounted such that their points of apparent sound origin (not necessarily their voice coils) are not physically aligned - time alignment. Signal delay units are used to "aim" loudspeaker arrays. Introducing small amounts of delay between identical, closely-mounted drivers, fed from the same source, controls the direction of the combined response.

delta-sigma modulation (also sigma-delta)
An analogue-to-digital conversion scheme Characterised by oversampling (q.v.) and digital filtering to achieve high performance at low cost, a delta-sigma A/D thus consists of an analogue modulator and a digital filter. The fundamental principle behind the modulator is that of a single-bit A/D converter embedded in an analogue negative feedback loop with high open loop gain. The modulator loop oversamples and processes the analogue input at a rate much higher than the bandwidth of interest. The modulator's output provides a very high rate bitstream and a digital filter is used to translate into more bits at a lower rate.

DIN
Deutsches Institut fur Normungt (German standardisation authority)

Directivity
Area of coverage of a speaker or microphone.

Dispersion
The spread or distribution or coverage of sound generated from a horn or loudspeaker.

dither
Noise (analogue or digital) added to a signal prior to quantisation which reduces the distortion and noise modulation resulting from the quantisation process. Although there is a slight increase in the noise level, spectrally shaped dither can minimise the apparent increase. The noise is less objectionable than the distortion, and allows low-level signals to be heard more clearly.

DSD(tm) (Direct Stream Digital(tm))
Joint trademark of Sony and Philips for SACD - the next generation CD-standard. A 1-bit, 64-times oversampled bitstream digital signal.

DSP
Digital Signal Processing

D Sub-25
A 25-pin D-shell connector standardised for RS-232 serial communications.

D Sub-9
A 9-pin version of the connector used for RS-232 communications.

ducking
A dynamic processor that lowers (or "ducks") the level of one audio signal based upon the level of a second audio signal (e.g. for paging)

DVD
A 12-centimeter (4.72") compact disc capable of holding video. The DVD standard specifies a laminated single-sided, single-layer disc holding 4.7 gigabytes, and 133 minutes of MPEG-2 compressed video and audio.

dynamic microphone
A microphone in which a wire (voice) coil is attached to a small diaphragm such that sound pressure causes the coil to move in a magnetic field, thus creating an electrical voltage proportional to the sound pressure

dynamic range
The ratio (in dB) of the loudest (undistorted) signal to that of the quietest (audible) signal in a unit or system.

EBU
(European Broadcasting Union)

electret microphone
A microphone design similar to that of condenser mics except utilising a permanent electrical charge, thus eliminating the need for an external polarising voltage (acronym for electricity + magnet). However an internal JFET amplifier is needed to overcome its extremely high output impedance, which requires (less) external power.

expander
A signal processing device used to increase the dynamic range of the signal passing through it.

Far Field
When the listener is more than twice the concerned wavelength away from the source.

FIR (finite impulse-response) filter
A commonly used type of digital filter (cf. IIR). Digitised samples of the audio signal serve as inputs, and each filtered output is computed from a weighted sum of a fixed number of previous inputs. An FIR filter can be designed to have completely linear phase (i.e., constant time delay, regardless of frequency). See SRP-F700.

FOH
Front Of House, used to describe the main (audience) sound reinforcement system

foldback
The original term for monitors, or monitor loudspeakers, used by musicians to hear themselves and/or the rest of the band during a live performance.

graphic equaliser
A group of filters using faders to control cut or boost. So-called because the positions of the faders represent the resulting frequency response of the unit.

groups (aka subgroup)
A mix of two or more signals (channels) treated as a set that can be varied in overall level from a single control.

Ground Loop
A loop formed when two or more points in an audio system that are nominally at ground potential are connected by a conducting path, sometimes resulting in audible hum or noise.

Haas Effect
(aka precedence effect), tells us that humans localise a sound source based upon the first arriving sound, if the subsequent arrivals are within 25-35 milliseconds they are ignored. This is true even when they are up to 10dB louder than the first.

headroom
A term related to dynamic range, used to express in dB, the level between the typical operating level and the maximum operating level (onset of clipping). For example, a nominal +4 dBu system that clips at +20 dBu has 16 dB of headroom.

high-pass filter
A filter having a passband extending from some finite cut-off frequency up to infinite frequency. Aka LCF (Low Cut Filter)

IEC
International Electrotechnical Commission

IEEE
Institute of Electrical and Electronic Engineers

IIR (infinite impulse-response) filter
A commonly used type of recursive (i.e. with output fed back to its input) digital filter. An IIR filter is more efficient than its FIR counterpart, but its weakness is not having linear group delay and possible instabilities.

IM (Intermodulation Distortion)
Non-linear distortion characterised by the appearance of output frequencies equal to the sums and differences of integral multiples of the input frequency components. For instance, an amplifier with high IM distortion amplifying two frequencies of 100 Hz and 2,000 Hz would also generate distortion components of 1,900 Hz and 2,100 Hz.

Impedance (Z)
The total opposition (in Ohms) to ac flow presented by a circuit. Usually just referred to as either "high" or "low"

Impedance Matching
The process in which the input impedance of a connected load (loudspeaker) is designed to be equal to the output impedance of the source (e.g. amplifier), thereby giving maximum transfer of energy, minimum reflection, and minimum distortion. In general the source should be of lower impedance that the load. is more impulsive in nature than it is steady state.

Infinite Baffle
A baffle (board) that effectively prevents all of a loudspeaker's rear sound waves from interfering with its front waves.

Insertion loss
A loss in gain of a system after a component has been added or inserted in the system.

Interleaving
The process of rearranging data in time. Upon de-interleaving, errors in consecutive bits or words are distributed to a wider area to guard against consecutive errors in the storage media.

ISO
International Standards Organisation

Jitter
A tendency towards lack of synchronisation caused by electrical changes. The difference between when a digital edge (clock) transition is supposed to occur and when it actually does occur, causing errors & hence distortion in DACs & ADCs (linearity).

limiter
A device which prevents an audio signal from exceeding a preset threshold setting.

Linkwitz-Riley crossover
filter design consisting of cascaded 2nd-order Butterworth low-pass filters, the Linkwitz Riley Crossover has flatter frequency & linear phase response.

Low-pass filter (LPF)
A filter having a passband extending from DC (zero Hz) to some finite cut-off frequency (not infinite).

Loudness Control
A volume control which compensates for the poorer frequency range of the ear when listening to lower sound levels. A loudness control boosts the LF & HF at lower signal level.

Matrix 1.
an array of rows and columns 2. A sub-mix of groups (q.v.) with its own master level control

MI
musical instrument

MIDI
(musical instrument digital interface

noise gate
When the incoming audio signal drops below a preset (the threshold point) a noise gate effectively shuts off the output by reducing the gain.

noise shaping
A technique used in oversampling low-bit converters and other quantizers to shift (shape) the frequency range of quantizing error (noise and distortion).

Notch Filter
A band rejection filter that produces a sharp notch (high Q) in the frequency response of a system, thus reducing the gain for a narrow range of frequencies.

Nyquist frequency
The highest frequency that may be accurately sampled. The Nyquist frequency is one-half the sampling frequency.

Octave
The interval between any two frequencies having a ratio of 2 to 1. Example: "A 440" is an octave above "A 220".

Omni-directional
Applied to microphones with a uniform pickup of sound from all directions.

oversampling
Sampling at a rate higher than the Nyquist (q.v.) sampling rate. This reduces the need for sharp (expensive) analogue filters to control aliasing (q.v.).

pan (panoramic) control
A control found on mixers, used to "move" the apparent position of a single source between the Left and Right stereo outputs. Fully anticlockwise corresponds to left output only, at the other extreme it is sent fully right. In the middle, the sound is sent equally to each output, but is reduced in level by 3 dB relative to its original value. As the sound is panned from one side to the other, it maintains equal loudness (power) for all positions.

parametric equaliser
A multi-band variable equaliser offering control of amplitude, centre frequency and bandwidth.

passive crossover
A loudspeaker crossover not requiring power for operation. Normally built into the loudspeaker cabinet

PFL
Pre Fade Listen, a signal taken before the main channel fader. Cf. AFL

phantom power
The term given to the standardised scheme of providing power supply voltage (and around 15mA current) using the same two lines as the balanced audio path. Three DC voltages are common (48 volts, 24 volts and 12 volts).

phase lock loop (PLL)
A circuit for synchronising a variable local oscillator with the phase of a transmitted signal. The circuit acts as a phase detector by comparing the frequency of a known oscillator with an incoming signal and then feeds back the output of the detector to keep the oscillator in phase with the incoming frequency.

pink noise
a random noise source having a flat amplitude response over each octave frequency band (cf. white noise)

PPM (peak program meter)
An audio meter displaying peak audio signals (cf. VU meter)

pre-emphasis
A high-frequency boost used during recording, followed by de-emphasis during playback, designed to improve signal-to-noise performance

Q
Quality factor. Filters. The selectivity factor, defined to be the ratio of the centre frequency f divided by the bandwidth BW.

Quantisation
The process of sampling an continuous analogue signal value and converting the sample into a predefined discrete numerical or digital value.

Quantisation Noise
The noise resulting from the error introduced by the imperfection in the A/D conversion process: the level of noise created by trying to represent an analogue signal of infinite resolution with a digital value of discrete and finite resolution.

RCA jack
(aka pin jack or phono connector) standard line (-10dBV 0.775V) level consumer audio and project studio sound equipment, and most recently to interconnect composite video signals.

R-DAT
see DAT (rotary head digital audio tape recorder )

RIAA
Recording Institute Association of America. Preamplifier (with EQ) used for magnetic cartridges on turntables.

RMS
Root Mean Square - RMS voltage is .707 times the peak voltage of a sine wave.

RS-232
The standard serial interface used on most personal computers.
(RS-232 pinout)

RS-422
A universal balanced line twisted-pair standard for all long distance (1000 m) computer interconnections, daisy-chain style.
(RS-422 pinout)

SACD - Super Audio CD format.
The Super Audio CD is a hybrid disc comprising two layers: a high density DSD layer on top, and a standard density CD layer at the bottom. The two layers are read from the same side of the disc; the CD laser reads the bottom reflective layer through the semi-transmissive DSD layer, while the other layer is read by a second laser delivering high-quality, multichannel sound without sacrificing backward compatibility. The DSD format uses three tracks: one for two-channel stereo; one for a six-channel mix; and an additional data track.

Sample rate conversion (SRC)
The process of converting one sample rate to another, e.g. 44.1 kHz to 48 kHz. Necessary for the communication and synchronisation of dissimilar digital audio devices, e.g., digital tape machines to CD mastering machines.

Sample-and-hold (S/H)
A circuit which captures and retains an analogue signal for a finite period of time. Used in ADC & DAC circuits

SCMS
Serial Copy Management System

Sensitivity
The minimum input signal required to produce a specified level of output

SIP
Solo In Place. Solo function with correct stereo (or mutlichannel) placement preserved.

Slew Rate
Refers to the ability of an amplifier's output to accurately follow the input waveform's level changes. Larger power amplifiers need a higher slew rate to maintain the same response to transients.

SMPTE
Society of Motion Picture and Television Engineers

S/N or SNR
(signal-to-noise ratio) A measurement of the level of signal compared to the noise level, measured in dB. Should be specified for a particular signal level (e.g. +4dBu), and gives the value for the noise floor.

solo monitoring
a single channel without affecting the main outputs

S/PDIF
(Sony/Philips digital interface format, also seen w/o slash as SPDIF)
A consumer version of the AES/EBU digital audio interconnection standard based on an optical fibre system called TOSLINK (q.v.) or 75? coaxial cable and phono connectors. (Not the same as SDIF - which is a previous Sony digital interface standard)

SPL
Sound Pressure Level
The unit of measurement of sound pressure expressed in dB re 20 ?Pa (the lowest threshold of hearing for 1 kHz). 0dB is therefore the threshold of hearing.

Splitter
A device which divides one signal input into multiple outputs.

THD+N
Total Harmonic Distortion plus Noise. The overall level of harmonics and noise remaining after a pure signal (e.g. 1kHz sine wave) input to the device under test has been processed and subsequently removed from its output signal.

TOSlink
TOShiba link
Consumer equipment fibre optic interface using square plastic connectors based upon the S/PDIF protocol, first developed by Toshiba

Tweeter
A high frequency speaker

VCA
(voltage-controlled amplifier)
An electronic circuit whose output voltage is a function of the input voltage and the gain set by a control voltage.

VU meter (volume units)
A special meter with averaging function designed to illustrate the perceived loudness of the audio signal. 0 VU is defined to be a level of +4 dBu for an applied sine wave. Does not show peaks (cf. PPM).

Wavelength
The distance required for the complete cycle of compressions & rarefactions that make up an audio waveform. Higher frequencies have smaller wavelengths. The wavelength varies with the speed of sound in air (& therefore temperature).

white noise
contains equal amounts of all audible frequencies at every frequency there is the same amount of power

Woofer
A low frequency speaker specialised in bass or low frequency reproduction.

word clock
The synchronising square wave signal that indicates the sampling frequency or rate of sample words over a digital audio interface. word length The number of bits in a word.

XLR
Registered trademark of ITT-Cannon.
The original model number series for their 3-pin circular connectors