Lexicon 20/20 AD manual Definition of Terms

Definition of Terms

AES : Audio Engineering Society.

aliasing : a form of distortion which occurs when an A/D converter’s sample rate is less than twice the highest fre- quency component of a signal.

amplitude : the maximum departure of the value of a wave- form from its average value.

attenuation : a reduction in the amplitude of an audio signal.

balanced signals : two equal and opposite signals run through a twisted pair. The advantage of balanced signals is that common mode noise tends to be added equally to both portions of the pair; it can be removed by subtracting the negative from the positive signal. (Common mode noise rejection cannot be performed on unbalanced signals, which consist of an audio signal and a ground return.)

bandwidth : the arithmetic difference between the upper and lower cut-off frequencies of an audio system.

bit : binary digit: the smallest unit of digital information, equivalent to the result of a choice between two alternatives (yes or no, on or off , 0 or 1). A series of bits make up a binary number called a sample word.

common mode noise :any signal which is common to both the negative and positive polarity of a balanced signal. In a typical audio signal, common mode noise is introduced over the length of the cable by outside interference. A typical example is line hum.

CMRR : Common Mode Rejection Ratio: measures the ability of a system to reject common mode noise. It is expressed as the ratio of common mode noise entering the system to the common mode noise leaving it.

dBfs : decibel full scale: this is the unit used to describe the amplitude of a digital audio signal where 0 equals digital full scale.

digital headroom : does not exist, see dBfs.

distortion : an unwanted change in a waveform as it passes through an electronic component or from one medium to another.

dither : a low level signal which is added to digital audio to reduce quantization noise, in effect, by replacing it with noise which is perceived as less offensive.

dynamic range : when used to describe digital-to-analog or analog-to-digital converters, dynamic range is the ratio of the full scale signal to the broadband noise (0-20 kHz) measured with a -60dBfs signal. In other words, it is the S/(N+D) measured with a -60 dBfs input signal and a 20 kHz low-pass filter.

EBU : European Broadcast Union.

emphasis : a form of equalization which increases high frequency signals in order to maximize the signal-to-noise ratio. De-emphasis is the inverse of emphasis.

equalization : the process of increasing or decreasing the amplitude of some frequency ranges of a signal with respect to others.

frequency : the number of vibrations per unit of time, ex- pressed in cycles per second (Hz).

harmonic distortion : the appearance of harmonics of the input signal at the output of a device.

link : the tying of two channels of the 20/20 AD compressor to a shared gain curve.

lock : two or more audio signals which have the same frequency and phase.

mS : millisecond: 1/1000 of a second.

oversampling : a method of sampling an audio signal at many times the audio bandwidth (typically, 64-256 times) in order to reduce the complexity of the anti-aliasing filter. The oversampled data is then digitally filtered to reduce the sample rate.

period : the interval of time required for a waveform to complete one cycle and begin to repeat itself. Period = 1/ frequency.

phase : the angular relationship between two signals, meas- ured in degrees, where 360° = the period of the waveform. When two identical signals are always of the same polarity, they are said to be in phase, or to have a phase of 0° . When they are always of opposite polarity, they are said to be out of phase, or to have a phase of 180° .

polarity : the positive or negative direction of an electrical force.

quantization : the process of converting an infinitely-varying waveform into a finite series of discrete levels. The inaccuracy introduced by this process is called quantization error.

rms amplitude : root mean squared amplitude: a way of assigning a DC power equivalent to an AC waveform. For a sine wave, this is 0.707 times the peak level.

sample rate : the periodic interval at which an analog audio signal is sampled, e.g. 44.1 or 48 kHz.

signal-to-noise ratio : the ratio between the maximum peak signal and the wideband noise in the absence of signal. This number becomes more meaningful when distortion is added. It is then expressed as S/(N+D).

S/PDIF : Sony/Phillips Digital Interface Format.

synchronize : to arrange events to occur simultaneously. In audio, the events are both the period and the phase of a signal.

THD+N : the ratio of the rms amplitude of the distortion and noise that is added to a signal to the rms amplitude of the signal itself. This can be expressed in percent or decibels. S/ (N+D) = 1/(THD+N).

time constant : the amount of time it takes for a signal to rise or fall 63% of its full scale. This only pertains to signals which rise and fall exponentially, such as the 20/20 AD meter time constants or compression gain curves.

word clock : a square wave used as a synchronization source. Its frequency equals the sample rate.

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