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Now looking at definitions starting with letter "d"

DBS : Direct Broadcast Satellite. See AC-1.

dbx™ noise reduction : a noise reduction system that uses a companding noise reduction to reduce noise. Dbx is connected into a recording system in the same way that a Dolby system is. It provides up to 30dB of noise reduction, but unlike Dolby noise reduction, the dbx system works over the entire audio frequency range, using a 2:1/1:2 compression/expansion ratio. Dolby-encoded and dBx-encoded tapes are incompatible. Systems using dbx noise reduction are typically more expensive than systems using Dolby."

DCA : Digitally Controlled Amplifier. Sometimes short for Digitally Controlled Attenuator. The DCA of a digital synthesizer modifies the amplitude of the signal generated by the DCO. It is the digital analog of a VCA.

DCAC : See DAT.

DCC : See Digital Compact Cassette.

DCO : Digitally Controlled Oscillator. The microprocessor-controlled sound generator used in a digital synthesizer. The DCO directly generates the original signal that is used as the fundamental for the sounds created by the synthesizer. The keyboard tells the DCOs what pitch to produce; the audio signal may then be altered by sound modifiers, including a DCW, DCA, differentiators and integrators, and various modulators and limiters. The digital equivalent of the analog VCO.

DDL : Digital Delay Line. See delay line.

de-emphasis : The complementary equalization which follows pre-emphasis. Sometimes redundantly called post de-emphasis.

de-esser : A special type of compressor that operates only at high frequencies, usually above 3kHz-4kHz. It is used to reduce the effect of vocal sibilant sounds. De-essers are usually used only for vocal music.

dead : Acoustically absorptive. The opposite of live. See also LEDE.

decade : The interval between two quantities plotted along an axis where the second quantity is ten times the first. A frequency ratio or interval of 10:1, as opposed to an octave, which is a 2:1 ratio. Sometimes the rolloff of a filter or equalizer is expressed in dB/decade, rather than in dB/octave. A rolloff of 20dB/decade is equal to A rolloff of 6dB/octave. The decade interval has no musical significance, but is used in the discussion of logarithmic quantities such as decibels.

decay : (1) The time it takes for a sound to reach minimum loudness; the end of a sound. (2) The second of the four segments of a typical ADSR envelope. The decay control determines the amount of time it takes for the envelope to fall from the peak reached at the end of the attack segment to the sustain level. If no additional energy is put into the sound source (e.g., a cymbal), then the decay is the time during which the sound falls from the loudest point back to silence. (3) The time taken for reverberation to die away. See decay time, reverberation time,RT-60.

decay rate : The number of decibels per second by which echoes or reverberation of a sound diminish once the sound has stopped. Depending on the sound source and environment, the decay rate may be linear, i.e., a constantly decreasing number of dB per second, or it may begin to decay slowly and then fall off rapidly, or the reverse. Also, various frequencies of the sound may decay at different rates.

decay time : See reverberation time.

Decca trees : A triangular array of omnidirectional microphones , a type of true spaced-microphone recording technique, where the central channel is distributed equally to left and right. This yields a very stable central image, avoiding the hole-in-the-middle which is problematic with many space-pair arrangements. A variant on the Decca tree places three cardioid microphones (L,C,R) in a triangle configuration. In all cases, the width of the tree is typically one-half to one-third the sound field width, and the center microphone is slightly closer to the performers. See binaural recording.

decibel (dB): A unit of measurement used to indicate audio power level, literally one-tenth of a bel, where the bel is a power ratio of 10:1. Technically, a decibel is a logarithmic ratio of two power measurements, which means that there is no such thing as a dB measurement in isolation. In order to measure a signal in dB, you need to know what power (watts, volts) it is referenced to and the impedance of the reference system.:

Number of dBs = 10 log (P1/P2),

where P1 and P2 are the two powers being compared, and where the log is base-10. Impre-cisely, 1dB is the smallest increment in loudness detectable by a careful listener. An increase of about 3dB is a doubling of electrical (or signal) power; an increase of 10dB is ten times more power, but is only a doubling of perceived loudness. Some commonly used power ra-tios, expressed in dB:


However,
Number of dBs = 20 log (V1/V2),

where V1 and V2 are the two voltages being compared, and where the log is base-10. This means that the answer is twice what it would be for a ratio of powers. In other words, dou-ble the voltage and the level goes up by 6dB; halve the voltage and the level goes down by 6dB. See Appendix A.

Commonly used reference levels are indicated by such symbols as:

decimation : A form of digital filtering whereby audio data is oversampled and then decimated to the required 44.1kHz. In practice, the sampling rate is 64 or 128 times 44.1kHz. A digital brick-wall filter is then applied to the data, resulting in a perfectly phase linear transformation. [This type of filter is impossible in the analog domain due to the phase-shift caused by very steep roll-off filters.] See FIR, IIR. After the data have been filtered below the Nyquist frequency, the next step is decimation where the data are re-sampled to produce an output stream of 44.1kHz, with the attractive result that the excess data thereby provides increased bit-resolution. See anti-aliasing filter, reconstruction filter, DSD.

deck plate : In a tape recorder transport, the heavy metal plate on which the headstack, rollers, and other transport components are located.

decoding : (1) In signal processing, restoring a signal to its original state by reprocessing the signal in a complementary manner, e.g., a NR system’s re-expansion of the signal during playback. (2) In digital recording, the entire process converting the encoded data stream back into an analog signal, including the process of error correction, i.e., digital-to-analog conversion.

deconvolution : A mathematical process for separating two signals that have been convolved. See convolution.

decrescendo : A musical term indicating a gradual reduction in loudness.

defeat switch : A control that can be used to mute a signal on a mixer.

definition : A qualitative term that denotes the clarity of a sound. A sound with poor definition may, like some woodwinds in their middle ranges, be easily mistaken for a similar sound. In recording, the apparent definition of a sound can be increased by boosting the frequency band characteristic to the specific sound of the instrument, and cutting other frequencies it has in common with other sounds in the mix.

delay : (1) The first stage of a five-stage D(elay)AD(ecay)SR envelope, which delays the beginning of the envelope’s attack segment. See ADSR. (2) An audio effect which temporarily suppresses the beginning of a sound, producing echo, chorusing, phasing, and flanging effects. A modulated digital delay effect which varies the time and/or intensity of the delay effect over time. See double tracking. (3) A signal processor used for flanging, chorusing, and echo, that holds its input for some period of time before passing it to the output, or the algorithm within a signal processor that creates delay. Also used in artificial reverberation systems and to provide delayed sound to certain loudspeakers in time-coherent sound reinforcement systems. (4) See MIDI delay.

delay line : Used to simulate an acoustic echo or reverberation. There exist both digital delay lines (DDL) and analog delay lines as well. The original delay lines were made by using tape recorders to record a signal while playing it back on the same machine. The distance between the record and reproduce heads causes a time delay; this technique is called tape delay. See tape delay.

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