| D/A | Digital to Analog
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| DAC | Digital to Analog Converter. A chipset or circuit or audio product whose primary function is to convert a digital signal into an analog form.
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| Damping | A process whereby the amplitude of a vibration or resonance is reduced. This may be required in tone arms or loudspeaker cabinets for instance. Resistors may be used in circuits to provide electrical damping. The pros and cons of damping have generated heated debate among audio enthusiasts.
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| Damping Factor | A measure of the control an amplifier exerts over a loudspeaker drive unit. Damping factor is a measure of amplifier output impedance relative to loudspeaker impedance.
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| DAT | Digital Audio Tape. This format introduced in the late Eighties makes use of a rotating drum containing a helical scan head similar to the sort used in video cassette recorders. Unfortunately disagreement between the recording and hi-fi industries led to a refusal by the major record companies to produce pre-recorded DATs or to sanction the release of a non-copyright protected digital tape recorder hardware. By the time a copyright protection system had been agreed, DAT was dead on its feet as a mainstream consumer format, though it is widely used by professionals and semi-professionals today.
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| db | abbreviation for decibel.
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| DC | abbreviation for direct current
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| DCC | Digital Compact Cassette is Philips' backwards compatible digital cassette format. In addition to playing and recording Digital Compact Cassettes, DCC recorders also play standard analogue cassettes. Hence the term backwards compatible. DCC tapes work to 16-bit 44.1kHz sampling. It also features Philips' PASC data reduction system. PASC selectively discards signals thought to be below the threshold of audibility. Soft signals audible in isolation may be masked by louder signals.
In such circumstances PASC eliminates the masked signals.
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| Decibel | A measure of loudness (abbr: dB). The decibel scale is such that 3dB represents a doubling of amplifier power from say 50W to 100W), while 10dB represents a doubling of perceived loudness.
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| Diaphragm | The surface of a loudspeaker drive unit. Most moving coil bass drive unit diaphragms are cones while moving coil tweeters are invariably domes. Ribbon and electrostatic drive units are flat.
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| Dielectric | The non conducting space/insulation between two conductors in a cable.
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| Digital | A sampled analogue waveform encoded in the form of on/off pulses. The frequency with which the analog waveform is sampled is its sampling frequency which, in the case of Compact Disc, is set at 44.1kHz (44,100 samples per second). The accuracy of sampling is determined by the word length of each sample. For Compact Disc it is 16-bit. Modern
professional digital recorders are capable of almost 24-bit resolution as of this date (12-15-96).
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| Diode | The thermionic diode invented in 1904, marks the start of the electronics era. It is the first device for controlling the flow of current in relation to applied voltage, and comprises two electrodes, the heated cathode (electron source) and anode (electron receptor).
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| Direct Current | Current that does not have a positive or negative value. Usually referred to as DC.
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| Directionality | The tendency in some loudspeakers to beam sound like a laser rather than radiate it equally in all directions. Horn, ribbon and electrostatic speakers tend to be more directional at high frequencies than well designed dome moving coil tweeters, a factor that in extreme situations can impose restrictions on listening and speaker position.
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| Distortion | Any loss or addition to the audio signal is a distortion. Various amplifier distortions have been identified, the most commonly measured being intermodulation, transient intermodulation and harmonic distortion.
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| Dither | A low level random noise added to a digital signal to mask highly audible forms of digital distortion.
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| DMM | Direct Metal Mastering. An LP disc mastering process in which silvering and electroplating stages are eliminated.
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| Dolby HX Pro® | A form of signal processing during recording which prevents self biasing of the signal. HX Pro thus protects loss of high frequency signals through unwanted compression, a useful feature in cassette decks.
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| Dolby Pro-Logic® | A more advanced form of Dolby Surround encoding and decoding in which there is an additional front center channel containing dialogue information.
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| Dolby Stereo® & Dolby Surround® | All Dolby Stereo encoded cinema sound tracks contain a surround channel. This may be recovered from video recorder sound tracks by a Dolby Surround Decoder.
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| Dolby® B, C & S | Three types of noise reduction system featured on cassette decks. The highest amount of processing occurs with Dolby S, the least with Dolby B. Dolby A is a noise reduction system sometimes used in professional recording situations.
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| Drive unit | One of the most important components in a loudspeaker. The drive unit turns electrical power, fed to it from an amplifier, into acoustic power. Most drive units operate over a limited frequency bandwidth. Tweeters handle treble frequencies, Woofers operate over the bass frequency range.
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| Dynamic range | A useful definition is the difference between the quietest and loudest sound of a music signal.
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| Dynamics | The level changes present in music.
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