DAB (digital audio broadcast) NRSC (National Radio Systems Committee) term for the next generation of digital radio broadcast.

DAC (or D/A, digital-to-analog converter) The electronic component which converts digital words into analog signals that can then be amplified and used to drive loudspeakers, etc. The DAC is the last link in the digital chain of signal processing

damping factor Damping is a measure of a power amplifier’s ability to control the back-emf motion of the loudspeaker cone after the signal disappears. The damping factor of a system is the ratio of the loudspeaker’s nominal impedance to the total impedance driving it. Perhaps an example best illustrates this principle: let’s say you have a speaker cabinet nominally rated at 8-ohms, and you are driving it with a power amp through 50 feet of 12 gauge cable. Checking the amplifier data sheet, you don’t find its output impedance, but you do find that its damping factor is 300. What this means is that the ratio of a nominal 8 ohm loudspeaker to the amplifier’s output impedance is 300. Doing the math [8 divided by 300] comes up with an amazing .027 ohms! Pretty low. Looking up 12 gauge wire in your handy Belden Cable catalog tells you it has .001588 ohms per foot, which sure ain’t much, but then again you’ve got 100 feet of it (that’s right: 50 feet out and 50 feet back — don’t be tricked), so that’s 0.159 ohms. Which is about six times as much impedance as your amplifier. (Now there’s a lesson in itself — use the correct  cable.) Adding these together gives a total driving impedance of 0.186 ohms — still pretty low — yielding a very good damping factor of 43 (anything over 10 is good enough, so you don’t have to get extreme about wire size).

DAR (digital audio radio) EIA term for the next generation of digital radio broadcasting standards.

DASH (digital audio stationary head) A family of formats for ensuring compatibility among digital multitrack studio recorders using stationary (as opposed to rotating) heads. The DASH standard, popularized by Sony and Studer, specifies 2 to 48 tracks, with tape speeds from 12 to 76 cm/sec.

DAT (digital audio tape recorder ) 1. A digital audio recorder utilizing a magnetic tape cassette system with rotary heads similar to that of a video recorder.

data compression See: digital audio data compression

DAW (digital audio workstation) Any of several software/hardware systems using a computer as the basis for creating, editing, storing, and playback of digital audio, using the computer's hard disk as the recording medium.

DB-25 connector A 25-pin D-shell connector standardized for RS-232 serial communications.

DB-9 connector A smaller 9-pin version of the connector used for RS-232 communications. First made popular by IBM in their AT personal computer.

DCC (Digital Compact Cassette) Philips's digital version of the standard analog cassette tape system. A DCC recorder plays and records digital cassettes, as well as playing analog cassettes. [Now discontinued.]

DCE (Data Communications Equipment) Within the RS-232 standard, the equipment that provides the functions required to establish, maintain, and terminate a connection, as well as the signal conversion, and coding required for communication between data terminal equipment and data circuit - e.g., a modem or printer. See: DTE. The main difference between DCE and DTE is the wiring of pins 2 and 3, thus the need for a null modem cable when tying two computers together.

decibel Abbr. dB Equal to one-tenth of a bel. [After Alexander Graham Bell.] The preferred method and term for representing the ratio of different audio levels. It is a mathematical shorthand that uses logarithms (a shortcut using the powers of 10 to represent the actual number) to reduce the size of the number. For example, instead of saying the dynamic range is 32,000 to 1, we say it is 90 dB [the answer in dB equals 20 log x/y, where x and y are the different signal levels]. Being a ratio, decibels have no units. Everything is relative. Since it is relative, then it must be relative to some 0 dB reference point. To distinguish between reference points a suffix letter is added as follows:

0 dBu A voltage reference point equal to 0.775 Vrms

. [This reference originally was labelled dBv (lower-case) but was too often confused with dBV (upper-case), so it was changed to dBu (for unterminated).]

+4 dBu Standard pro audio voltage reference level equal to 1.23 Vrms.

0 dBV A voltage reference point equal to 1.0 Vrms.

-10 dBV Standard voltage reference level for consumer and some pro audio use (e.g. TASCAM), equal to 0.316 Vrms. (Tip: RCA connectors are a good indicator of units operating at -10 dBV levels.)

0 dBm A power reference point equal to 1 milliwatt. To convert into an equivalent voltage level, the impedance must be specified. For example, 0 dBm into 600 ohms gives an equivalent voltage level of 0.775 V, or 0 dBu (see above); however, 0 dBm into 50 ohms, for instance, yields an equivalent voltage of 0.224 V -- something quite different. Since modern audio engineering is concerned with voltage levels, as opposed to power levels, the convention of using a reference level of 0 dBm is obsolete. The reference levels of 0 dBu, or -10 dBV are the preferred units.

0 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. See: Overs. The Full Scale value is fixed by the internal data converter design, and varies from model to model.

de-emphasis See: pre-emphasis

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 Related to a signal that has been delayed in time in reference to it origination.  May refer to any of the following: 1. A signal processing device or circuit used to delay one or more signals by a controllable amount of time. This feature is 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. Good delay circuits are frequency independent, meaning the specified delay is equal for all audio frequencies (constant group delay). Delay circuits based on digital sampling techniques are inherently frequency independent and thus preferred. 2. MI. Digital audio delay circuits comprise the heart of most all "effects'' boxes sold in the MI world. Reverb, flanging, chorusing, phasers, echoing, looping, etc., all use delay in one form or another and are therefore some times referred to as Delays. 3. Sound Reinforcement. Acousticians and sound contractors use signal delay units 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. 4 Slight delay that sometimes occurs when transferring video, audio and data signals over long distances. When offensive, may be corrected using a digital echo canceling circuit.

DHCP (Dynamic Host Configuration Protocol) - Servers that permit multiple devices to share a limited group of IP addresses by assigning and reserving a temporary address form the pool of addresses to a device as needed.

Digital A device, process or protocol based on digitization of information  to enable storage, transportation or processing.

digital audio data compression Any of several algorithms designed to reduce the number of bits (hence, bandwidth and storage requirements) required for accurate digital audio storage and transmission. Characterized by being "lossless" or "lossy." The audio compression is "lossy" if actual data is lost due to the compression scheme, and "lossless" if it is not. Well designed algorithms ensure "lost" information is inaudible.

digital audio The use of sampling and quantization techniques to store or transmit audio information in binary form. The use of numbers (typically binary) to represent audio signals.

digital clipping See: 0 dBFS

digital filter Any filter accomplished in the digital domain.

digital hybrid See: hybrid

digital signal Any signal which is quantized (i.e., limited to a distinct set of values) into digital words at discrete points in time. The accuracy of a digital value is dependent on the number of bits used to represent it.

digitization Any conversion of analog information into a digital form.

DIN Acronym for Deutsche Industrie Norm (Deutsches Institut fur Normung), the German standardization body. Often used to describe European connectors.

Diode A two terminal electronic device that will pass reasonably unimpaired electrical current in one direction but provide a high degree of resistance to current flow in the other direction.  Diodes are often found in Rectifier (converts AC current to intermittent DC current) circuits.

disc The term used for any optical storage media. Originally popularized to refer to phonograph records. From Latin discus, the term refers primarily to audio and video storage systems, such as compact discs, laser discs, etc., but the advent of CD-ROMs and computer optical storage units blurs this distinction. Compare with disk

discrete Constituting a separate thing; distinct, or a set of distinct things.

discrete Fourier transform (DFT) 1. A numerical method of calculating the coefficients of the Fourier series from a sampled periodic signal. 2. A DSP algorithm used to determine the Fourier coefficient corresponding to a set of frequencies, normally linearly spaced. See: Fourier theorem.

disk The term used for any magnetic storage media such as computer diskettes or hard disks. From Greek diskos, the term refers primarily to non-audio digital data storage, but the advent of hard disk digital audio recording systems fogs this up somewhat. Compare with disc

Distortion  any departure of the output signal waveform from that which should result from the input signal waveform's  Distortion may result from many mechanisms, including nonlinearities in active and passive components.

distribution amplifier A splitter with buffer amps on each output.

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

DIY Acronym for do-it-yourself, usually referring to various hobbies, especially audio-related.

Dolby Digital Dolby's name for its format for the digital soundtrack system for motion picture playback. Utilizes their A C-3system of digital compression. The signal is optically printed between the sprocket holes. Now being introduced to Home Theater on laser disc and DVD. Dolby Digital may use any number of primary audio delivery and reproduction channels, from 1 to 5, and may include a separate bass-only effects channel. The designation "5.1" describes the complete channel format. Surround decoder systems with Dolby Digital automatically contain Dolby Pro Logic processing to ensure full compatibility with the many existing program soundtracks made with Dolby Surround encoding.

DOS (pronounced "doss") (disk operating system) A software program controlling data in memory, disk storage, running programs and I/O management.

downward expander See: expander

dropout An error condition in which of a signal are incorrect or lost from a digital medium. Occurs with digital and analog - audio or video.

DSD™ (Direct Stream Digital™) Joint trademark of Sony and Philips for their "Super Audio CD" proposal for the next generation CD-standard. Sony and Philips have split form the DVD ranks to jointly propose their own solution comprised of a 1-bit, 64-times oversampled direct-stream digital Super Audio CD format. The Super Audio CD is a hybrid disc comprising two layers: a high density (HD) DSD layer in the middle, 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 HD layer, while the middle layer is read by the HD laser delivering high-quality, multichannel sound without sacrificing backward compatibility. The HD layer has three tracks: The innermost is for two-channel stereo; the middle is a six-channel mix; and the outer is for such additional infomation as liner notes, still images and videro clips. Maximum playing time is 74 minutes.

DSL (Digital subscriber Line) Command "Last Mile" technology used to piggyback reasonably high speed data services over standard twisted copper telephone (POTs) lines between the telephone switching station and individual users residences.

DSP (digital signal processing) A technology for signal processing that combines algorithms and fast number-crunching digital hardware, and is capable of high-performance and flexibility.

DTE (Data Terminal Equipment) Within the RS-232 standard, the equipment comprising the data source, the receiver, or both - e.g., personal computers or terminals. See: DCE. The main difference between DTE and DCE is the wiring of pins 2 and 3, thus the need for a null modem cable when tying two computers together.

ducker A dynamic processor that lowers (or "ducks") the level of one audio signal based upon the level of a second audio signal. A typical application is paging: A ducker senses the presence of audio from a paging microphone and triggers a reduction in the output level of the main audio signal for the duration of the page signal. It restores the original level once the page message is over.

duplex Pertaining to a simultaneous two-way independent transmission in both directions. This may be accomplished with two channels, one in each direction, or in digital circuits by time sharing a single channel. Often referred to as "full duplex" which is redundant. See also: half-duplex.

DVD (latest info says "DVD" no longer stands for anything! It used to mean "digital versatile disc" - and before that it meant "digital video disc") A new type of 12-centimeter (4.72") compact disc (same size as audio CDs and CD-ROMs) that holds 10 times the information. Capable of holding full-length movies and a video game based on the movie, or a movie and its soundtrack, or two versions of the same movie - all in sophisticated discrete digital audio surround sound. 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. It is backwards compatible, and expandable to two-layers holding 8.5 gigabytes. Ultimately two discs could be bounded together yielding two-sides, each with two-layers, for a total of 17 gigabytes. There are four versions: DVD-Video (movies), DVD-Audio (music-only), DVD-ROM and DVD-RAM (games and computer use). The DVD-Audio standard is still being defined.

DVI (Digital Visual Interface) A recent connection standard for linking a computer video card to desktop, projection or wall mounted displays using a digital signal.  Also defines a standard 24 pin connector for DVI connections.

DWDM ( dense wavelength-division multiplexing ) A technology that puts data from different sources together on a single optical fiber, with each signal carried at the same time on its own separate light wavelength. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a lightstream transmitted on a single optical fiber. Each channel carries a time division multiplexed (TDM) signal. In a system with each channel carrying 2.5 Gbps (billion bits per second), up to 200 billion bits can be delivered a second by the optical fiber. DWDM is also sometimes called wave division multiplexing (WDM).

dynamic controllers (or dynamic processors) A class of signal processing devices used to alter an audio signal based solely upon its frequency content and amplitude level, thus the term "dynamic" since the processing is completely program dependent. The two most common dynamic effects are compressors and expanders, with limiters, noise gates (or just "gates"), duckers and levelers being subsets of these. Another dynamic controller category includes exciters, or enhancers.

dynamic microphone A microphone design where a wire coil (the 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. Works in almost the exact opposite of a dynamic loudspeaker where an electrical voltage is applied to the voice coil attached to a large cone (diaphragm) causing it to move in a magnetic field, thus creating a change in the immediate sound pressure. In fact, under the right circumstances, both elements will operate as the other, i.e., a dynamic loudspeaker will act as a microphone and a dynamic microphone will act as a loudspeaker -- although not too loud.

dynamic range The ratio of the loudest (undistorted) signal to that of the quietest discernible (often the system noise level) signal in a unit or system as expressed in decibels (dB). Dynamic range is another way of stating the maximum S/N ratio. With reference to signal processing equipment, the maximum output signal is restricted by the size of the power supplies, i.e., it cannot swing more voltage than is available. While the minimum output signal is determined by the noise floor of the unit, i.e., it cannot put out a discernible signal smaller than the noise. Professional-grade analog signal processing equipment can output maximum levels of +26 dBu, with the best noise floors being down around -94 dBu. This gives a maximum dynamic range of 120 dB - pretty impres/sive numbers, which coincide nicely with the 120 dB dynamic range of normal human hearing (from just audible to uncomfortably loud).

dyne A unit of force, equal to the force required to impart an acceleration of one centimeter per second per second to a mass of one gram. Used to Define dB-SPL