Glossary of terms [Introducing Digital Audio]

A-D Analog to digital conversion Address A location on a magnetic or optical disc or in a memory. Each address location is identified by its address number, usually referred to as the address . Aliasing The generation of spurious frequencies caused by using a sampling rate that is less than twice the highest frequency of signal present . For example, if a 15 kHz signal is sampled at 25 kHz, there will be a strong 5 kHz component, equal to the difference between 25 kHz and 2 x 15 kHz .

Analog Using an infinite number of signal levels between maximum and minimum (usually zero) . Asynchronous Not tied to a fixed rate of repetition . An asynchronous signal can occur at intervals which do not coincide with a fixed-rate clock pulse . ATF Automatic track following, the system used in the R-DA T player to ensure that the rotary heads follow the recorded track. This uses a set of signals that is recorded along with the digital data and which are passed to the servo controls to ensure that the tape is correctly positioned with respect to the heads.

Audio Referring to the frequency range of human hearing, in the range of 30 Hz to 20 kHz.

Azimuth The angle of the slit in a tape-head to the line of a perpendicular drawn to the tape edge, normally zero. A difference in azimuth between recording and replaying heads will cause a large loss of signal amplitude . This is used deliberately in rotating-head machines to ensure that interference between adjacent tracks is minimized.

Bandpass A filter circuit which passes a range of frequencies, rejecting frequencies below the lower limit of the band and above the upper limit . A 25Hz to 15kHz bandpass is often applied to audio signals . Bias An additional voltage or signal applied so as to use the linear part of a device or recording system. Voltage or current bias is required for semiconductors, AC signal bias for analog magnetic recording systems . Digital systems require no biasing.

Binary Using only two digits, levels or codes, as opposed to the scale of ten used in normal counting.

Bit A binary digit, 0 or 1. The position of a bit in a binary number determines its significance (its value as a power of 2). Bitstream A technique pioneered by Philips in which a binary number is represented by a set of signals of two possible levels . The average value of these signals corresponds to the analog voltage which was encoded by the binary number . Byte A set of 8 digital bits, the normal unit of memory used in small computers . CD Compact disc, a recording made using optical markings on a plastic

disc, formed and also read by a laser beam. The markings are digitally coded to represent samples of the amplitude of sound waves.

Checksum A number derived from arithmetical actions on data and used to check that the data has not been corrupted after transmission or recording and replay.

Clock A pulse which is repeated at a steady rate in order to control the timing of actions . The clock pulses are usually derived from crystal

controlled oscillators, and rates of up to 33 MHz are used in computing circuits.

CMOS Acronym of complementary metal oxide semiconductor, applied to ICs which use both N-channel and P-channel MOSFET technology in device pairs Combinational circuit A circuit whose output depends on the combination of inputs that is present at any instant . Also known as a logic or gating circuit.

Counter A circuit whose output depends on the number of pulses at a input, so that the state of the output can be used as a number count.

CRC Cyclic redundancy check, a system of recording a checksum number along with data in order to detect and in some cases correct any corruption of the data.

D-A Digital to analog conversion, the essential transformation at the player which permits the sound wave to be reconstructed from the digital number data . DASH Digital audio stationary head, a system of recording digitally

encoded sound using a large number of parallel tracks on tape .

DAT Digital audio tape, any system of recording digitally coded sound onto tape, using either multiple heads and fast tape speeds (S

DAT) or revolving heads and slow tape speeds (R-DAT) . Demultiplexer A circuit which separates combined streams of data into separate streams . Differentiator A circuit whose output represents changes in signal level only, with no response to steady signal levels.

Digital Using only two digital levels, labeled as 0 and 1.

Dither Addition of random signal (noise) in order to avoid perfectly steady signal conditions . Dropout A tape fault in which a small piece of magnetic materials is missing or faulty, causing a loss of signal at that point . Dynamic memory A form of memory which depends on storing charge in small IC capacitors . Because of leakage, the stored charge needs to be refreshed at intervals of a few milliseconds . Dynamic memories can be built in very large sizes, of the order of IM - 4M bits per chip.

EFM Eight to fourteen modulation, a method of recoding a byte of data so that the 0s and 1s fall into a pattern that is more suitable for recording.

Electrostatic Depending on electric charge. Any objects rubbing against each other will be electrostatically charged, and if both are insulators, the voltage levels obtained in this way can be very large, of the order of several kilovolts .

EPROM Electrically programmable read-only memory, a memory chip which can be programmed by electrical signals, see PROM. Excess-3 Code A form of binary code in which 3 is added to each number before binary coding. In this code, there is no zero number (0 codes as 001 1), some arithmetic actions are simpler, and the sum of a single digit number and its complement is always 9.

Fanout The ability of a digital IC to drive others, expressed as the number of inputs that can be driven from a single output.

FET Field-effect transistor, a switching and amplifying device that controls current flow in a ribbon of semiconductor by means of an electric field. See also MOSFET. Field In TV, one half-set of picture scan lines, either the odd-numbered lines or the even-numbered lines . Filter A circuit which will pass one range of frequencies, the passband, and reject all others (the stop-bands). See also lowpass, highpass, bandpass . Flip-flop A circuit whose output can be switched to either state (0 or 1) by an input.

Frame In TV, one complete set of picture scan lines, composed of two fields.

Frequency Number of pulses or repetitions of a waveform per second.

Gate A digital circuit whose output depends on the combination of inputs that are present .

Glitch

In general, any false digital signal. In digital audio, a transient that occurs mainly in the D-A conversion which can be removed in a low-pass filter.

Gray code A system of coding numbers using 0 and 1 digits, but in which the change from one number to another is always small, one digit only, as a count is made.

High pass A filter circuit which passes only frequencies above a limit, rejecting lower frequencies.

Integrator A circuit which has little or no response to sudden changes in input, but smooths the input into a steady or slowly-changing output.

Interpolation Addition of information by taking an average value between two adjacent items . Lambda SLC A technique for digital to analog conversion, due to Denon, and meaning 'ladder-form multiple bias DIA super linear converter' . This uses 8 times oversampling to produce a 20-bit signal which is split into two streams to reduce the effect of cross-over biasing.

Laser Acronym of 'light amplification by stimulated emission of radiation' . A device which for an electrical input generates light of a single wavelength and with no sudden changes of phase . Linear Any system in which a graph of output plotted against input produces a straight line.

Logic A system for producing answers from questions. In digital circuits, a logic circuit will produce an output when the inputs are in a pre

determined pattern.

Lowpass A filter circuit which passes only lower frequencies, rejecting frequencies above the limit. A 15 kHz lowpass filter is often used in audio work to attenuate or reject higher frequencies which could cause trouble in circuits . MASH The name used for the Technics multi-stage noise shaping circuits, a form of bitstream D-A conversion.

Memory A circuit which retains one of two possible states once set. Memory can be volatile or non-volatile . MFM Modified frequency modulation, a method of coding binary data, particularly for computer magnetic discs, in which a 1 is coded by a transition (change of direction of magnetization), isolated zeros are ignored and a transition is placed between pairs of zeros. This reduces the DC content of the signal as compared to NRZ coding.

MFM has been superseded by EFM for digital audio work.

MOSFET Metal-oxide-semiconductor field effect transistor, a form of FET (q.v.) in which the control is applied to a gate electrode which is isolated from the semiconductor by a thin film of silicon oxide. No current is taken by the gate, so that MOSFETs can operate at very low power levels . In addition, the construction is particularly easy to use in integrated circuits.

Microprocessor An IC which carries out digital actions such as logic gating, counting and shifting in response to inputs of instructions (the program) and data .

MIDI Acronym of 'musical instrument digital interface', an agreed standard for linking electronic musical instruments to each other and to computers in order to control several instruments at once.

Modulation The alteration of signals into a form that can be transmitted or recorded.

Monochrome Of one single frequency, like the light from a laser. Also applied to black/white TV. Multiplex Any system of encoding more than one signal on a line . This includes frequency-multiplex, using several carriers at one, combined phase and amplitude modulation as used for color TV, and time-multiplex, in which the signals for different channels are sent at different parts of a timing cycle.

NMOS Acronym of 'negative-channel metal-oxide-semiconductor' . One of the three possible forms of MOSFET ICs, the others being PMOS and CMOS. The NMOS IC uses predominantly N-channel MOS

FET technology.

Noise Unwanted signal of any kind, usually of random frequency and amplitude . Noise immunity A measure, usually in terms of volts, of the ability of a digital circuit to ignore noise signals.

Noise shaper A circuit, used in bitstream D-A converters, whose action is to shift the frequency of noise in a digital signal so that on conversion the noise will be outside the audio range . This is done by reducing the number of parallel bits used to carry the data, increasing the number of serial bits and so the frequency of the digital signal . Non-volatile Applied to memory that retains information when its power supply is switched off . This is true of all magnetic memory, and also of ROM and some types of CMOS RAM. NRZ Non-return to zero, the conventional digital magnetic recording system in which magnetization in one direction represents logic 1 and magnetization in the opposite direction represents logic 0.

NTSC National Television Standards Committee, the US body which agreed the original compatible color TV standards in 1952.

Optical Making use of light, including infra-red and ultra-violet frequencies which are invisible to the human eye . Oversampling A method of increasing the performance of a D-A converter by inserting additional pulses into the digital signal between the original pulses. The effect is similar to that which would be produced by using a faster sampling rate in the original recording.

Parity A system of checking data by recording an extra bit in each byte.

The extra bit is used to check the number of 1s in the byte by determining if this number is odd or even.

PCM Pulse-code modulation, a system of converting analog signals to digital in which the amplitude of the analog signals is converted to a binary number, rather than to pulse amplitude, pulse frequency or pulse position. All digital audio systems use PCM. Photoresist A material which is used in the manufacture of printed circuit boards, I Cs and compact discs . The photoresist is normally soluble in an alkaline solution, but it becomes insoluble when exposed to light, so that a surface which has been exposed through a photographic negative can be 'developed' in an alkaline solution to leave a pattern of hardened resist. The exposed surface that is left can then be chemically etched, leaving a pattern which will remain when the remaining photoresist is removed.

PROM Programmable read-only memory, an IC which contains data that cannot be lost when the power-supply is turned off. In its most familiar form (EPROM), the PROM can be programmed by using signals of higher level than normal, and these signals will be retained until the PROM is 'washed' by exposure to ultra-violet light.

Pulse A short-duration change of electrical voltage or current from one level to another and then back again.

Pulse code modulation see PCM, above.

Quantization Analysis of a waveform into a number of set levels, the essential preliminary to any analog to digital conversion.

Quantum A unit of anything. Originally applied to the quantity action (energy x time) in Planck's Quantum Theory, and used in digital work to describe a unit of signal level . Quartz crystal A crystal of quartz cut and with metal film deposited as to be used in an oscillator whose frequency will be precisely set by the dimensions of the crystal.

Race hazard A problem of gate circuits, in which spurious signals are formed because the inputs to a gate arrive at slightly differing times . Race hazards can be eliminated by ensuring that all signals are synchronized to a master clock signal . RAM Random access memory, used now to refer to read-write memory, since all modern memory systems use random access. RAM is often volatile so that its contents will disappear when power is switched off.

Random Corresponding to no fixed pattern, such as numbers drawn from a hat . R-DAT Rotary-head digital audio tape, the system adopted for domestic DAT systems, using two rotating heads spinning in a plane that is slightly angled to the tape. This allows a low tape speed to be used along with a high head-to-tape speed; the same system as is used for video recording.

Red Book A data book which contains the specifications for the CD and other digital audio systems, provided to manufacturers who take out a license for using the system.

Redundancy The addition of excess information so that the loss of some data can be made up by using the rest of the data . Reed-Solomon A coding system that uses redundancy to detect and correct errors in data transmission.

Refresh The action of applying pulses to a dynamic memory chip to re

write the 1 bits of data . Register A temporary store for digital data, in which data can also be manipulated using addition, subtraction and bit shifting.

ROM Read only memory, a form of non-volatile memory which uses fixed connections in a chip to provide data that cannot be altered . Rotary head The video-recording system that uses a rotating drum carrying two or more heads which sweep across the tape at a small angle, typically 5° to 8°. This allows a high rate of scanning to be combined with a low tape speed.

Sampling The system of measuring the amplitude of a waveform at fixed intervals so that the samples can be converted into digital signals.

Saturation A state of complete magnetization of a magnetic material, in which no increase of magnetizing effort can produce more retained magnetism.

SCMS Serial copy management system, a form of coding used on DAT

machines which prevents a digital tape from being used to create another tape and so mass-produce perfect copies . S-DAT Stationary head digital audio tape, another name for DASH (q.v. ). Sequential circuit A circuit in which the output depends on the number of inputs rather than on their state at a given time . All counter circuits are sequential . Shift register An array of flip-flops connected so that the output bits are shifted from one flip-flop to the next when a clock pulse is received on all the units . Side band A range of frequencies produced when a signal frequency is modulated or sampled.

Side beam A part of the laser beam of a CD player, used to detect whether or not the main beam is aimed correctly. Two side-beams are used to ensure that the main beam is held in the centre of the track.

Significance The number that the position of a digital represents - the '2' in 120, for example represents the number of tens, and the T represents the number of hundreds . Smoothing The conversion from a waveform with a sawtooth or spiky outline into a smooth waveform, a process of integration.

Splicing Joining cut edges of tape so as to make an edited version in which the joins are undetectable.

Static memory A form of memory in which data is held as long as power is supplied, but without any need to refresh the data . Switching The sudden change from one state to another, such as conducting to non-conducting or non-conducting to conducting.

Synchronous Acting in time to a set of clock pulses which are delivered at regular intervals . Truth table A table of inputs and their corresponding outputs for a combinational circuit . TTL Transistor-transistor Logic, a form of construction used for early digital ICs, in which the inputs are to the multiple emitters of a transistor. The system is still used in modified form for logic circuits . Ultrasonic Using frequencies above the normal limit of audibility (about 20 kHz) . Very high ultrasonic frequencies, extending to tens of megahertz, are used in some equipment.

Video Relating to images, applied to image recording and transmission.

Volatile Applied to memory systems, meaning that all data will be lost when the power supply is switched off.

Waveform The graph of voltage or current plotted against time for an electrical signal . White noise A noise signal in which all frequencies are present to an equal extent, not weighted in any way (as pink noise would be) .

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