facsimile

fading Accidental or deliberate variation of signal amplitude. Random amplitude variations occur in the signal input to a radio receiver as a result of the vagaries of radio-wave propagation and AGC systems are used in receivers to minimise the effects of such variations.

    Deliberate variations of signal amplitude are employed in sound and vision mixing equipment where programme material is introduced by bringing the signals up to standard amplitude (known as fading up or fading in). Similarly programme material is removed by reducing the signal amplitude to zero (known as fading out).

fader Control used for fade-in and fade-out effects. See fading.

failure Cessation of the ability of a component or equipment to carry out the required function.

failure rate The number of failures per unit time.

fall time A measure of the steepness of the trailing edge of a pulse waveform. More specifically it is the time taken for the instantaneous amplitude to change from 90% to 10% of the peak value as illustrated in Figure F.1.

    The ability of a circuit to bring about changes in voltage or current as required in the reproduction of pulses is determined by the high-frequency response of the circuit and the following simple relationship exists between fall time and upper frequency limit f_max:

Thus to reproduce a pulse with a 0.1-μs fall time an amplifier requires an upper frequency limit of at least 5 MHz.

    In a simple RC or RL circuit, decreases in voltage or current are exponential in form and for such a change the fall time is simply related to the time constant according to the approximate relationship

fan in In logic circuitry the maximum number of outputs that may be connected in parallel at the input to a logic element whilst still permitting normal operation of that element.

fan out In logic circuitry the maximum number of inputs which can be connected in parallel at the output of a logic element whilst still permitting normal operation of that element.

Farad The unit of capacitance, symbol F. A capacitor is said to have a capacitance of 1F when 1 coulomb of charge on each plate sets up a potential difference between them of 1 Volt. The farad is an inconveniently large unit and the microfarad (μF) and the picofarad (pF) are more commonly used.

Faraday cage An earthed electric screen of wire mesh or parallel conductors which shields equipment within it from the effects of electric fields but is transparent to magnetic fields.

Faraday’s law Of electromagnetic induction states that the EMF induced in a circuit is directly proportional to the rate of change of the magnetic flux linked with the circuit.

fatal error An error which necessitates the immediate termination of the current application, but does not necessarily affect other tasks in the computer.

fax Same as facsimile (2).

feed Of an active device the mean current taken by the device from the DC power supply.

feedback The return of a portion of the signal at any stage in an amplifier to an earlier stage so as to augment or reduce the signal at that stage. If the returned signal is in phase with the signal at the earlier point the feedback is said to be positive and its effect is to increase gain. If the returned signal is in phase opposition to the signal at the earlier point the feedback is said to be negative and its effect is to reduce gain. See negative feedback, positive feedback.

feeder In general any transmission line carrying electrical power. In particular the type of line used to connect an antenna to a transmitter or a receiver.

ferrite A homogeneous non-metallic material with high permeability and high electrical resistance with the general formula MFe₂O₄ where M is a divalent metal such as nickel, cobalt or zinc. Ferrite cores have very low eddy-current losses making them suitable for use in RF inductors and transformers and in the yokes of deflection coils. Ferrites are of ceramic nature and cannot be sawn or drilled. They are produced in the form of cups, rods and rings from which magnetic circuits can be constructed.

ferrite bead store Same as bead store.

ferromagnetism A property of certain materials subjected to a magnetising field which causes induced magnetism which greatly aids the applied field. Such materials are strongly attracted to a magnetic pole and have high effective permeabilities which are greatly dependent on the applied magnetising field. Iron, cobalt, nickel and certain alloys are typical examples of ferromagnetic materials. See diamagnetism, paramagnetism.

fibre optics The study of the transmission of light along thin fibres of transparent material. Light can be transmitted along such a fibre by a succession of internal reflections and transmission is improved if the fibre material is clad in a material of lower refractive index. Transmission is unaffected if the fibre is bent and thus optical fibres provide a method of conveying light from one point to another and around bends if necessary.

    By using an assembly of a very large number of very fine fibres it is possible to convey optical images from one point to another and around bends. This is useful in surgery for internal examinations of the body and there are also television applications.

    Because of the enormous bandwidth obtainable with optical fibres they are also used as a means of communication over long distances.

fidelity The degree to which a transmission system or part of a system reproduces at its output the essential characteristics of the signal applied to its input. The term is often applied to audio equipment such as amplifiers and loudspeakers.

field In TV the part-picture composed by the lines described in one downward sweep of the scanning agent. In twin-interlaced scanning, as used in most TV systems, two vertical sweeps are needed to cover all the lines of the picture and each field is thus a half-picture. If the lines are numbered in sequence from the top to the bottom of the picture, the scanning agent first covers lines 1, 3, 5, etc (this being known as the odd field) and then returns to the top of the picture to cover lines 2, 4, 6, etc (this being known as the even field). The field was formerly known as a frame.

field blanking In TV the suppression of the picture signal during the interval between successive fields.

field-effect transistor (FET) A transistor consisting essentially of a channel of semiconductor material, the resistance of which can be controlled by the voltage applied to one or more input terminals (gates).

    A feature of the FET is that the input resistance of the gate is very high so that input current is negligible. FETs are therefore voltage-operated devices, a property they share with the electron tube. FETs differ from bipolar transistors in that the output current is carried by only one type of charge carrier, e.g. electrons in n-channel devices. See insulated-gate field-effect transistor, junction-gate field-effect transistor.

field emission The release of electrons from an unheated surface under the action of a strong magnetic field.

field frequency In TV the number of vertical sweeps made by the scanning beam in one second. For interlaced scanning it is equal to the product of the picture frequency and the number of fields per picture. In most TV systems the field frequency is approximately equal to the frequency of the supply mains. In Europe this is 50 Hz.

field sync signal In TV the signal transmitted at the end of each field to initiate vertical flyback of the scanning beam in receivers so keeping field scanning at the receiver in step with that at the transmitter.

    In most TV systems the field sync signal consists of one or more pulses (each longer than the line sync pulse) and so arranged that the continuity of the line sync pulses is not interrupted. See Figure F.2.

field time base In TV the circuits responsible for generating the signals causing vertical deflection of the scanning beam.

filament The cathode of an electron tube which is heated to give the required electron emission by passing an electric current directly through it.

    For low-power tubes such as those used in battery-operated equipment, such cathodes consisted of a fine wire consuming say 0.1 A at 1. 4 V or 2 V and coated with metallic oxides to yield copious electron emission at a low temperature.

    For tubes of higher power such as mains rectifiers the filament was often in the form of a tape consuming say 2 A at 5 V. For high-power transmitting tubes the filament is of pure tungsten or thoriated tungsten consuming up to several hundred amps at say 30 V.

file In computers and data-processing equipment a series of bytes given a unique identity (filename) for storage in a filing system.

fileserver A computer in a Local Area Network which incorporates a high-capacity hard disk on which storage space is provided for the users of the network. In practice some storage space may be declared ‘public’ this contains files which any network user may access. Other storage space is dedicated to individual users of the network, access to these being protected by a password system. The fileserver may also control other shared resources such as a network printer.

filing system In computers and data processing equipment a system which stores programs and data as files in an orderly manner which permits their rapid location for retrieval. The storage media most commonly used are magnetic disks.

filter A network which passes signals with frequencies within certain bands (passbands) with little attenuation but greatly attenuates signals within other bands (stopbands). The block symbol for a filter is given at Figure F.3.

firing voltage The minimum direct voltage applied to a gas-discharge tube which will initiate the discharge.

firmware Software resident in read-only memory in a computer system.

first generation computer See digital computer.

fixed-point arithmetic, computation or representation In computers a form of presentation in which a number is displayed as a single set of digits, the decimal (binary or other) point being fixed in position with respect to the set.

fixed disk Same as hard disk.

fixed storage Same as read-only memory.

flag In computers and data-processing equipment a bit or byte used to signal the occurrence of a particular condition or event, e.g. an error.

flare In an optical or TV image, spurious areas caused by scattering of light in the camera or picture tube.

flash arc In an electron tube an arc between the electrodes causing a violent increase in cathode emission which, if maintained, can destroy the tube. The effect is thought to be caused by irregularities on electrode surfaces which cause local concentrations of electric field sufficient to ionise the residual gas.

flash EPROM A type of EPROM which can be erased and re-used. Unlike EEPROM, however, the whole memory chip must be erased at once. See erasable, programmable, read-only memory.

flashing Same as reactivation.

flashover Arc discharge between two conductors or between a conductor and earth caused by excessive voltage or a breakdown of the insulation. It may occur between the electrodes of an electron tube.

flash test Application for a brief period of a voltage considerably greater than the working voltage of a component or equipment to test its insulation resistance.

flat random noise A noise signal in which the components have approximately equal amplitudes over the frequency range of interest.

Fletcher-Munson curves Equal-loudness curves plotted in terms of sound intensity and frequency (Figure F.4).

    Each curve is an equal-loudness contour, i.e. it shows how the sound intensity necessary to give a particular value of loudness level varies over the audio band. The lower contours show that at low loudness levels considerably more power is needed to make very low frequency signals and very high-frequency signals as loud as signals around 1 and 2 kHz. This is another way of saying that the human ear is most sensitive at frequencies around 1 and 2 kHz.

    As loudness level is increased, however, the contours become flatter showing that the sound intensity needed to give constant loudness does not vary greatly with frequency. In some audio amplifiers the volume control is made frequency discriminating to allow for the change in shape of the contours with change in loudness level.

    The number of phons for each contour is numerically equal to the intensity level at the point where the contour crosses the 1-kHz ordinate; this stems from the definition of the phon.

flicker In TV, unwanted regular variation in the brightness of the reproduced picture. Flicker can be annoying when the field frequency is low and, in fact, this consideration sets a lower limit to the field frequency which can be used. It was for this reason that interlaced scanning was adopted because this permits a high field frequency (minimising flicker) whilst allowing a low picture frequency (minimising bandwidth).

flicker effect In electron tubes, random variations of the output current causing noise which is inversely proportional to frequency.

flip-flop (US) Same as bistable circuit.

floating Not connected to any source of potential. For example the potential of the open-circuited grid of an electron tube could be described as floating.

floating-carrier modulation A system of amplitude modulation in which the carrier is modulated in the normal manner and is simultaneously controlled by the envelope of the modulating signal so that the carrier amplitude is at all times just large enough to accommodate the modulation envelope.

    Thus the modulation depth remains constant at nearly 100% no matter what the amplitude of the modulating signal. Such a system gives more efficient transmitter operation than if the carrier amplitude is kept constant as in conventional amplitude modulation.

floating-point arithmetic, computation or representation In computing a form of presentation in which a number x is displayed as two numbers y and 2 such that x = yb² where b is usually 2 or 10. y is known as the fixed-point part, z as the exponent and b is the base. As an example the number 12500000 could be written 125,5 (representing 125 × 10⁵).

floppy disk (diskette) A demountable data storage and distribution medium used in microcomputers and word processors. The magnetic disk is enclosed in a rectangular plastic sleeve which, on the earlier 8-inch and 5¼-inch disks, was flexible, hence the term floppy. The 3½-inch disks now most widely used are housed in rigid plastic cases which are more robust and incorporate a metal shutter over the aperture for the read/write heads, affording better protection against grease and dust. Each side of the disk carries 80 tracks spaced at 135 tracks per inch, giving a storage capacity, depending on the operating system and disk quality, of 720 kbytes, 800 kbytes, 1.44 Mbytes, 1.6 Mbytes or 2.88 Mbytes.

floptical disk and drive A magnetic data storage system using media which resemble 3½-inch floppy disks, but which use an optical track to allow more accurate tracking of the read/write heads. Storage capacity is 21 Mbytes per disk. The drive can also read and write to standard 3½-inch floppy disks.

flow angle Same as angle of current flow.

Floyd and Steinberg error diffusion See dithering.

fluctuation noise Same as random noise.

fluorescence The emission of light from materials irradiated by energy of a higher frequency or bombarded by electrons. The effect differs from phosphorescence in that the light emission lasts only for the duration of the stimulus.

flutter Distortion in sound reproduced from disk, film or tape and caused by undesired rhythmic speed variations on recording or reproduction. It is usually caused by eccentric or unbalanced driving of the film, tape or turntable. The variations occur at a rate exceeding approximately 20 Hz and cause a roughness in the sound quality. See wow.

flux A term used to describe a flow of particles, photons or the lines of force of an electric or magnetic field.

flux density The number of particles, photons or lines of force of an electric or magnetic field which pass through unit area of a surface normal to the direction of the beam or field.

flyback In cathode ray tubes the rapid return of the electron beam to its starting point at the end of each trace. In TV there is a horizontal flyback at the end of each scanning line and a vertical flyback at the end of each field.

focusing In electron optics the process of converging the beam to minimum cross section (ideally to a small spot) on the target of the tube.

foldback A protective system for power suppliers which ensures, in the event of a sustained overload, that the output current is automatically reduced to zero thus reducing the possibility of damage to the supplier and/or to the load.

forbidden band In an energy-level diagram a range of electron energies lying between the permitted bands such as the conduction band and the valence band.

    No electron can have an energy corresponding to that of a forbidden band; it must have lower energy and be, for example, in the valence band or higher energy and be in the conduction band. Electron energy is therefore in steps and if sufficient energy can be given to electrons in the valence band to lift them into the conduction band, the element can be made conductive. See Figure E.12.

forced-air cooling A method of removing heat from a component or equipment by blowing air against it or against finned radiators in good thermal contact with it.

forced oscillation See oscillation.

format (1) In data transmission the way in which the code characters are grouped into blocks. (2) A process known as formatting must be applied to magnetic storage media such as floppy and hard disks before they can be used to store data and programs. The process lays down the pattern of tracks (sometimes called cylinders) divided into sectors which the operating system uses to locate files and to check for errors and also creates the root directory.

form factor Of an alternating quantity the ratio of the root mean square value to the mean value of the positive or negative half cycle. For a sinusoidal waveform the RMS value is 0.71 (1/√2) and the mean value 0.63 of the peak value. The form factor is hence 0.71/0.63 = 1.11.

Forth See computer language.

FORTRAN See computer language.

forward automatic gain control An AGC system in which the gain of transistors is reduced by use of forward control bias. An essential feature of the circuit is a resistor in the collector circuit (R₁ in Figure F.5) which causes the collector voltage to fall as the forward bias increases. The transistors used for this application are so designed that their collector characteristics become more crowded at low collector voltages thus decreasing gain.

forward current In a rectifier or semiconductor diode the current which flows through it in the forward direction.

forward power dissipation In a semiconductor diode the power dissipated within the diode by the forward current.

forward recovery time Of a semiconductor diode in a specified circuit the time taken for the forward current to reach a specified value after the application of forward voltage.

Foster-Seeley discriminator A detector for phase- or frequency-modulated signals in which the centre tap of the tuned secondary winding of a transformer is coupled to the primary winding and in which two diodes in series opposition are connected across the secondary winding, their net output giving the modulation-frequency output.

    A typical circuit diagram of a Foster-Seeley discriminator is given in Figure F.6. The effect of the two types of coupling between the primary and the secondary windings is that an increase in input-signal frequency from its unmodulated value causes the voltage at one end of the secondary winding to increase whilst that at the other end decreases. One diode therefore yields a larger output than the other. When the frequency of the input decreases from the unmodulated value this process is reversed and the second diode now gives the larger output and it is of opposite polarity to that of the first diode. Thus a modulation-frequency output is obtained from the detector.

    A disadvantage of this form of discriminator is that it responds to amplitude modulation of the input signal. In a receiver therefore it must be preceded by one or more limiter stages to give protection against unwanted noise signals. See amplitude limiter.

Foucault current Same as eddy current.

Fourier analysis A mathematical method of determining the number, amplitude, frequency and phase of the components of a given repetitive complex waveform.

    The French mathematician Fourier showed that any repetitive waveform could be synthesised by adding together a number of sinusoidal waves of suitable frequency, amplitude and phase. The frequencies of the waves are simple multiples of the repetition frequency (i.e. the fundamental frequency) of the original waveform.

four-quadrant multiplier A multiplier which operates normally irrespective of the sign of the input signal. See two-quadrant multiplier.

fourth generation computer See digital computer.

Fowler-Norheim tunnelling See EEPROM.

fractal An image, usually mathematically generated, in which the detail appears similar at any level of magnification. A fern leaf provides an example from Nature of a fractal pattern. Fractal-based techniques offer exciting possibilities for data compression.

frame (US) In TV and cinema film practice, one complete picture.

frame frequency (US) Same as field frequency.

frame grabber Same as video digitiser.

frame-grid tube An electron tube with a control grid of fine pitch, wound with fine wire and situated very close to the cathode so giving higher values of mutual conductance than is possible with grids of more conventional construction. Mutual conductances as high as 15 mA/V can be obtained in a small receiving tube by using frame-grid construction.

framing In digital data transmission systems a procedure used to ensure that the receiver can identify the subchannel to which each set of data belongs.

Franklin oscillator An oscillator in which the frequency-determining circuit is coupled via very small capacitances to the input and output of a two-stage electron tube amplifier. Because of the double phase inversion in the amplifier, the output is in phase with its input so giving the positive feedback necessary for oscillation. By making the coupling capacitances to the tuned circuit only just large enough to sustain oscillation the effect of the amplifier on the LC circuit can be minimised so giving good frequency stability. A circuit diagram of a Franklin oscillator is given in Figure F.7.

free oscillation See oscillation.

freeware Software which may be freely copied, distributed and used subject to conditions described in a text file that forms part of it. Unlike public domain software, its originator retains the copyright and may demand payment if, for instance, a software distributor wishes to incorporate it or a part of it into a commercial package.

freeze frame (1) On a video tape recorder, the facility to stop the tape temporarily during playback while the head continues scanning, thus producing a still picture. In practice this facility often has a time limit after which normal playback is resumed; this is to prevent excessive wear of the tape surface. (2) In computing, a system of primitive animation in which successive frames appear too infrequently to give the illusion of true movement.

frequency band In general the range of frequencies between specified upper and lower limits. In particular one of the following frequency ranges which are agreed internationally.

frequency changer A circuit which accepts a modulated signal at a particular carrier frequency and transfers the modulation to a different, usually lower, carrier frequency. Such circuits are used in superheterodyne receivers to transfer the modulation of all received signals to the intermediate frequency. Most frequency changers consist of an oscillator (at a frequency f₁), the output of which is fed, together with the input signal (frequency f₂), to a mixer stage the output of which contains the required difference frequency (f₁–f₂). Many practical frequency changers use separate active devices for oscillation and mixing although it is possible to make one device serve both purposes as in the self-oscillating mixer. The block symbol for a frequency changer is given in Figure F.8.

frequency converter (US) Same as frequency changer.

frequency deviation In angle modulation the peak difference between the instantaneous frequency of the modulated wave and the carrier frequency.

frequency discriminator Same as discriminator.

frequency divider A circuit which accepts a signal with a frequency f and gives an output at a frequency f/n, where n is an integer. See counter.

frequency division multiplex (FDM) The process of transmitting two or more signals along a common path by using different frequency bands for each of them. Each signal modulates a separate carrier and the carrier frequencies are spaced to avoid mutual interference between sidebands.

frequency modulation (FM) Methods of modulation in which the frequency of the carrier wave is made to vary in accordance with the instantaneous value of the modulating signal.

    Figure F.9 (a) illustrates a carrier wave of constant amplitude and constant frequency and Figure F.9 (b) shows the effect of frequency modulation. The amplitude of the modulating signal determines the extent of the change in carrier frequency and the frequency of the modulating signal determines the number of times per second the carrier frequency is swept above and below its nominal value.

    Frequency modulation is used in sound broadcasting in the VHF bands and in most TV broadcasting services for the associated sound channel. An important feature of frequency modulation is that the carrier amplitude is unaffected by modulation: thus by making frequency modulation receivers immune to amplitude changes in the received signal much interference can be eliminated so making frequency-modulated reception noise-free.

frequency multiplier A circuit which accepts a signal with a frequency f and gives an output at a frequency nf, where n is an integer. The commonest examples of frequency multipliers are frequency doublers and frequency triplers.

    One form of frequency multiplier is a device with a markedly non-linear input-output characteristic (e.g. an active device operating in class C). This generates an output rich in harmonics of the input frequency and the desired component can be selected by a circuit resonant at the required frequency.

frequency pulling The displacement of the frequency of an oscillator towards the frequency of an external applied signal. The effect becomes more marked as the frequency of the applied signal approaches that of the oscillator.

frequency range The range of frequencies over which a component or equipment operates satisfactorily. For example an AF amplifier may have a frequency range of 30 Hz to 15 kHz.

frequency response The variation in output amplitude of a component or equipment as the frequency of a constant-amplitude input is varied over the working frequency range of the equipment. The response is usually displayed as an amplitude/frequency curve and shows up any attenuation distortion of the equipment.

frequency shift keying (FSK) A form of frequency modulation in which, without interrupting the carrier, its frequency is switched between two predetermined values termed the mark and space frequencies.

frequency stability A measure of the constancy of the frequency of an oscillation. The frequency stability of an oscillator is important in many applications, e.g. when it is used as the carrier source in a transmitter which must stay on its assigned carrier frequency within very close tolerance.

frequency standard A highly stable crystal-controlled oscillator the output of which can be calibrated against National Standard Frequency radio transmissions.

frequency swing In angle modulation the difference between the maximum and minimum values of the instantaneous frequency of the modulated wave.

fringing Distortion of the electric field at the edges of a parallel-plate capacitor. The lines of force are normal to the plates over most of the area of the plates but at the edges they tend to bulge outwards, a feature that may be important in the design of electrostatic deflecting plates in a cathode ray tube.

front end (1) Of an equipment, that section of it which handles the input signal(s). (2) Of a computer application, the user interface, i.e. the screen display and input facilities with which the user interacts.

front porch In a TV signal the period of blanking level immediately preceding the line sync signal. See Figure F.10.

full adder Same as adder.

full motion video Smooth animation in a full-colour picture occupying the whole screen. This contrasts with the jerky freeze-frame animation or fractional-screen pictures offered in some computer animation systems.

full radiator An ideal radiator and absorber of radiation. Its radiation in any part of the spectrum is the maximum obtainable from any radiator at the same temperature. The nearest practical form of full radiator is a cavity with opaque walls maintained at a constant temperature and with a small opening for observation. It was formerly known as a black-body radiator.

full-wave rectification Rectification in which power for the load is taken from the AC supply during both half-cycles of the supply. One circuit for a full-wave rectifier is given in Figure F.11. Diode D₁ conducts during positive half-cycles of the secondary voltage and diode D₂ during negative half-cycles, the diodes being so connected that current flows through the load in the same direction during both half-cycles. As shown in Figure F.12 a bridge arrangement of rectifiers is often used for full-wave rectification, this circuit avoiding the need for a centre tap on the transformer secondary winding.

function In computing, a subroutine which is named and which returns either a numeric value or a character string. The usefulness of functions lies in that, once defined, they can be treated in much the same way as variables; for example, an instruction to display a function executes the code in the function and then displays the value it has returned.

function keys On a computer keyboard the keys normally numbered F1 to F12 (and sometimes to F24). They have no fixed function, but operating systems and programs may assign temporary functions to them. For example, F1 is often used to call up an on-line help system.

fundamental frequency Of a regular complex waveform the repetition frequency of the wave.

    Fourier analysis of a complex wave yields a series of components the frequencies of which are multiples of the repetition or fundamental frequency of the wave. It is possible for the fundamental frequency component to be missing but in a complex sound signal it is the fundamental frequency, whether present as a discrete component or not, which determines the pitch of the sound.

fuse A protective device which heats and melts so interrupting a circuit when the current exceeds a certain value which depends on the material and the cross-sectional area of the fuse. The graphical symbol for a fuse is given in Figure F.13.