vacuum tube A tube which has been exhausted to a high degree of vacuum so that the amount of gas present in the envelope is too small to have any effect on the electrical characteristics. The tube may be sealed or continuously pumped. Currents in a vacuum tube are carried by electrons not ions.

valence The force which binds atoms together to form a molecule.

valence band In an energy-level diagram the range of energies corresponding to states which can be occupied by the valence electrons binding the atoms together.

valence electrons Electrons occupying the outermost orbits of an atom and which are shared when atoms combine to form molecules. See covalent bonds.

value (1) The numerical level of the primary property of a passive component, for instance, the number of Ohms of resistance offered by a resistor. (2) In computing, the contents of a variable. (3) In the HSV colour model the overall level of the light emitted by the colour.

valve Term originally applied to a device which allowed current to pass through it in one direction only (i.e. a diode) but later extended to include vacuum devices with grids which could amplify and oscillate, and gas-filled devices such as thyratrons. The term has been replaced by electron tube which is used throughout this dictionary.

vapour cooling Of a high-power electron tube a method of removing heat which utilises the latent heat of vaporisation of water. The external anode has thick copper walls containing vertical passages and is immersed in distilled water which boils in the passages, the latent heat required to turn the water into steam being taken from the tube.

varactor A reverse-biased junction diode the capacitance of which can be controlled by adjustment of the bias voltage. Such diodes are used in parametric amplifiers, for automatic frequency control and for tuning in radio and TV receivers.

variable In computing, a block of data held in a reserved area of memory and identified by an unique name. The data held may be numeric or may consist of a text string and may be accessed by programs; instructions to output the variable yield its current contents, not its name. See also array.

variable-capacitance diode See varactor.

variable-carrier modulation Same as floating-carrier modulation.

variable-mu tube An electron tube in which the control grid is wound with a pitch which varies along its length so permitting the mutual conductance to be controlled by choice of grid-bias value. A typical I_a–V_g characteristic for a variable-mu tube is shown in Figure V.1: for a small value of grid bias as at A the slope (and gain) is high whereas for a point such as B the slope (and gain) is low. An advantage of these tubes is that they can accept a larger input signal at B than at A for comparable distortion levels. Such tubes were used in situations where it is required to control gain by choice of bias value as in automatic gain control systems.

variable-reluctance pickup A pickup in which the movements of the reproducing stylus vary the reluctance of a magnetic circuit and generate corresponding EMFs in a stationary coil. The moving-iron pickup is an example of a variable-reluctance pickup.

varicap diode Same as varactor diode.

variocoupler A mechanical arrangement permitting control of the degree of magnetic coupling between two coils. For example one coil may be fixed and the other brought up to it to increase the mutual inductance between them.

variometer A variable inductor consisting of two coils, one of which can be rotated within the other so as to vary the mutual inductance between them. The coils are connected in series and the mutual inductance can add or subtract from the series inductance depending on the position and sense of the rotatable coil.

varistor A resistor of semiconductor material with a non-linear current-voltage characteristic.

vector graphics A system of computer graphics in which lines and shapes are stored as co-ordinate-based data. See graphics.

velocity factor Of a transmission line the ratio of the velocity of an electromagnetic wave along the line to its velocity in free space. Depending on the construction of the line, values of velocity factor lie between 0.6 and 0.97.

velocity modulation The process of periodically altering the velocity of an electron stream by subjecting it to a high-frequency electric field which alternately accelerates and decelerates the beam. If the period of the variation is comparable with the transit time of the electrons in the space concerned, the electrons subsequently gather into bunches. Bunching makes possible microwave amplification and oscillation in klystrons and travelling-wave tubes. See Applegate diagram.

velocity microphone (US) Same as pressure-gradient microphone.

vented enclosure A cabinet housing a loudspeaker unit, closed except for a forward-facing vent, cabinet and vent forming a Helmholtz resonator tuned, by choice of dimensions, to a low audio frequency. Radiation through the vent reinforces that from the diaphragm at low frequencies so extending the effective frequency response of the loudspeaker.

vertex Same as node (1).

vertical amplifier In oscilloscopes the circuits which amplify the signals responsible for vertical deflection of the beam. Also known as a Y amplifier.

vertical blanking Same as field blanking.

vertical-groove metal-oxide-semiconductor transistor (VMOS) An insulated-gate field-effect transistor so constructed that the current flow is nearly perpendicular to the plane of the layers as in planar bipolar transistors, a groove-shaped gate giving very short channel lengths. Such transistors thus have very high switching speeds and low drain-source resistance in the on-condition. Large source and drain areas permit the transistors to give substantial output power. A simplified diagram of the structure of a VMOS transistor is given in Figure V.2.

vertical hold See hold control.

vertical polarisation Property of an electromagnetic wave in which the plane of polarisation of the electric field is vertical.

vertical timebase In a cathode ray tube the circuits generating the signals which give vertical deflection of the beam. In TV this is usually termed the field timebase.

very-large-scale integration (VLSI) See monolithic integrated circuit.

vestigial sideband transmission (VSB) A system of amplitude modulation in which one sideband is transmitted in full, only part of the other sideband being transmitted (usually that corresponding to the lower modulating frequencies).

    The system is used for transmitting the vision component of a TV signal and has the advantage that it occupies less bandwidth than if both sidebands were transmitted in full.

vibrator converter An equipment in which a tuned reed, maintained in vibration by a low-voltage DC supply, is used to generate DC at a higher voltage. The equipment is, in effect, a vibrator inverter in which the alternating output voltage is rectified to give the required DC supply often by contacts on the vibrating reed itself so avoiding the need for a separate rectifier.

vibrator inverter A device in which a tuned reed, maintained in vibration by a low-voltage DC supply, is used to generate alternating current usually at higher voltage. The interrupted current from the low-voltage supply is applied to a transformer which generates the required alternating voltage. Such equipments are used, for example, for powering mobile equipment normally designed for operation from mains supplies.

video cassette recorder (VCR) A video tape recorder which uses cassette tapes. In practice domestic VCRs also incorporate a UHF or VHF television tuner, allowing television broadcasts to be recorded off air without the need for a separate television receiver. A VCR usually incorporates a dedicated microcomputer which can be programmed to record future television broadcasts automatically.

video digitiser An equipment, often contained in an expansion card, which converts video frames to bitmaps. It is also sometimes called a frame grabber.

video disk A disk containing video data, usually in a digital format. The first video disks used a recording technique similar to that in Compact Disc, but were 12 inches in diameter and could accommodate a 1½-hour feature film. With advances in compression and storage technology, such as MPEG, it is now possible to store a feature film on a 5 inch compact disk.

Video Graphics Array (VGA) A VDU display standard introduced by IBM on its PS/2 computer in 1987 and since adopted as an industry standard. The highest resolution in the original VGA standard was 640 × 480 pixels with 16 colours.

video memory The RAM which stores the image on a VDU. The video memory requirement depends on the video mode since it is proportional to the number of pixels in the display and to the logarithm (base 2) of the number of colours to be displayed. The original VGA standard (640 × 480 × 16 colours) required 640 × 480 × 4 = 1,228,800 bits = 150 kilobytes. A computer fitted with 1 Mbyte of video memory can support a display of 640 × 480 pixels in true colour (24 bits per pixel) which requires 900 kilobytes or a display of 800 × 600 pixels at 16 bits per pixel (65,536 colours) which requires 937.5 kilobytes.

video mode or screen mode The specification of a VDU display. The data include: the number of pixels across the screen, the number of pixels down the screen, the nominal width and height of a single pixel, the maximum number of colours that may appear on the screen without a change of palette and the refresh rate, that is the number of frames per second. Most computers allow a choice of screen modes to suit various purposes.

video on demand A proposed system in which subscribers may demand any of a wide range of video, audio or computer material from a remote service provider. The requested material is then transmitted in compressed format over a landline, assumed to be a fibre-optic cable, to the subscriber’s terminal. At the time of writing pilot schemes are in operation.

video signal In TV the signal obtained by combining the picture signal with the synchronising signal. The way in which the signals are combined is illustrated in Figure B.18 (a). See picture signal, sync signal.

video still camera An electronic camera which takes individual frames and stores them as bitmaps. These are subsequently uploaded to a computer for processing and printing as computer graphics.

video tape recorder (VTR) An equipment which records video and audio, usually in analogue format, on tape. See also video cassette recorder.

videotex A system whereby signals sent over telephones lines can be displayed as letters, figures or diagrams on the screen of a TV receiver. The system uses the same symbols and techniques as teletext and British Telecom is now using such a system under the name Prestel to make available to telephone users some hundreds of thousands of pages of information, e.g. on stock exchange prices, railway timetables, sports results.

videotron Same as monoscope.

vidicon A low-velocity TV camera tube with a photoconductive target. The construction of a vidicon tube is illustrated in Figure V.3. The target consists of a transparent signal plate on which is deposited a layer of photoconductive material. The scanned face of the target is stabilised at electron-gun cathode potential as in any low-velocity tube and the signal plate is given a small positive bias. Current thus flows longitudinally through the target thickness and the magnitude of this current at any point on the target depends on the target resistance which in turn depends on the amount of light in the optical image at that point on the target. These longitudinal currents set up a charge image on the scanned face of the target and this is neutralised by the scanning beam once per picture period. The target has a significant capacitance between front and rear faces and this is charged by the longitudinal currents in the interval between successive scans by the beam so that the charge image grows during the picture period: in other words the tube has the charge storage which is essential to achieve adequate sensitivity.

    Early vidicon tubes suffered from lag, i.e. there was an appreciable delay between a change of light input and the corresponding change in target resistance but intensive work on photo-conductive materials has resulted in satisfactory sensitivity and lag. Modern tubes (plumbicons) have multilayer targets of lead oxide.

    The vidicon is a particularly simple and compact tube and it is not surprising that it is the standard tube for inclusion in colour TV cameras.

viewdata Same as videotex.

virtual cathode A region between the electrodes of an electron tube which contains an accumulation of electrons and can be used as a source of electrons for nearby electrodes. Such regions occur when the electron stream through the tube encounters a retarding field, e.g. between the screen grid and suppressor grid of a pentode. See phantastron, space charge, transitron.

virtual memory Memory which appears to be available to a computer, although it does not exist as physical RAM. Usually virtual memory is a file on a hard disk accessed via a buffer. Using virtual memory a computer can process files that are larger than the physical RAM fitted in the machine.

virtual reality (VR) An interactive computer simulation of a three-dimensional environment, such as a building, a town, a ship or a region of outer space, stored as a mathematical model. Using vector graphics techniques the computer repeatedly generates in real time the instantaneous view of the environment as seen by a hypothetical observer moving around within it. In the most sophisticated form of VR users wear special headsets containing stereoscopic LCD displays, a binaural sound system and sensors which detect movements of the head. Movement and interactive functions are controlled through further sensors in gloves, or even a whole body suit, worn by the user. These give him the illusion of being inside the virtual world, able to explore it and to interact with it in various lifelike ways. Simpler versions of VR use a conventional monitor and pointing device. VR has many serious applications in such fields as medicine, architecture, town and country planning and training (such as flight simulators). Its leisure uses, however, have attracted much publicity, especially the fantasy games in which several players interact with each other within the virtual environment. See also telepresence.

virus A portion of computer program devised by a malevolent programmer. Usually viruses are embedded in applications software and they replicate themselves, by embedding themselves in other applications, when the host application is run or even when its icon appears in a directory display. They spread from one system to another when the applications harbouring them are copied and transmitted to other systems. Some viruses are comparatively harmless, perhaps displaying a humorous message on a certain day, but others cause serious damage, overwriting or corrupting valuable data and programs.

vision carrier In TV transmission the carrier wave which is modulated by the video signal.

vision frequency In TV transmission the frequency of the vision carrier.

vision pickup tube Same as camera tube.

vision signal In TV transmission the carrier wave modulated by the video signal.

visual display unit (VDU) In computers or data-processing equipment a cathode ray tube used for displaying characters or graphical information under program control. Often a keyboard is associated with visual display unit for displaying the input data.

visualisation The use of computers to create high-quality images of objects that do not exist except as mathematical models. In industry such techniques are used to create photographic-quality images of proposed products (such as automobiles) from engineering drawings. Sometimes ray tracing is used, but sophisticated vector graphics applications also yield acceptable images, depending on the nature of the subject.

voice coil (US) Same as speech coil.

voice recognition Computer technology that recognises and is able to respond to spoken words picked up using a microphone and converted to digital format. It has many applications in security systems and in providing facilities for the handicapped who cannot use a keyboard or pointing device.

volatile Liable to disappear. Data stored in normal RAM is volatile in that it will disappear if the equipment is switched off or its power supply is interrupted. Data stored in battery-backed RAM or on magnetic media such as a floppy or hard disk is non-volatile and is not lost at power-off.

voltage The value of an electromotive force or potential difference expressed in volts.

voltage amplifier A circuit incorporating one or more active devices and designed to amplify voltage waveforms. The term distinguishes such amplifiers from those designed to amplify current waveforms or to deliver power to a load. It is the ratio of source impedance to input impedance and of output impedance to load impedance which determines whether a device or amplifier is best regarded as a current amplifier, power amplifier or voltage amplifier. If both ratios are small compared with unity the signals transferred from source to input and output to load are best regarded as voltage waveforms. The associated signal current waveform is of minor interest in a voltage amplifier and is usually of small amplitude. Because electron tubes and field-effect transistors have a high input impedance and a lower output impedance they are usually regarded as voltage amplifiers.

voltage-controlled oscillator (VCO) An oscillator of which the frequency can be controlled by adjustment of an input voltage. Such an oscillator forms part of phase-locked-loop systems. The oscillator may be an LC type generating a sinusoidal waveform, the frequency being controlled by a varactor connected across the oscillatory circuit. Alternatively it may be an astable multivibrator the duration of the unstable state (and hence the free-running frequency) being controlled by the base bias voltage.

voltage-dependent resistor (VDR) A non-linear resistor the resistance of which decreases markedly with increase in applied voltage. The current-voltage relationship for a voltage-dependent resistor is of the form I = kV″ where k is a constant and n lies between 3 and 7. Such resistors are usually of silicon carbide pressed with a ceramic binder into disks or rods and fired at about 1200°C.

voltage divider Same as potential divider.

voltage dropper A resistor connected in series with a power supply to drop the voltage applied to the load to a desired value. An example occurs in radio and TV receivers where the electron-tube heaters are connected in series. Where the total heater voltage is less than the mains voltage a resistor is inserted in the circuit to act as a voltage dropper. From Ohm’s law the required resistor value is given by (volts to be dropped)/current.

voltage feedback A system in which the negative feedback signal is directly proportional to the voltage across the load. Frequently the feedback signal is derived from a potential divider connected across the load as shown in Figure V.4, the potential divider having a total resistance large compared with the load value. The effect of voltage negative feedback is to improve linearity, decrease gain and to decrease the effective output resistance of the active device driving the load, i.e. it tends to make this device a constant-voltage source.

voltage gain Of an active device or amplifier the ratio of the output voltage to the input voltage. Where the voltage gain depends on the value of the load resistance it is necessary to specify the load value when the voltage gain is quoted. The gain may be expressed as a ratio or in decibels as 20 log₁₀V_out/V_in.

voltage gradient In a conductor the difference in potential per unit length. In an electric field the difference in potential per unit distance along the normal to the lines of force at the point in question.

voltage level See level.

voltage-multiplier rectifier A combination of rectifiers and capacitors which produces an output voltage approximately equal to an exact multiple of the peak value of the alternating input voltage. As an example Figure V.5 gives the circuit diagram for a voltage doubler. When D₁ conducts C₁ is charged to the peak value of the alternating input. On the next half cycle D₂ conducts and C₂ is also charged to the peak value of the input. C₁ and C₂ are connected in series and their voltages add so yielding an output voltage equal to twice the peak value of the alternating input.

    By using a circuit of the type shown in Figure V.6 it is possible to obtain an output of n times the peak value of the input voltage where n is the number of rectifiers (and the number of capacitors) employed. The particular example illustrated is, of course, a voltage tripler.

voltage-reference diode A semiconductor device which when carrying a current within a specified range develops across its terminals a particular voltage of specified accuracy which can be used for reference purposes.

    The device consists of two Zener diodes connected in series back-to-back as indicated in the graphical symbol shown in Figure V.7. If the applied voltage has the polarity shown here and is increased sufficiently, diode D₁ is reverse biased and breaks down at its Zener voltage, D₂ being forward biased and so acting as a low-value series resistor. D₁ has a positive temperature coefficient and D₂ is designed to have an equal negative coefficient so that the device as a whole has a breakdown voltage independent of temperature provided the bias current through the device is maintained constant.

voltage-reference tube A cold-cathode gas-filled tube which when carrying a current within a specified range develops across its terminals a particular voltage of specified accuracy which can be used for reference purposes.

voltage-regulator circuit See regulation.

voltage-regulator diode A Zener diode used for voltage regulation. A simple circuit is shown in Figure V.8. The supply voltage must exceed the Zener breakdown voltage so that the voltage across the diode is at breakdown value. The series resistor R_s is so chosen that even when the load takes its maximum current there is still some current through the diode so maintaining the voltage across it at the Zener value.

voltage-stabiliser circuit See stabilisation.

voltaic cell A cell in which chemical action takes place between two electrodes of different materials immersed in an electrolyte and gives rise to an EMF between the electrodes.

voltmeter An instrument for indicating or recording the magnitude of a voltage in volts. For high voltages the measurement may be in kilovolts (kV) and for small voltages in millivolts (mV) or microvolts (μV). The resistance of a voltmeter must be high so that, when connected across a circuit, it takes negligible current and so has little effect on the voltage being measured.

volume (1) See programme volume. (2) In some computers volume refers to a storage medium. This might be a floppy disk or a whole hard disk or a partitioned area on a hard disk.

volume compression See compression.

volume expansion See expansion.

volume indicator or volume meter See programme meter, VU meter.

volume unit A transmission unit for measuring programme volume. For a sinusoidal signal one volume unit equals one decibel. The VU meter is calibrated in volume units.

von Neumann’s principle John von Neumann can be regarded as the father of the modern computer because it was he who in 1944 suggested that the program should be stored electronically within the computer itself so that it could be executed at a speed limited only by that of the logic circuits of the computer. Internal storage also meant that the program could be modified as a result of processing. Hitherto the program had been stored externally on cards or tape and was fed into the computer at low speed. ‘Stored program control’ is thus the essence of von Neumann’s principle.

VU meter A voltmeter for measuring programme volume. It consists of a moving-coil instrument of a specified type fed from a bridge rectifier which is, in turn, fed from the programme line via a series resistor. Unlike the peak programme meter the VU meter needs no source of power for its operation other than the signal input. The VU meter measures the total energy of the input signal and not the peak value and its scale is markedly non-linear. It is, however, less expensive than the peak programme meter and is extensively used for measuring programme volume particularly in audio tape recording.