Philips tech. Rev. 36, No. 11/12 MODULATION

V. Modulation in telecommunication

Modulation and telecommunication are closelyinterrelated. Apart from the simplest wire or cableconnections for telephony and telegraphy, telecom-munication is unthinkable without modulation. It isalso true to say that all known modulation systemswere really developed for telecommunication purposes.In this part we shall therefore give a survey of the

The principal forms oftelecommunication are shownin fig. 81. There are four groups: broadcasting, tele-phony, fixed links and mobile systems. Under 'broad-casting' we shall presently discuss a few more mod-ulation aspects of AM and FM sound broadcasting,television broadcasting and broadcasting via satellites.We shall not return here to telephony, as such; the

wavelengthfrequencyband

300Hz

sound

broadcastingpicture

L- ~A~M ~I~

1p

ooTV

satellite broadcasting

telephony ,..------------ï~ TOM It.: .1

radio

fixed linkscable

lmicrowave links Ioptical 0communication

I mobile links SSB FM

Fig. 81. Forms of telecommunication. Broadcasting: AM radio in bands 5,6 and 7; FM radioin band 8, television in bands 8 and 9. Satellite broadcasting - with direct transmission fromsatellite to individual receivers - is intended for television and sound broadcasting (band 10).Band n is the band from 0.3 to 3 times IOn Hz; the frequency JOn Hz thus lies at the centre ofthis band. Telephony: frequency-division multiplexing of speech signals to form 'basic, super,master and supermaster groups', and time-division multiplexing of signals coded in the formof pulses. Fixed links: transmission paths between two fixed stations for broadcasting andtelephony (andalso for telegraphy, telex, data transmission, ... ). Cable links are, very generally,used for short distances but are also in use for very large distances, especially in telephony(transoceanic cables). Radio links below 30 MHz are slightly directional and have a relativelylong range; those in bands 9 and 10 (microwave links) are strongly directional and have arelatively short range (about 50 km), unless they include communication satellites. In thelatest form of optical communication a type of cable link is again used for guiding the electro-magnetic waves (here light waves), the cables in this case being glass fibres. Mobile systems areused for communications with and between pedestrians, cars, ships, aircraft, etc. For fre-quencies below 30 MHz these generally use SSB modulation, but at higher frequencies FM ispreferred, although AM is also still in use.

specific uses of modulation in the principal forms oftelecommunication in current use. Other areas of ap-plication touched on earlier were the storage of infor-mation (FM in magnetic recording of video signals,PFM and PDM with the 'VLP' record) and signalprocessing (PAM with charge-transfer devices). Thereare other areas where modulation is used, but not assuch an essential feature: range-finding and navigationsystems for traffic on land, at sea and in the air, remotecontrol and telemetry," but we shall not be concernedwith these here.

modulation methods used in telephony - frequency-division multiplexing of SSB signals and time-divisionmultiplexing of pulse-coded signals - have alreadybeen dealt with at some length. We shall, however,take a closer look at the 'fixed links' between twostations. These are not unrelated to broadcasting andtelephony but are integral links in these systems, forexample between studio and transmitter in broadcast-ing, and between two exchanges in telephony. Fixedradio links operate either at frequencies below 30 MHzor in the UHF and SHF bands (microwaves); theyare

353

O.1ym

354 F. W. DE VRIJER Philips tech. Rev. 36, No. 11/12

only weakly directional in the first case and highlydirectional in the second. The microwave links havebeen given a new dimension by communication satel-lites. Very much higher up in the spectrum we haveoptical communication, which in its modern form isstill in the experimental stage. Finally we shall discusssystems where at least one of the stations is mobile.Before going further with this, two comments should

be made on the subject of transmission by cable, butwe shall leave the matter at that. In conventionalbroadcasting the transmission generally takes place infree space, but the systems that have a single centralantenna for small residential estates, suburbs, or towns('relay television') form an exception. In relay systemsthe 'transmitter' is a relay station that receives thesignals from elsewhere through a large antenna orin some other way. The subscriber to such a systemgenerally has a wider choice of programmes, andpictures and sound of better quality, than is possiblewith individual reception. To permit the use of thesame receivers, the signal is generally similar to theconventionally transmitted signals, and for that reasonthese systems need not be considered here. The oldform of 'rediffusion system', in which an audio sgnalwas distributed in the base band by cable, has now al-most disappeared from the scene.

'Fixed links' are often cable systems. For small-scale communications, for example within a group ofbuildings, this is nearly always the case. For telephonyin particular there are also cable links that are tens,hundreds or thousands ofkilometres long (transoceaniccables), over which FDM and TDM groups are trans-mitted (the TDM groups in the baseband, see p. 349).The exponential attenuation of the signals is com-pensated by regularly spaced repeaters.

Broadcasting

AM sound broadcasting

For sound broadcasting in bands ~, 6 and 7, thefamiliar 'long-wave', 'medium-wave' and 'short-wave'bands, the modulation system used is conventionalAM. The radio signals in this wavelength range areable to propagate over great distances by bending andreflection from the ionosphere, so that one transmitterof sufficient power covers a large area. To avoid inter-ference between the large number of transmissions thefrequency spectrum is rigorously divided up into manynarrow bands, and the audio bandwidth allocated formany transmitters is limited to 5 kHz, which meansthat each transmitter takes up 10 kHz of the r.f. spec-trum.It was long argued that the replacement of conven-

tional AM by SSB modulation would allow broad-

casters to use twice as much audio bandwidth, makingit possible either to increase the quality of the trans-missions or to increase the number of transmitters.The old counter-argument that an SSB receiver ismuch more expensive than a receiver using simple peakdetection is much weakened by the continued advancesin electronics (ICs, LSI). There remains, however, thefact that there are still large numbers of conventionalreceivers in use, and these receive SSB signals severelydistorted. As a transitional measure the use of specialSSB systems has been proposed, where the carrierwould not be completely suppressed and with the side-bands pre-distorted in such a way as to compensate forthe distortion caused in the conventional receiver('compatible single sideband', CSSB) [191. This system,however, causes distortion in a real SSB receiver. Thewhole question is still being studied by the CCIR.

FM sound broadcasting

For FM sound broadcasting in band 8 a much widerbandwidth is available. In the first place, band 8 (likeevery other band) is nine times as wide in linear fre-quency measure as all the lower bands put together,and it is linear measure that counts for audio band-width. In the second place, these shorter waves undergoless bending and they are not reflected by the iono-sphere, which means that the range of the transmittersis shorter and that two transmitters a few hundredkilometres apart can have the same frequency withoutinterfering with one another. Advantage is taken of thisfrequency latitude to broadcast FM transmissions ofhigh sound quality. In general each transmitter isallocated an r.f. bandwidth of 200 kHz. In this band-width a broad audio spectrum (15 kHz) can then becombined with a frequency deviation of say 75 kHz,thus giving strong noise suppression (see (16) andp.323).

We shall touch brieflyon some extensions andexperiments for which there was and still is room inFM sound broadcasting, and which depend on mod-ulated subcarriers. In the first place the system has beenextended to include stereo sound [201. In conformitywith a recommendation made by the CCJR, the signalSet) of a stereo FM transmitter is made up as shown infig. 82a. From the 'left-hand audio signal' L and the'right-hand audio signal' R the sum and difference areformed. A subcarrier at 38 kHz is modulated in nSB,with suppressed carrier, by the difference signal. Theinformation signal pet) for the transmitter modulatoris the sum of the sum signal L + R, the modulatedsubcarrier and a synchronous pilot signal at 19 kHz;fig. 82b shows the spectrum of pet). The pilot signalserves for the synchronous demodulation of the mod-ulated subcarrier in the FM receiver. It is also some-

Philips tech. Rev. 36, No. I 1/12 MODULATION 355

L.R LI L-R Ro 15 I 23 3853kHz

19 -f

Fig. 82. Stereophony in FM radio. a) Diagram showing the corn-position of the FM signal 5(1). L left-hand audio signal, R right-hand audio signal. The difference signal L - R DSB-modulatesa (suppressed) subcarrier at 38 kHz. b) Spectrum of the informa-tion signal p(l) for the frequency modulator FM; fl(r) is the sumof the sum signal L + R, the modulated subcarrier and a pilotsignalof 19 kHz synchronized with this carrier. The pilot signalserves as a reference during the reconstitution of L - R from thesubcarrier in the FM receiver.

times used in stereo transmisslons to activate the(L - R) channel in the receiver; without a pilot signalthis channel is closed in these receivers. The systemgives compatability with mono receivers, which onlyreceive the sum signal L + R. At a frequency deviationof 75 kHz the required bandwidth is now about260 kHz (2 x (53 + 75) = 256). The total frequency

p(f)L,\ [\ [\V V,

R

L-R on 38kHz

Fig. 83. The information signal p(l) in fig. 82 for cases where Land R are sinusoidal signals of identical amplitude and are inphase (a) or in antiphase (b). The vertical strips in (b) representthe subcarrier. In both cases the amplitude of pet), and hence thefrequency deviation of 5(1) in fig. 82, have the same magnitudeas for a mono transmitter with 2R or 2L as the informationsignal. It can be shown that the sarne applies approxirnately forall other combinations of Land R.

deviation in this system is not much greater than for amono transmitter, which, for the same Land R, trans-mits only L + R (jig. 83). The signal-to-noise ratio inthe CL + R) channel may thus be about as good as inthe mono transmitter. In the CL - R) channel, how-ever, fmax is about three times higher, and the mod-ulation index is thus three times smaller, so that thegain in signal-to-noise ratio is about ten times less thanin the (L + R) channel. For stereo, therefore, thecoverage of an FM transmitter is smaller than formono. Stereo reception will therefore often require agood outside antenna where for mono reception asimple inside antenna is sufficient.

The idea of using a single FM transmitter to broad-cast extra signals on subcarriers in addition to the mainsignal is currently finding application in various ways.l n the United States some FM transmitters broadcasta continuous programme of background music thatfrequency-modulates a subcarrier at 67 kHz (some-times 41 kHz in the case of mono transmitters); thelistener can choose between the main programme or thebackground music. In the Netherlands the Dutchbroadcasting organization and Philips are carryingout joint experiments in which FSK (65-67 kHz) isused for sending digital information together with abroadcast programme; the digital information is thendisplayed on a panel in the receiver. I n Germany sometransmitters send a digital signal, modulating a carrierat 57 kHz, to indicate when traffic information is to begiven. In a car radio tuned to the transmitter this signalthen switches the programme on again which the driverhad switched off. A problem with systems of this kindis the possible crosstalk that such signals can produceon the CL + R) and (L - R) signal, as a consequenceof nonlinearities and delay errors in the receiver. Asubcarrier of 66 kHz, for example, can interfere withthe pilot signal at 19 kHz to give a signalof 47 kHz,which produces a 9-kHz tone in the (L - R) channel.Introduetion of the system therefore calls for somecaution, and this matter is also being studied by theCClR.

Television broadcasting

Television broadcasts are found in bands 8 and 9(see fig. 81). Tt was decided at the time, for economy ofbandwidth and the possibility of peak detection, toadopt vestigial-sideband amplitude modulation (VSB-AM, see p. 317) for transmuting the video signal. Inpractice the transmitter sends out the conventionallymodulated carrier (carrier plus two sidebands), with

[19J Th. J. van Kessel, F. L. H. M. Stumpers and J. M. A. Uyen,E.B.U. Rev. No. 71A, 12, 1962.

[20] See for example N. van Hurck, F. L. H. M. Stumpers andM. Weed a, Philips tech. Rev.26, 327, 1965 and 27, 62, 1966.

356 Philips tech. Rev. 36, No. 11/12

Semitrans 102 modem, acoustically coupled to a telephone. This modem isused for feeding digital signals into the public telephone system. The twolevels of the binary signa! are coded as two audio frequencies (frequency-shift keying or FSK); these are transmitted by the microphone and aresupplied to the modem at the receiving end via an acoustic connection tothe telephone. Two pairs of frequencies are used - 980/1180 Hz and1650/1850 Hz - so that simultaneous two-way communication is possible;the maximum transmission rate is 200 bits per second.

Philips tech. Rev. 36, No. 11/12 MODULATION 357

the lower sideband almost completely suppressed(fig. 84). The filter with the symmetrical responsecharacteristic of VSB modulation (the 'Nyquist char-acteristic') is in the receiver. The sound modulates (AMin France, FM elsewhere) a separate carrier, which is6.0 MHz above the video carrier in the system used inBritain. For colour transmissions the colour informa-tion is carried on a subcarrier in the video band. Thesystem for modulating the subcarrier depends on whichcolour-television system is used (QAM for NTSC andPAL, FM for SECAM) tei,

In television broadcasting, experiments designed totransmit more information by means of additionalsubcarriers are also in progress. In particular, trials arebeing carried out with a second sound channel. Thiscould for example be used for a second language whenan event of international importance is being reported.In Germany this is done with a second sound carrier242 kHz above the first. In Japanese trials the 'secondsound' frequency-modulates a subcarrier at about32 kHz, which, together with the 'first-sound', fre-quency-modulates the sound carrier ('FM-FM system')

Satellite broadcasting

In addition to broadcasting by 'Earth' stations, wenow have broadcasting by satellite, i.e. by a transmitterin a space satellite that covers a part ofthe Earth wherethe signals can be received by individual sets. Signals at12 GHz in band 10 are now being used experimentallyfor such transmissions. The problem of sufficient signalstrength is much greater here than in the case of asatellite for a 'fixed link'. In a fixed link the Earthstation can use a very large parabolic antenna; forindividual reception of satellite broadcasts, on the other

y

~ : ~f--- f..f.3MHz----+l'~------6MHz------~

Fig. 84. Spectrum of a television signal. In principle, vestigial-sideband amplitude modulation (VSB-AM, p. 317) is used fortransmitting the video signal (carrier frequency fv). The trans-mitter does not, however, transmit the actual VSB signal but aconventional AM signal most of whose lower sideband is sup-pressed. A filter in the receiver, preceding the actual detector,provides the skew-syrnmetric VSB response (dashed line). Thevideo signal consists of the luminance signal Yand the chromin-ance signal C (a subcarrier at 4.43 MHz modulated by the coloursignals R - Yand B - Y). Special measures are needed to ensurethat the effect of C on Y and vice versa does not give picturedistortion 181. The sound usually modulates a separate carrierat a frequency fA (in Britain fA - fv is equal to 6 MHz).

hand, a parabolic 'dish' of about 1 metre in diameterwould seem to be about the maximum. For a satellitein the geostationary orbit (see p. 359) to cover a countrylike Germany, France or Great Britain with a tele-vision programme, an 80-kW transmitter would benecessary if the conventional AM-VSB system wasused. This would not be a practical proposition in asatellite. If FM was used, however, with adequatebandwidth, then 500 W would be sufficient; this poweris feasible in a satellite. In the CCIR a system has been .proposed that would use FM with a deviation of8 MHz in a 27-MHz band. Efforts are now beingmade to reach international agreement on the use ofthe 12-GHz band for television and sound broad-casting.

Carson's rule (16) gives the required bandwidth of 27 MHzdirectly from the deviation of 8 MHz and the video bandwidthof 5.5 MHz. The fact that this combinaton also gives the desiredimprovement in SIN is partly because the eye is much moresensitive to noise at the lower frequencies than at the higherfrequencies in the television picture. This is accounted for in theSIN calculations by means of a weighting factor; this is to theadvantage of FM, which of course gives less noise at the lowervideo frequencies than at the higher ones ('triangular noise',p. 322). To simplifya little, the eye is only sensitive to noise atfrequencies up to 1 MHz in the video signal. Withfmax = 1 MHzand /1f = 8 MHz we find IX = 8, and for the improvement inSIN compared with AM we thus find a factor of 3 X 82 Ri 190(see p. 323), which broadly agrees with the factor of 160 betweenthe figures mentioned above of 80 kW for AM and 500 W for FM.

Fixed links

The conventional 'fixed radio links' - the connec-tion formed by radio signals at frequencies up to about30 MHz between two fixed stations - used to be theprincipal means employed in telecommunication (tele-phony, telegraphy, telex) over long distances. The con-tact, which depends on the signal being reflected fromthe ionosphere, is not in general very reliable becauseof the varying and to some extent unpredictable degreeof ionization in the various layers of the ionosphere.Many such links have therefore been replaced by cablesor satellite links. Where they are still used, the optimummodulation systems have generally been introduced,i.e. SSB and ISB (see p. 313 and 316). Formerly con-ventional AM was usual.

Microwave links

Radio signals at frequencies in bands 9 and 10 (wave-lengths from 1 cm to 1 m) can readily be beamed withthe aid of parabolic reflectors, and the microwavebeams thus obtained nowadays play an important partas links in transmission paths for FM radio, televisionand telephony. Because of beam bending and diffrac-tion, the range of the quasi-optical beam is in principle

358 F. W. DE yRIJER Philips tech. Rev. 36, No. 11/12

somewhat farther than the horizon. With masts about50 m high, ranges of about 50 km can be covered.Repeater stations are used to cover greater distances.The scattering of microwaves by the troposphere alsomakes direct contact possible between two stationsseveral hundreds of kilometres apart; this method isoften employed for military communication.

Frequency modulation is the obvious method to usehere, since power is more of a problem than band-width. At the transmitter the power of the microwavebeam is no more than 20 W, and is often much less.For two microwave links more than a few hundred kmapart the same frequency can be used. Furthermore,FM provides the signal that is least vulnerable to thenonlinearity of the amplifiers and the considerableamplitude fluctuations that often occur (fading due tofluctuating propagation conditions).

Fig. 85 shows a conventional modulation system anda possible distribution of the successive signals over thespectrum. The frequencies and bandwidths quoted arepractical examples. The information signal frequency-modulates a carrier at 70 MHz in a bandwidth of30 MHz. The 30-MHz band is then transposed to3884 MHz by SSB modulation of a (suppressed) car-rier at 3954MHz; this is known as 'up-conversion'.This stepwise modulation is necessary because directfrequency modulation of carriers in the GHz band

5:.rt)

Q

Fig. 85. Transmission of television signals and multiplexed tele-phone signals over a microwave link. a) Modulation, b) spectraof the various signals. The information signal Pl(t) frequency-modulates a 70-MHz carrier with a bandwidth of 30 MHz (FM).The resulting signal SI(t) is transposed to a microwave frequencyby lower-sideband modulation ('up-conversion'). TWT travel-ling-wave tube. Fi filter that passes only the lower sideband.SI' the signal after Fç, The microwave beam B contains micro-wave signals SI', S2', S3', ... , which are obtained with differentSHF carrier frequencies and are spaced some distance apart fromeach other in the spectrum.

- obtained by multiplication from crystal oscillations- is not a practical proposition. Signals obtained inthis way are frequency-division multiplexed on a singlemicrowave beam, with the GHz carrier frequenciesspaced by about 30 MHz or a multiple of this value.The '4-GHz band' (3800-4200MHz) was the first to bereserved for fixed microwave radio links. There isample room in this band for 2 X 12channels of 30MHz,some of which are used as service channels. The factorof 2 is due to the use of horizontally as weUas verticallypolarized beams. When the beams are used for tele-phony, the signal for each direction is sent via a dif-ferent channel. At a given repeater station the fre-quencies of the incoming beams must differ from thoseof the outgoing beams. It is also necessary to havecareful allocation of the frequency channels among thevarious beams in each region of a few hundred kilo-meters in diameter to avoid mutual interference. Inmany countries the 4-GHz band is becoming crowded.For microwave radio links higher-frequency bandshave already been allocated and some are now in use.Nowadays a substantial proportion of trunk calls

and international calls are made via microwave radiolinks. The telephone signals are multiplexed by fre-quency division on channels of bandwidth say 30 MHz.In television, microwave beams are used for transmit-ting the signals from studio to transmission mast, andalso, for example, in 'Eurovision' transmissions, fordistributing international programmes over WesternEurope. For 625-line television a frequency deviationof ± 8 MHz is often used; this leaves room for a tele-vision signal with a baseband width of nearly 8 MHzin the 30-MHz band. Prior to the main modulation thesound is made to frequency-modulate a subcarrier at afrequency of typically 7.5 MHz, added to the videosignal. For international links systems are used withfour sound channels on subcarriers of 7.020, 7.500,8.065 and 8.590 MHz to transmit speech in four lan-guages simultaneously.

During a microwave transit the signal accumulates the'triangular noise' peculiar to frequency modulation. To distributethis noise more uniformly over the speech channels in telephony,pre-emphasis is applied to the frequency-division-rnultiplexedgroup before modulation, with an attenuation of 4 dB at thelowest frequency up to an amplification of 5 dB at the highest.

Pre-emphasis is also applied to television signals (from -11 dBat the lowest frequency to +3 dB at the highest), but not so muchfor noise reduction as to limit certain colour defects in the picture.This may be briefly illustrated with the aid ofjig. 86a. This showsthe video signal as a function of time during part of a line periodwhen it passes through three colour areas. It consists of aluminance signal (dashed) with the colour subcarrier superimposedon it; the phase and amplitude of the subcarrier determine thecolour. Colour errors easily occur in this situation because, owingto nonlinearities in the microwave-link equipment, the phase andamplitude of the colour signal are affected by the level of the

Philips tech. Rev. 36, No. 11/12 MODULATION 359

îm

~~~o,=i--~---mm-f-f

aFig. 86. a) Video signal during part of a line period while passingthrough three different colour areas. The dashed line indicatesthe luminance signal, with the chrorninance signal superimposedon it. As a result of nonlinearities in the equipment, the level ofthe luminance signal influences the phase and amplitude of thechrominance signal. Pre-emphasis gives a relative reduction ofthe differences in levels and thus reduces the errors. Sharp transi-tions in the luminance (b), however, may lead to overshoot of theinstantaneous frequency of the frequency-modulated signal if thepre-emphasis is too great (c).

luminance signal ('differential phase and amplification errors') 1.8l.These errors can be reduced by a relative reduction of the 'leveldifferences' before frequency modulation. This is just what pre-emphasis does, since the level variations represent the lowestfrequencies in the signal. Strong pre-emphasis is not permissiblehere, however, because ifthe high frequencies are over-emphasizedovershoot will occur at an abrupt change in the luminance signal(sharp transition between light and dark); see fig. 86b,c. Theinstantaneous frequency of the VH F or UHF signal may thengo outside the passband, giving rise to annoying distortion andnorse.

Communication satellites

To enable them to function as 'repeater stations' ina fixed microwave link, communication satellites areput into the geostationary orbit, i.e. an orbit directlyabove the equator at a height of about 35900 km(about 2.8 Earth diameters). At this distance the orbitalperiod is equal to the Earth's period of rotation. Giventhe right orbital direction, the satellite will thus bestationary with respect to the Earth's surface. To helpcompensate for the attenuation (beam spread) due tothe great distance, parabolic antennas with diametersfrom 10 to 25 m are used at the Earth stations. Greatdistances can be bridged with one satellite becausemore than a third of the Earth's surface is 'visible'from the satellite. The Earth transmitter sends adirected beam to the satellite, which then retransmitsthe received signal back to the Earth at another carrierfrequency. Here again, the transmitted signals aremainly television signals and FDM groups of (900 or1800) telephone chan nels.

A modern communication satellite has variousrepeaters ('transponders') with antennas that can beoriented ('pointed') separately. The lntelsat IV type ofsatellite - the first version was launched in 1971 and isin orbit above the Atlantic - has twelve transponders,each with a bandwidth of 36 MHz, two antennas thatcover the whole visible part of the Earth, called 'globalantennas', and two more closely beamed antennas,

called 'spot' antennas. The spot antennas are used forthe busiest routes. The global antenna serves for theless busy routes. Two Earth stations in the area ofcoverage, communicating through a global antenna,keep a complete transponder busy during the period ofcommunication.

An exception is the transponder reserved for'SPADE' (Single Channel Per Carrier Multiple AccessDemand Assignment Equipment); this operates in anIntelsat IV above the Atlantic. The name indicatesthat each of the 800 channels of bandwidth 45 kHzinto which its band of 36 M Hz is divided may betemporarily used on demand by any Earth stationfor a connection that is not so frequently required.SPADE is also an exception in the modulation methodused. The signals are coded in PCM of 64 kbit/s andthen transmitted by four-phase PSK in a bandwidth of38 kHz so that each channel retains 7 kHz as a safetymargin ('guard bands'). A separate inforrnation chan-nel is used for requesting and assigning SPADEchannels.

The fact that the velocity of light is not infinitelyhigh is beginning to cause some difficulties in telephoneconversations by satellite. In a station-to-station routeover one satellite the transit time of the signals is aslong as a quarter of a second. For this reason it hasbeen agreed that a telephone channel shall never includemore than one satellite.

Optica! communication

Telecornrnunication by modulating a beam of lightis in itself very old; an example is the naval signallinglamp. Modern techniques allow the use of higher mod-ulation frequencies. It is now more than ten years sincea television signal was first transmitted on a light beamover a distance of a few kilometres. The light sourceused - an LED or a laser - was intensity-modulatedand the signal was detected by a photomultiplier or asolid-state photodetector. This method had someinherent drawbacks and was not therefore taken up.Today, however, a promising development is takingplace in which communication is established by themodulation of light conducted through glass fibres (21J.

The potential advantages ofthis system, compared withthe coaxial cable, include a very large bandwidth, smalldimensions, low losses and insensitivity to electricalinterference.

It is possible to transmit an analog signal along glassfibres, but since nonlinearities in the transducers aredifficult to avoid the obvious method is to use binarycoded signals and to modulate the light source by theresultantbitruns(on = 'I',off= '0'). Thebandwidth,

[21] See the issue of Philips Technical Review on this subject:Vol. 36, No. 7, 1976 (pp. 177-216).

360 F. W. DE VRIJER Philipstech. Rev. 36, No. 11/12

and hence the bit rate, are limited by transit-time dis-persion of the light in the fibre. This is mainly attribut-able to the fact that the light travels along differentpaths through the fibre. In some types of fibre (the'single-mode fibre' and the 'graded-index fibre') thiseffect is absent or greatly reduced, and bit frequenciesof gigabits per second are possible.

Mobile systemsWe have already seen that SSB modulation is greatly

to be preferred to conventional AM for voice com-munication by radio between mobile units, such asships, aircraft and automobiles, or between such aunit and a fixed station. With SSB modulation therequired bandwidth is twice as small and the requiredtransmitting power some 13 dB lower. For frequenciesbelow 30 MHz conventional AM has therefore beenalmost completely superseded by SSB modulation.

Above30 MHz frequency modulation is now widelyused for mobile communication systems. Amplitude

variations are of course unavoidable and very large inmobile installations, but with FM the information iswell protected from the effects of such variations.Another advantage is that different FM transmittersat relatively short distances apart can use the samechannel, since there is only a small range of field-strength ratios in which they can interfere with oneanother at the individual receivers (p. 324). This is animportant consideration, for example, in mobile tele-phone systems. A commonly used bandwidth per chan-nel is 25 kHz. For speech (baseband width 3.4 kHz)the frequency deviation may then be as much as 8 kHz,so that in addition there is an improvement in signal-to-noise ratio compared with AM (modulation indexat least 2).However, the advantages of SSB are also leading to

the growing use of SSB modulation for frequenciesabove 30 MHz. The necessary stability in the localoscillators in the transmitter and in thereceiver can beobtained even for mobile equipment and at higherfrequencies by the use of recent techniques.

Philips tech. Rev. 36, No.ll/12 361

Ground station of the Netherlands Post Office at Burum providing a linkwith communication satellites. In the traffic via the 'Intelsat' satellite,orbiting at 35 900 kilometres above the Atlantic, certain frequency channelsare allocated to the Netherlands at certain times for telephone communica-tions with North America. The up-link uses frequencies of about 6 GHz,the down-link frequencies of about 4 GHz. Frequency modulation is em-ployed, and the telephone signals are grouped in the conventional mannerby frequency-division multiplexing (FDM).

362

MODULATION

Philips tech. Rev. 36, No. 11/12

INTRODUCTION. . . . . . . . . . . . 307

I.MODULATION OF A SINUSOIDAL CARRIERConventional amplitude modulation . . . . . . .Double-sideband modulation with suppressed carrier (DSB)DSB modulation compared with conventional AM 312

Single-sideband modulation (SSB) . . . . . .Generation of an SSB signal 315Frequency-division multiplex (FDM) 316

Related methods of amplitude modulation.Frequency and phase modulation .....Modulation and detection 319·Bandwidth 321Signal-to-noise ratio 322Pre-emphasis 323Threshold effect 324Applications; magnetic recording of video signals 326

309309311

313

316318

II. MODULATION OF PULSE TRAINS 329Some basic concepts . . . . . . . . 329Pulse-amplitude modulation (PAM) 330Time-division multiplex (TDM) 330Nyquist's theorem 331

Pulse-frequency and pulse-duration modulation (PFM and PDM);the 'VLP' record . . . . . . . . . . . . . . . . 332

lIL QUANTIZATION AND CODING OF ANALOG SIGNALS 337Pulse-code modulation (PCM) . . . . . . 337Differential pulse-code modulation (DPCM) 338Delta modulation (DM) . 339Effect of bit errors . . 339Predictive coding. . . 340Other coding methods 341

IV. TRANSMISSION OF DIGITAL SIGNALS 343Baseband transmission . . . . . . . . 343Systems with no d.c. components 346Clock extraction 348Equalization 348Applications 349

Modulation systems for digital signals; data transmission 349

V. MODULATION IN TELECOMMUNICATION 353354Broadcasting .... . . . .

AM sound broadcasting 354FM sound broadcasting 354Television broadcasting 355Satellite broadcasting 357

Fixed links . . . . . . .Microwave links 357Communication satellites 359Optical communication 359

Mobile systems .. . . . . . . . . . . . . . . . . . . . . . 360

. . . . . . 357

Volume 36, 1976, No. 11/12 pages 305-362 Published 1st June 1977