1962/63, No. 11/12 353

MODULATION, YESTERDAY AND TOMORROW

by F. de JAGER *).

Problems of today's communications

Fifty years ago there was no need for a theory ofmodulation. The problem of transmitting speechor music by means of radio waves had been solved,and most of the physical problems involved werewell understood. The people putting these principlesinto practice found themselves in the happy situa-tion of having available an enormous frequencyspace, which was very quiet and extensive like thedeep blue sea.

Nowadays, however, the situation has changed.The evergrowing need for more channels has filledup this once silent frequency space and we now havea large number of transmitters radiating power inthe form of television, speech, radar and controlsignals. Indeed we are not far from the well-knownsituation in a cocktail party where everybody israising his voice in trying to overcome the back-ground noise generated by other people. In tele-communication nowadays this man-made noisepresents a far more serious problem than the smallatmospheric disturbances, with which we wereconfronted in the early days of telecommunication.Now we are forced to study the theory of modulationmore seriously, aiming at a more effective trans-mission of information signals.

Three problems arise from this. Firstly, how tocommunicate in the presence of background noise;secondly, how to use minimum power; and thirdly,how to occupy the minimum bandwidth of the totalfrequency spectrum available. These are the prob-lems from a' theoretical point of view. Connectedwith this are the practical problems of how to realizethe correct physical principles of modulation, pre-ferably without running into very complex appara-tus and high costs.

Fortunately, during the last decade, the rapiddevelopment of electrical components has suppliedus with a great variety of small, cheap and reliableunits, such as transistors or magnetic cores, whichallow the realization of complex technical processesour ancestors would never have dreamed of. (As anexample we need only mention modern high-speedelectronie computers.) So, in a way, we are in thesituation of a child who has received on his birth-day a nice box of building-blocks and now has todetermine what should be built with it. In this

*) Philips <Research Laboratories, Eindhoven.

621.376

respect modulation theory is the art of combiningthese building-blocks in an efficient way. This, ofcourse, is only possible by keeping in mind thecorrect physical and mathematical principles, sothat, in the end, we are studying physics again.. It !pay be of interest to look in more detail at

some of the principles which have influencedmethods of modulation in the past and - as far aswe can foresee them - to discuss some < futuredevelopments.

Early methods of modulation

The original and to some extent most naturalway of transmitting information signals is byvarying the amplitude of a high-frequency sinus-oidal wave. This is called amplitude modulationand it is still applied in broadcast transmittersof long and medium wavelengths. In 1933, however,it was discovered by Armstrong that, generallyspeaking, a disturbing noise component could lesseasily affect the frequency than the amplitude ofthis wave. So Armstrong kept the amplitude at aconstant value and transmitted the informationsignals by varying the frequency. This was calledfrequency modulation and it proved to be a veryefficient way of reducing noise. The method isapplied so generally now (for instance in f.m.broadcast transmission, for sound transmission intelevision, etc.) that it is hard to believe that in theearly days of telecommunication these ideas wereconsidered to be revolutionary.In a physical sense the underlying principle is

very simple. Originally the realization required alarge amount of apparatus, spread out over a largetable in the laboratory. Nowadays it is not difficultto contain the whole system in a matchbox. Thisillustrates how a new technical principle, whichinitially demands a rather complex realization, maybe realized in a convenient manner after a fewyears development of the suitable components. Inthis respect we may be optimistic for future devel-opments of new ideas. Indeed, the continuing pro-gress in solid-state devices permits us to perform. more and more functions with a still smaller numberof molecules.

Pulse modulation: introducing non-linear techniques

The methods of modulation considered so farall used sine waves for the transmission of infor-

354 PHILIPS TECHNICAL REVIEW VOLUME 24

mation, either by varying the amplitude or thefrequency. It is, however, equally possible to modu-late short pulses, either in amplitude or time. Aremarkable fact is that; though this possibility wasvery well understood theoretically, the practicalrealization of this only occurred after a rather longtime. Probably this is a consequence of the differentnon-linear techniques which were required for thispurpose, and indeed a great deal of this circuitrywas only developed during World War 11. Besides,the shift from sine-wave techniques to pulse tech-niques implied more than simply a change ofequipment. By tradition, all definitions were basedon linear systems and their corresponding, well-known, linear differential equations. Now, with thenew pulse circuitry, many circuits contained anon-linear element which could no longer he con-sidered as a small deviation from the linear model,but whose non-linearity was of a very fundamentalnature.

In fact, this introduetion of a non-linear elementin a circuit which for the rest is linear, gave rise tovery difficult mathematical problems which haveonly been partly solved. In this relation may bementioned the name of Balthasar van der Pol, whowas at this laboratory during many years and whoinvestigated many fundamental problems of thiskind 1). The fascinating point in these combinedlinear and non-linear circuits is that va'rious effectshave been discovered which have led to importanttechnical applications. For instance, parametrieamplification of very high frequencies belongsto this class. Here the variation of a linear networkleads to an energy transfer between different fre-quencies that can be used for amplification.

Regarding the possibility of transmitting infor-mation by means of modulated pulses, a close anal-ogy was found to modulated sine waves. Again, themodulation in amplitude was very simple, but didnot reduce noise. Modulating the pulses in theirposition in time, however, could reduce the back-ground noise, just as was found with the method offrequency modulation, mentioned earlier. Analysisof both systems showed that the improvement wasproportional to the bandwidth used for trans-mission.

For some time this led to the belief that anyimprovement in signal-to-noise ratio required aproportional increase of bandwidth. However, thisconclusion proved to be wrong in the light of the de-velopment of modern information theory as founded

1) Balth. van der Pol, Selected scientific papers, North-Holland Publ. Co., Amsterdam 1960.

by Shannon and others 2). On a purely mathematicalbasis, Shannon derived an upper limit for the im-provement to be gained if the most efficient codingwere used. It showed clearly that this improvementmight be far greater than simply proportional andin fact should be of an exponential nature.

Pulse-code modulation: translating speech into num-bers

Though we are still far from reaching Shannon'supper limit (which, by the way, would involve verylong time delays), there is now one method known inwhich this exponential improvement with band-width can be realized. This is the modulation methodknown as pulse-code modulation. The underlyingidea is that signals like speech or music should notbe transmitted in the form in which they are de-livered by a microphone, but that they should betransmitted as numbers. This means that at thetransmitting end the signals must be analysed indigital form, the digits then being transmitted aspulses which may have either the value 1 or 0, andat the receiver the digits being used for reconstruct-ing the original waveform (jig. 1).

/'" ----[IJ--. I . I I • I •• ---ffi--FFig. 1. Functional diagram of a pulse-code-modulation link:C coder, for converting speech into a sequence of "1" and"0" pulses; D decoder, for reconstructing an output signalwhich is nearly identical to the original one.

The principle could easily be compared withcommunicating the information contained in agraphical curve, by means of a table representingthe horizontal and vertical coordinates. Indeed inpractice the speech signals are measured within smallintervals of time and the scale of amplitudes maybe divided into, for instance, 128 different levels.Each of these levels can be characterized by a num-ber expressed by 7 digits in the binary scale, becauseexactly 128 different numbers can be distinguishedin this way.With alittlebit of sophisticated circuitrythese numbers can be converted into voltages again.Using this type ofmodulation a maximum deviationof 1% or smaller can now be guaranteed in the out-put signal, whereas the rather large deviation of50% can he tolerated in the received pulses (jifS. 2).The efficient use of bandwidth in this case may bedemonstrated by the fact that with the addition ofone digit to a group of, say, 7 binary digits, the totalnumber of available amplitude levels is doubled,

2) C. E. Shannon, A mathematical thecry of communication,Bell Syst. tech. J. 27, 379-423 and 623-656, 1948.

1962/63, No. 11/12 MODULATION METHODS 355

whereas the effect on the resulting pulse frequency isvery small, namely in the ratio 8 to 7. In these sys-tems we do not have an exact reproduetion of theoriginal signals, but the remaining difference (whichis often called "quantization noise") can be madevery small by choosing a high enough pulse fre-quency (50 to 60 kc/s).Transmitting information in the form of numbers

has another advantage in the case of a large num-ber of links put in tandem. The received pulses canbe regenerated in each section so that small noisecomponents in the different sections can be elimi-nated before they have an accumulating effect. Inthe more conventional modulation systems, wherethe information is transmitted by varying a phys-ical parameter in proportion to the original signal,there is no possibility of distinguishing betweensignal and noise and in that case noise entering thedifferent sections will be necessarily accumulated.By using a special type of computer for coding

and decoding speech, a more efficient use of band-width can thus be made for obtaining noise reduc-tion than with the older methods of modulation.The question naturally arises whether this is an aca-demic result only or whether it can be put intopractice. During the last few years it has become ap-parent that an important practical problem can besolved in this way. This concerns the overcrowdedtelephone connections between telephone exchangesin large cities. A large number of telephone con-nections use the same cable (up to 900 pairs inone cable). The laying of new cable in these cities,however, causes so much trouble that the growingneed for more telephone connections should prefer-ably be met by making use of the existing cablenetwork.

Fig. 2. Regeneration of a pulse series which is affected by noise,in a P.C.M. system. The incoming signal is sampled at theproper instants of time and, in accordance with the level ofthe detected signal, a unit pulse is, or is not, regenerated. Aslong as a disturbance does not exceed a given maximum, itcan be completely eliminated.

Nat~rally one could extend the number of con-nections by using higher frequencies. To this endthe mutual interference between different pairsin a cable should be eliminated, as it wouldotherwise cause serious "crosstalk" between thedifferent channels. Unfortunately, however, thiscrosstalk is about proportional to the frequency.Using conventional methods of noise reduction theinterferences could be suppressed by enlarging thebandwidth proportionally, but if the interferencesalso increase in this proportion the method is notvery effective. So a more rapid improvement iswanted, and this is furnished by pulse-code modu-lation. In practical systems, as have been installedrecently in the U.S.A. 3), pulse frequencies of aboutIt million pulses per second are used in the cables.As a result the number of telephone connectionscan be extended by a factor 12 and at the sametime the quality of the new connections may heequal to or even better than that of the olderones.

Delta modulations introducing a memory

The realization of a coding procedure using thebinary code will always require some form of acomputer. The question arises whether the conver-sion from a continuous signal into a series of"1" and "0" pulses, and the reversed procedure,could not b~ effected by means of a far simplerdevice. This problem was investigated some yearsago by Schouten and others of this laboratory. Itwas, found that, instead of using a binary code witha large number of digits, the sequence of "1" and"0" pulses could be arranged in such a manner thatthis pulse series only needs to be applied to theinput of a simple electrical network or filter in or-der to generate the required signal at the output.This method is called delta modulation, and it isbased on a simple control mechanism 4).

In order to achieve the correct sequence of thepulses three things must be taken into account: thepresent deviation between the original and theapproximation signal, the way in. which precedingquantization errors can still be corrected and thechanges to be expected in the information signal.So it might he said that each decision oftransmittinga "1" or a "0" pulse is based, in fact, on the present,the past and the future. These functions can all heperformed by a linear network, functioning as a

3) C. G. Davis, An experimental pulse code modulation systemr for short-haul trunks, Bell Syst. tech. J. 41, 1-24, 1962

(No. I).4) J. F. Schouten, F. de Jager and.T. A. Greefkes, Delta modu-

lation, a new modulation system for telecommunication,Philips tech. Rev. 13, 237-245, 1951/52. <,

356 PHILlPS TECHNICAL REVIEW VOLUME 24

memory, which is put in a non-linear feedback loop,thus making the circuit self-correcting 5) (figs. 3and 4).

In analysing this method of delta modulationthe way of transmitting information can, I think,best be illustrated by the following model. Ifthe original information signal corresponds to awinding road, the transmitter may be seen as acar which is driven along this road. Here the pulses

Fig. 3. Functional diagram of a delta modulation linie A"quantized" feedback circuit at the tra nsmit.ting end produces"1" and "0" pulses. As a consequence of the feedback action,the signal at the output of the filter F resembles the originalvery closely. The same filter can thus be applied at the receiv-ing end for converting the pulse series into an identical approxi-mation signal.

--------0

Fig. 4. Approximation of a continuous signal (a) by means of aquantized signal (b), obtained by changing the slope at rel!ularintervals of time. Decisions on changing this slope are not basedon the instantaneous deviations between original and approxi-mation signal, but on their predicted values.

given to the steering wheel force the car to followthe road. This indeed implies a feedback loopbecause the driver watches the road ahead, predictsthe movements of the car, and decides from theexpected discrepancies whether or not a new pulsehas to be applied to the steering wheel. If nowidentical pulses are given to the steering wheel ofa hypothetical car, which is running through adesert, then a fairly good reproduction of the paththat is followed by the fust car can be obtained.It was found that speech lends itself very well to

this kind of coded transmission. Quite recently,however, the method has also been applied to nor-mal television by Balder and Kramer of this labora-tory 6). The whole computer, which can make 100million decisions per second, is contained in asmall box (fig. 5). This very high speed, by the way,

5) F. de Jager, Delta modulation, a method of P.C.M. trans-mission using the I-unit code, Philips Res. Repts 7, 442·466, 1952.

6) J. C. Balder and C. Kramer, Video transmission by deltamodulation using tunnel diodes, Proc. IRE 50, 4.28-431,1962 (No. 4).

is only attained by making use of tunnel diodes:very small elements of semiconductor type, whichcan be switched in one or the other direction 111

about one thousandth of a micro-second.

The TAS! system: using the time distrihution of

speechWith all modulation methods considered so far,

the waveform of the signal that is to be transmittedis arbitrary to a large extent. It should only be limit-ed in amplitude, of course. In modern modulationsystems, however, more and more account is takenof the statistical properties of the signal, either intime, frequency or amplitude. For instance, speechis never a continuous stream of information but itconsists of a large number of very short signals, oftenno longer than one tenth of a second. So the timeduring which signals are really present is very muchreduced and one could consider using the silent timeintervals for transmitting other information signals.This in fact has been done on circuits using atransatlantic telephone cable. In a large group oftelephone conversations the signals are split up intoparts and only those parts carrying information aretransmitted, after being labelled in such a way thatthese elementary parts containing information canbe directed to the correct receiver. The system wascalled Time Assignment Speech Interpolation and itwas put in service on transatlantic cables byWestern Electric in 1960. The result is that theoriginal capacity of 36 channels could be enlargedto 72 channels 7).

Of course systems like these are very complex,but on the other hand they are very economical too.

Fig. 5. Tunnel·diode delta modulator for coding televisionsignals (see 6».

') J. M. Fraser, D. B. Bullock and N. G. Long, Over-allcharacteristics of a TASI system, Bell Syst. tech. J. 41,1439-1454, 1962 (No. 4).

Now we enter the studyof the speech signalsthemselves. In analysingspeech it is found that aspeech signal can be char-acterized by a number ofparameters like intensity,pitch, spectral distribu-tion, etc. One thing isclear now beyond anydoubt. The maximumspeed ofvariation of theseparameters is far lowerthan the speed allowedby the normal telephonechannel. This fis due simply to the fact thatthe rate of speech is limited by the velocity ofmuscular movements. Thus the available bandwidthof a telephone channel is being used in a rather in-efficient way. This can even be expressed quantita-tively. According to Shannon's coding theorem anormal telephone channel of 3 kcfs bandwidth and50 dB signal-to-noise ratio has an upper limit oftransmitting information of about 50 000 bits persecond. Our senses, however, are not able to detectmore than about 50 bits per second. Nevertheless,for the exact reproduction of a speech waveform abandwidth of 3 kcfs is required and this means that alarge part of our speech signals does not carry newinformation but consists of repetitions only. Thisphenomenon may be observed in any oscillogram ofspeech (fig. 6). Of course, in everyday life this largeamount of redundancy is very important, for itenables us to disregard many disturbances.The technical aspect of this redundancy is that,

if we are able to distinguish the proper physicalparameters of speech, we could, bytransmitting them,carry a speech signal in far less bandwidth than weare accustomed to do now. Theoretically the reduc-tion could be more than a hundred to one, but weshould he very glad to achieve a ten-to-one ratio.

1962/63, No. 11/12

Indeed, the installationcosts of a transatlanticcable is of the order of 40million dollar, so that todouble the number ofchannels it is far cheaperto build a somewhat com-plicated apparatus thanto lay a new cable.

Speech bandwidth com-presslons studying speechparameters

MODULATION METHODS

To

-~- .

ER

s

Fig. 6. Waveform of the word "ALTERS", showing the periodicity in speech signals.(Taken from H. Fletcher, Speech and hearing in communication, p. 33, Van Nostrand,New York 1953.)

This again would be very economical for use ontransatlantic cables, and also in those frequencyregions of radio transmission which are now over-crowded.

The study of these speech parameters dates backalready to the eighteenth century, when Wolfgangvon Kempelen built a famous Speaking-machine,using bellows and horns, for reproducing speechsounds 8). It seems that the reproduetion of thesesounds was fairly good, as it is reported that hismachine could even speak Latin. A more up-to-datespeaking machine however was constructed anddemonstrated by Dudley 9) in 1939 (fig. 7). Thiselectrical machine could be operated manually, butthe necessary parameters could also be extractedautomatically from real speech by means of an"analyser" at the transmitting end which was con-nected to the "speech synthesizer" at the receivingend. This "vocoder", as it was named, was able to

8) H. Dudley and T. H. Tarnoczy, The speaking machine ofWolfgang von Kempelen, J. Acoust. Soc. Amer. 22,151-166, 1950. See also a forthcoming issue of Philips tech.Rev. devoted to the Institute for Perception Research.

9) H. Dudley, Remaking speech, J. Acoust. Soc. Amer. 11,169-177, 1939.

357

358 PHILIPS TECHNICAL REVIEW

@ RESONATORS

NE\U:5~KÇ--""

® VIBRATORS

"'" B- PERIODIC----------VOCAL CORDS BUZZER--------

CD POWER SUPPLY____ ---- LUNGS ELECTRICAL----,..

Through this researcha large amount of infor-mation relating to thesespeech parameters isavailable now. There areprobably two reasons whyit has not been possible,so far, to construct areal high-quality vocoder.Firstly, our hearing mech-anism makes use of avery great number ofnerve fibres which areused for the most partin parallel. Constructingan electrical model wouldthus require so many cir-cuits that it is technicallyunattractive. (Possiblythe progress in micro-miniaturization may beof help here.) Secondly,however, we have arrivedat the far more seriousproblem that, though wehave analysed speechsounds and the mechan-ism of the ear very care-

fully, we really do not know how the receivedsignals are combined and interpreted by thehuman brain. As long as this is not known, thedesign of new vocoders will necessarily proceedin a haphazard way (which, eventually, may leadto a solution), but we must not be surprised tofind that the analysis of real speech is farmore difficult for a machine than for a human

CONTROLS:

C VOICED

VOCAL SYSTEM OF MANSYNTHET~ SPEAKER

transmit speech by means of a great number ofparameters, each varying so slowly that a bandwidthreduction of a factor 10 could be obtained. The prin-ciple on which it was based was: splitting the fre-quency band ofnormal speech into a great number ofchannels, measuring the power in each channel andthen regenerating a signal having a correspondingpower spectrum at the receiving end. Most of thespeech compression systems of today still use thesame principle 10); seefig. Ba and b. The results of thisfirst vocoder were very promising, though theremade speech suffered from a lack of naturalness.

Pattern recognition: the missing linkExperiments on speech band-compression sys-

tems were continued, but up to now it has beenfound extremely difficult to achieve natural quality.However, this problem must be solved first, beforeapplication to the public telephone system ispossible. Indeed, any telephone subscriber expectsto be answered by a human voice, and not by amachine reproducing words in a monotonous, imp er-

.sonal way.

10) F. H. Slaymaker, Bandwidth compression by means ofvocoders, IRE Trans. on Audio AU-8, 20-26, 1960.

Fig. 7. Functional comparison of synthetic speaker with the human vocal system. (Takenfrom H. Dudley, R. R. Riesz and S. S. A. Watkins, A synthetic speaker, J. FranklinInst. 227, 739-764, 1939, fig. 5.)

VOLUME 24

observer.These problems belong, in fact, to the more gen-

eral problem of pattern recognition: how to buildmachines which can read and identify informationalelements like printêd letters, numbers, or acousticpatterns as encountered here. This is a relativelynew field of research and it will require the combinedeffort of mathematical, physical and biologicalresearch to solve these problems.

Now suppose one discovers what the "informa-tion-bearing elements" of speech really are and thatone knows how to develop a system of speech band-width compression based on this. There still wouldarise a curious problem: what will a speech com-pression system do with the crackling noise ofpaper that sometimes accompanies the speaker'svoice into the microphone? The system is taughtto reproduce speech signals only and so it will

1962/63, No. 11/12 MODULATION METHODS 359

Fig. 8. Sketch of the apparatus in a vocoder system. (Taken from Slaymaker '0).)a) Transmitting end.b) Receiving end.

convert any noise of this kind into speech. Thesame effect, by the way, is met in experimentswith frequency-band compression systems for tele-vision, where noise is not demodulated as noise butas a new type of signal. Practical difficulties of thiskind will limit the compression ratio of high-quality vocoders.As regards practical application of vocoders in

communication an intermediate solution may befound, perhaps, in transmitting a small part of thespeech spectrum in its natural way and completingthis with an additional transmission of parametersrepresenting the larger part of the spectrum. Infact it has been found that especially the lower-frequency region from about 300to 800 cis lends itself

a

conveniently for direct transmission, if it is combinedwith a parametric transmission of the remainingpart of the frequency band. The signals of the firstfrequency range then constitute a natural base forthe reconstructed speech signals and the difficultproblem of pitch determination is avoided. Theconstruction of a vocoder is much easier in thisway, though the resulting ratio of compression isalso much lower.In spite of the above-mentioned difficulties in

vocoder systems it must be admitted that someprogress has been made during the last few years inrelation to low-quality vocoders. If intelligibilityand not naturalness is of primary importance itis now possible to transmit speech signals withonly 2000 "1" or "0" pulses per second. In mili-tary applications these pulses can be re-coded forthe purpose of security and then be transmittedvia the normal telephone network. This is a more

reliable operation than trying to obtain security byhandling the speech wave in its original form.

Compandors: using the amplitude distribution ofspeech

In the foregoing examples we have seen how thestatistical properties of the time and frequencydistributions of speech can be used for increasingthe number of channels. From an engineering pointof view it is worth-while to ask what can be donewith the statistics of the amplitude distribution. Itis found then primarily that this knowledge can beused for obtaining a better noise reduction. General-ly speaking, the large range of different amplitudesoccurring in speech or music can be compressed into

CHANNELSBANO-PASS

• FILTERS MODULATORS'APPROX. 200'V

10---

30----'

4 C~-,':'-

TO

b

a smaller range during transmission, and after recep-tion it can be expanded again. This especiallyfavours the very small information signals whichotherwise would have been lost in the noise. Devicesof this kind, using a compressor and a correspondingexpandor, are called compandors 11). They areapplied more and more in telephony on circuitswhich would otherwise be useless. In fact a non-linear characteristic is necessary for this purpose, butit must be used in such a way that non-linear distor-tion of the signal is avoided. This seems a contra-diction at first sight, but it is attainable by usingdifferent time constants. There are two possibilities:either the signal itself, or a separate pilot signal maybe employed in controlling this non-linear mechan-ism. The first case is applied to normal compan-dors, where the compression ratio is relatively low,

11) N. Valentini, The dynamics compressor-expandor (corn-pandor) in telephony, Telettra No. 2, 12-22, Sept. 1954.

360 PHILlPS TECHNICAL REVIEW VOLUME 24

mostly two to one in a decibel scale. The secondcase - that of the pilot compandor - can be usedwith a very high compression ratio and allows trans-mission via circuits suffering from a very highnoise level. As the operation of pilot compandorscan be made independent of the received signallevel, these are very suitable at the same time forcompensating "fading" effects.

Though it is not our object to go into the detailsof a compandor, its effect can best be illustrated bymeans of the following example. If we suppose thatthe peak value of a speech signal is only two orthree times as large as the mean value of the noise,then it will be clear that many parts of the speechsignal are drowned by the noise. By using a oompan-dor the noise can now be decreased more or less inproportion to the mean amplitude of the speech,so that also signals of small amplitude - oftencarrying much information - can rise above thenoise. Indeed it has been found that if the accom-panying noise is modulated in the same rhythm andin proportion to the amplitude of the speech, thennoise and speech voltages may be of the same orderbefore intelligibility is destroyed 12).

An important aspect of noise reduction by meansof these compandors is that practically no extrabandwidth is necessary here, contrary to the oldermethod of frequency modulation where a propor-tional increase of bandwidth is required. This meansthat compandors can also be used in the future formaking available more telecommunication channelsusing radio waves.

Further, the combination of a compandor withother methods of modulation, such as pulse-codemodulation, has turned out to be very fruitful. Inthis case it is only the quantization noise, and nottransmission noise, which is of importance and whichis reduced in the output signal: this enables us toapply lower pulse frequencies. If we may mentiondelta modulation again, it was found quite recentlyby Greefkes of this laboratory that the feedbackcircuit which is already used in delta modulationcan, at the same time, perform the function of acompandor 13). In that case two different informs-tion signals are transmitted by the generated pulseseries, one relating to the detailed structure of thespeech wave, and the other to the level of thespeech. Referring again to the model of a car follow-ing a road, it might be said that the sensitivity ofthe steering mechanism is adjusted here to the mean

12) F. de Jager and J. A. Greefkes, "Frena", a system of speechtransmission at high noise levels, Philips tech. Rev. 19,73-83, 1957/58.

13) Belgian Patent No. 620450.

curvature of the road. With this system of "con-tinuous delta modulation", as it was called, it wasfound possible to reduce the necessary pulse frequen-cy from about 60 to 30 kcls and even with the ratherlow pulse frequency of 16 kcls a 20 dB change in thelevel of the input signal could still be tolerated.

In this way compandors are used in a process of"adaptation" that is largely analogous to adapta-tion in the biological sense. It enables us to handleinformation signals of widely varying signalstrengths, in more or less the same way as the inten-sity of light entering the human eye is controlledby a process of adaptation. We can srill learn muchfrom these biological processes, for instance as re-gards the time constants involved. and the researchon different aspects of perception and adaptationwill, therefore, be of great value for the design ofoptimum communication systems for use in thefuture.

Compatibility

In the laboratory it has been shown that a greatvariety of oompandors is useful for noise reduction,each having its specific advantages. In "point-to-point" communication one is free to choose any ofthese. However, for general application, one isfaced now with the difficult problem of "compati-bility". It is desirable that most transmittersand receivers can communicate without muchalteration of apparatus and this was comparativelyeasy when only amplitude and frequency modu-lation were employed, but with the introduetion ofnew ideas the technical developments have divergedso much that achieving compatibility is an almosthopeless task. This is one of the main reasons whymany improvements in the method of modulationhave been held back for a long time before beingput in practice. The need for more and bettercommunication links, to be expected in the nearfuture, will soon force us, however, to apply onlythe best principles that are known 14).

Considering the last mentioned aspect of com-patibility, an important problem is found in broad-cast transmission. Most of the transmitters applyamplitude modulation and all receivers are con-structed for this type of modulation. However, inthis case half of the bandwidth is wasted by usingthe symmetrical waveform that is generated in thenormal way. It has already been known for 40years that single-sideband transmission is to bepreferred as regards power and frequency demands,

14) G. Jacobs, Radio interference - suicide or challenge, IRETrans. on radio frequency interference RFI-4, No. 2,21-23,1962.

1962/63, No. 11/12 MODULATION METHODS 361

and so this method is commonly applied in systemsof carrier telephony. However, a somewhat morecomplicated receiver is necessary in this case andthis difficulty has excluded the use of single-side-band transmission for broadcast purposes. Severalsolutions have been proposed for generating a"compatible" single-sideband signal capable ofbeing received by all normal broadcast receivers,but the distortion involved is usually too large forhigh-quality reproduetion of music. Van Kesseland others from this laboratory have recently foundan elegant solution to this problem, based on acareful mathematical analysis. A normal broadcasttransmitter need be only slightly changed here inorder to produce single-sideband components ofsuch phase and amplitude that any receiver detectsthe signal wave form without distortion 15). Theresult is a more effective use of transmitter powerand a better reproduction of the higher frequenciesin any conventional receiver.

Television

Another field in which it is worth-while to look forthe essential information is that of television.Although television pictures are two-dimensional,their information is transmitted from point to point,which requires the rather large bandwidth of about5 Mc/s. This bandwidth (sufficient for more than1000 telephone channels) would permit us to changethe whole television picture 25 times per second,which would neither be perceived nor enjoyed byany human observer. Considering the statistics ofthe signal we find, again, that most parts of thesignal are repeated many times. In reducing theamount of available information one is faced, how-ever, with the difficulty that the observer's attentionmay be directed to any part of the picture, and thisrequires the picture quality to be maintained at asufficient high degree over the whole area.For reducing bandwidth one can in this case either

use the correlation which is found to exist in time,or in place, of the succeeding signals, or one canmake use of the slowness with which brightness.variations can be perceived by the human eye.Several systems have already been investigated, forinstance by Teer in this laboratory 16). Some resultshàVe been found with respect to colour television .For instance, it was found possible to add colour

15) Th. J. van Kessel, F. L. H. M. Stumpers and J. M. A.Uyen, A method for obtaining compatible single-sidebandmodulation, E.B.U. Rev. Part A, No. 71, 12-19, 1962.

16) K. Teer, Investigations into redundancy and possible band-width compression in television transmission, P.hiIips Res.Repts 14, 501-556, 1959, and 15, 30-96, 1960.

information without increasing the bandwidth. Inthat case a number of signals, containing informa-tion about the different colours, can be transmittedsimultaneously in the same frequency band, and itis possible to suppress mutual interferences bychoosing special phase and frequency relations of thesubcarriers used 17). Thanks to the integrating effectofthe human eye the picture appears natural. As thesystem is compatible, a picture in black and whitecan still be obtained with a conventional receiver.

Data transmission

So far we have considered the modern aspects oftransmIttmg continuously varying informationsignals such as speech, music or television. Duringthe last decade, however, a growing interest is foundin the transmission of telegraphic signals, especiallyin the form of high-speed telegraphy or data trans-mission. Indeed, with the growing application ofcomputers, equipment for data-processing, etc., alarge amount of digital information is available nowwhich often needs to be transported over long dis-tances. By means of the thousands of pulses thatcan be transmitted per second via a telephone cir-cuit, the speed of transmitting this information canbe much higher, of course, than with a normal tele-phone conversation.

So we find more and more that the telephonenetwork, originally designed for speech, is used nowby machines which are talking in digitallanguages.An air-line reservation system, answering questionswithin one second, represents a typical example ofthis kind. Reliability is often of primary impor-tance in these cases and so error-detection anderror-correction schemes are usually applied. Just asthe redundancy which is present in normal speechwas found to be very useful for a reliable trans-mission, we may apply a redundancy in this digitalinformation by introducing check-bits. It was point-ed out by Golay 18), and in more detail by Ham-ming 19), that by applying special codes, these check-bits could not only be used for detection of errors,but also for automatic error correction. Nowadays agreat interest is taken in the scientific aspects oferror-correcting codes and it is a remarkable factthat many ideas which have originated in a branch. of pure science, namely number theory, find herepractical applications 20).

17) K. Teer, Colour-television transmission, Electronic andRadio Engr. 34, 280-286, 326-332, 1957.

18) M. J. E. Golay, Notes on digital coding, Proc. IRE 37,657, 1949.

19) R. W. Hamming, Error detecting and error correcting codes,Bell Syst. tech. J. 29, 147-160, 1950.

20) W. W. Peterson, Error-correcting codes, M.LT. Press, 1961.

362 PHILlPS TECHNICAL REVIEW VOLUME 24

In this way modulation theory is now closelyrelated to the storage of information in memories,the way of handling this information by means oflogical circuits and the problem of performing thesefunctions in the most effective manner. Again thestudy of statistics has entered the field, though it isnot the statistics of the signal to be transmitted, butthe statistics of the transmission path which is nowof primary importance.The transmission of pulses through the normal

telephone network, however, presents its own diffi-culties. This network has been designed for speechtransmission and many peculiarities which do nothave the slightest effect on speech (such as the sup-pression of very low frequencies or the influence ofphase distortion) may do much harm in the case ofpulse transmission.If a new telephone network were to be designed,

then the requirements for digital transmission wouldcertainly be taken into account. But the large in-vestments in telephone circuits all over the worldforce us to work the other way round. So we have tolook for modulation methods which permit thetransmission of digital conversations via speechcircuits, in a more effective manner than is mostlyused nowadays. One might even say that the possi-bility ofusing and connecting data-processing equip-ment in the future depends on the development ofbetter methods of modulation, giving faster andmore reliable operation. At present there is muchresearch in this field. Consider speed, for example; asystem was built in this laboratory for transmitting4000 bits per second, using only part of the avail-able bandwidth in a normal speech circuit andtransmitting information at a rate very close to thetheoretical maximum 21); see fig. 9.

Many problems, however, remain to be solvedbefore the specific troubles on telephone connectionsare overcome. Some of them are related to the prob-lems of radio transmission, others are of a quitespecific nature. For instance the large and un-predictable phase distortions which are encounteredon switched telephone connections present seriousdifficulties. In this case the disturbing voltages aregenerated by the signal itself, and so improving thesignal-to-noise ratio by increasing the signal poweris of no use here, as the interfering voltages aregrowing proportionally. So we must look for othermeans which are effective and not too complicated.

21) F. de Jager and P. J. van Gerwen, CO-modnlation, a newmethod for high-speed data transmission, IRE Trans. oninformation theory IT-8, S 285·S290, 1962 (No. 5).

Looking into the futureComparing modulation today with that of fifty

years ago, we find that we can do much more; but weare also faced with many more problems. Develop-ment in the next fifty years may be expected, Ithink, to follow two different trends. The first is themore practical application of theoretical resultsobtained in "information theory". This will probablylead to the design of systems that approach moreand more the theoretical limits, thereby increasingreliability and using less power and less frequencyspace than is found in existing systems. (In satellitecommunications for instance this is very urgent. Bymaking use of special amplifiers - masers - onehas already nearly reached the physical limit ofattainable signal-to-noise ratio.) The second aspectis the closer relation between man and machine,such that the possibilities of adaptation andcontrol in electrical systems can be used in relationwith the sensory behaviour of man. For the designof these adaptive systems we need many moreresults of the research in "perception" (as in-vestigated for instance by Schouten and his co-workers at the "Instituut voor Perceptie Onder-zoek" 22)). The combination of the results in thesetwo fields - information theory and perception -will probably be the most important characteristicof future development. So far the combination of thetwo has presented some difficulty, which may be dueto the fact that problems and results in the firstfield are usually considered as "exact", whereas theresults in the second field are of a more "subjective"

I I II

II

I

II

I1

I,I

I1

I1I

I I III

I

I1

I I 1,\

1

\I,, '\ / I

\\ I 24kc/s

,\ ;

,\ ." ~

I,~

../....._-- I "

35dB

30

25

20

15

10

5

oo 0,5 1,5 2 2,5 3 3,5kcjs

Fig. 9. Bandwidth used for transmission of data signals at aspeed of 4000 bits/sec, compared with the bandwidth (dottedline) of a normal telephone channel (see 21».

22) J. F. Schouten, The Institute for Perception Research, toappear in a forthcoming issue of Philips tech. Rev. devot-ed to this Institute.

1962/63, No. 11/12 MODULATION METHODS 363

nature. But if enough people can he found who areinterested in both of them, we may expect revolu-tionary developments in modulation systems forthe future.

During many years engineers have transmittedelectrical signals carrying information, withoutconsidering the type of information contained inthem. Now research is directed towards finding the"information-hearing elements" of these signals.This is not a question of academic interest only. In-deed, the need for improving modulation systems isso very urgent, in view of the acute' expansion intelecommunications, that engineers are forced tostudy these fundamental aspects of human commu-nication more seriously. The solution of theseproblems, however, will require more mathematical,physical and hiological analysis - together with theart of good guessing.

The large numher of' communication channelsspread over the world (and sometimes even reachingother planets) may he looked upon as an enormousnervous system which mankind has huilt in only.half a century and which is going to he used moreand more. By means of modulation theory we tryto discover how to use these nerves most efficiently.

Summary. In this article it is shown how the ever increasingdemands on communication links have led to the Introduetionof new methods of modulation. After considering frequencymodulation, pulse code modulation and delta modulationspecial attention is given to those methods ofmodulation whichmake use, to a large extent, of the statistical properties of theinformation signals: bandwidth compression, compandors.Modern research is directed here towards finding the informa-tion-hearing elements in these signals. Finally some problemsrelated to compatibility, and to the transmission of digitalinformation, are considered.It is expected that in the future the combination of results

found in information theory and in perception research willleadto the design of still more effective methods of modulation.