Alec H. Reeves, OBE, ACGI, DIC, MIEE
Standard Telecommunication Laboratories,
Harlow, England
British Affiliate of International Telephone and Telegraph Corporation
Twenty-five years after its invention, it can be said that pulse code modulation
has little past as yet; the real interest is in its future. This future depends a great deal on how well
or how badly its main planning problems are tackled during the next decades or so. There is little
or no agreed view of the technical and more general points involved in this planning. It is
therefore vital that such points should be freely discussed now, to try to avoid irreversible wrong
decisions.
I'm hoping that my own views given in this article will do something to stimulate such discussion
- and the fiercer the criticism, the better!
Pulse Code Modulation (PCM), or Coded Step Modulation (CSM) which I
think would have bean an apter name, is a good example of an invention that came too early. I
conceived the idea in 1937 while working at the Paris Laboratories of International Telephone and
Telegraph Corporation. When PCM was patented in 1938 1 and is 1942
2 I knew that no tools then existed that could make it economic for general civilian use.
It is only in the last few years, in this semiconductor age, that its commercial value has begun to
be felt.
In 1937 I realised, though, that it could be the most powerful tool so far against interference on
speech - especially on long routes with many regenerative repeaters, as these devices could easily
be designed and spaced in such a way as to make the noise nearly non-cumulative.
The quantising noise was foreseen - and also the fact the effects on the listener could not a priori
be calculated, as it would be neither constant and additive, nor of a nature to produce simple,
fixed harmonic content for a given waveform and volume as with simple non-linear distortion. It
was clear that nothing but subjective tests could provide the information needed for design - and
it is strange that even now some PCM planners and committees seem to place a magical reliance
on calculated QN power, before those few subjective tests as are now in progress have even been
completed, let alone interpreted for the designer! It is strange too that these very necessary,
systematic, subjective tests were not started by someone much sooner; it was probably the doubt
about the need for PCM itself that delayed them.
My guess of thirty-two levels for constant speech volume was not far out - though I made a bad,
avoidable mistake in not realizing that these levels should be logarithmically companded.
PCM was invented mainly for line-of-sight microwave links or link sections, where in 1938 the
needed extra band-width would have been cheap and easily obtainable - not for more limited
frequency-bands, as in cables, which are now in fact the main fields of application. It is this change
of aim for PCM, for quite sound reasons, that has caused most of the technological difficulties so
far in its application.
Having got it patented, for understandable reasons I then let the invention slip from my mind until
the end of the war; and it was in the U.S.A. during World War 2 that the next step in PCM's
progress was made. It was in the Bell Telephone Laboratories that this next, important stage was
undertaken: a team under Ralph Bown, comprising among others Harold S. Black, carried out
practical design studies on PCM for the U.S. Army Signal Corps. It is right that in the records this
early Bell work should be stressed; for it was the first time that the principles underlying the new
system were translated into hardware.
It was no doubt the results of their war-time work that caused the Bell Laboratories to take the
next further steps forward. Suitably cheap, long-life components were still not available; the US
Bell System took a long shot, eventually proved correct, in deciding that PCM-steered efforts
would be justified at that date as a reasonable bet for future civil networks.
Apart from patents, the first public disclosures concerning PCM came from Bell Laboratories in
19473,4. In these articles Goodall and Black described many of the basic principles,
and some coder and decoder circuits are given. During the same year, 1947, ITT engineers
published three papers relating to noise and distortion in Pulse Count and PCM systems.
35,36,37 The next publication5 is a landmark; it is the first account, by Bell
Laboratories, of an experimental PCM multichannel link meeting toll quality. The stress was on
technical feasibility: in 1948 the economics of the method could be left to the future. In the same
issue of BSTJ is the first description 6 of the Bell Laboratories' electron-beam
coder tube, which in a more sophisticated form is proving useful to this day. In principle it is
elegant and simple; but it suffers from the disadvantage of a fairly short life, and needs a feedback
loop for sufficiently accurate beam alignment which problem has since been solved up to a point
quite neatly.
The next article7 is also a landmark - for in it Shannon, with Oliver and Pierce,
explained the philosophy of PCM in terms of previous theoretical work by Nyquist and Hartley
but in more usable form, thus making possible a little later his important contributions to
Information Theory, a branch of science vital to telecommunication and computer engineers alike.
At the end of that same year, too, Reiling8 discussed the use of companding in
PCM - a point, as I have already said, that might well have been foreseen in 1937.
By December 1948, then, most of the main factors required for efficient long-distance PCM were
well realized, together with its advantages and basic theory. The date marks the end of an
important phase.
After 1948 there was a growing emphasis on PCM studies in a number of other countries, e.g.
Britain, France, Japan and Germany, as well as the USA. Anything approaching a complete
review of it is impossible in this brief article, the examples that follow, chosen partly at random,
being intended merely as typical-of this world-wide effort.
Though invented by E.M. Deloraine in 1945 25 it was in 1951 and 1952 that a
different digital method for speech, the "delta" system, first claimed serious attention.
9,10 As is now well known, the delta method has a basic advantage for the particular nature
of speech waveforms in that it has less redundancy than PCM, in the effective number of levels
per signal-frequency period. On the debit side, however, the coding means is inefficient as there
are only two possible output levels per sample. As will be explained later, there are good gounds
for thinking that the speech networks of the future may well use a combination of PCM and delta.
That is why this non-PCM method is included in this PCM article.
Japan, now one of the most active countries in the PCM field, also started serious studies on the
subject during this period, in 1951, at the Electrical Communication Laboratories of the Nippon
Telegraph and Telephone public corporation. In the first few years the work here and elsewhere
in that area included 24channel systems by coding tubes, together with improved quantised
feedback methods to stabilize their beam alignments. The "reflected binary pattern" technique for
coding was invented by Dr. Kiyasu during this phase.
After the start of the practical semiconductor era, in about 1954, PCM planners and circuit
designers began to re-think their projects; for at last suitable tools to justify the relative
complexity of the terminal equipment were not only on the horizon but practically within their
grasp. PCM progress for civilian uses, after a good start, was nevertheless slow. I think there
were two main reasons:-- (a) the vast capital already locked up in the world's existing telephone
plant, which is at first sight not readily compatible with PCM - a type of factor decisive severa1
times before in delaying other improvements, as Deloraine pointed out in 1956 11;
and (b) the fact that while PCM was being developed the older analog methods were themselves
being improved. It is arrays a healthy cold shower, and a challenge, for a pioneer to have to
remember that his "wonderful new system" must compete not just with current equipment but
with improved versions of the older art at the time his invention is in production and that if he is
too slow he may never catch up at all!
Feedback coding, an older idea, was explored more fully by B.D. Smith in 1953 12;
and additionally a digitally companding type was discussed by J.G.H. Davis in 1962
13. The possible advantages, especially in accuracy, of companding digitally as part of the
coder itself rather than externally by a separate device, ware by then beginning to be
realized.
In 1956 an interesting new type of parallel coder, elegant and simple in conception, was invented
by A. T. Starr 14. It used a square-loop ferrite core, a reliable passive element, as
a near approach to a true level-decision device. Experiments showed, however, that on account
of magnetic break-through it would be difficult to get beyond about 32 levels usefully, not now
considered enough for straight PCM with a practical range of input volume.
Soon after about 1950 a further point was beginning to be realized: that if in electronic exchanges
the speech information was in digital form a number of switching problems could be simplified.
A little later it was realized, too, by PCM planners that their new system would fit much more
easily in the first instance into the local areas, where the parts had only to be compatible internally,
than into the toll routes where it must interconnect with conventional systems.
Considerations of this kind led Laboratoire Central de Telecommunications, the Paris
Laboratories of ITT, in 1958 to introduce PCM as a basic feature of their studies into electronic
switching methods15 and led ITT (in their London Standard Telecommunication
Laboratories), and AT&T in their "TI"system, to begin to develop and introduce a self-contained
24-channel method on individual junction cable pairs into such local areas16,17
Operation or experimental operational results have been most promising; and for at least this
application it is proving economical already, even without integrated circuitry, which make the
possible savings look still better.
Since 1961 Japan too has been very active in this PCM short haul field, mainly on a 24-channel
per group basis. The rapidly growing industry of this country and the consequent fast increase in
telephone demands has made it one of the most promising areas for PCM's early application and
further development. The first field trials of such a 24-channel system were satisfactorily
completed in 1964.
Japan's work during this period also included a method, due to Prof. Osatake, for transmitting the
code digits on a parallel basis within exchanges, rather than the serial means more usually
employed. It is now being applied to the switching system. Considerable economies are claimed
in high-capacity exchanges. In my opinion this version, though specifically foreseen in the first
PCM patent1, has in general been rather neglected.
In 1963 a new type of digitally companding system was disclosed18, based on
sending the information in two parts, as is done when expressing the characteristic and mantissa
of a logarithm. In 1963, too, what seems to be a new circuit principle was invented, applicable
basically to other problems as well as to PCM coding, called the "equilibrium"
method19. Potentially for PCM it has nearly the speed capabilities of a parallel
coder, while it is believed that the circuitry can be still simpler and cheaper than in the serial
variety. Improved methods for digital companding by the "equilibrium" process have also been
studied20.
One further coder idea should be mentioned here: it is a many level, time-counting version
believed operable at high bit rates, that does not suffer from the disadvantage of needing a very
high speed at the input stage of a binary counter21. Because time is the coded
parameter as in the first coder designed1 it is believed that a high degree of linearity
can be obtained cheaply in a coder-decoder combination. Such linearity would of course be
essential in any PCM equipment that was used for coding an FDM group or supergroup directly,
without prior separation and demodulation.
Much work has also been done in the past sixteen years on codes other than Simple Binary, for
two main purposes:- (a) to reduce the effects of single-digit errors from any cause and (b) to
reduce re-repeating errors caused by occasional code groups having heavy low frequency
components, in combination with cables or cable pairs in which attenuation falls steeply as the
frequency falls. The effort on (b) has led to a number of special codes and coders, examples of
which are the Alternate Mark Inversion, ternary type used in the AT&T "TI" equipment, and the
ITT "low-disparity" code for the same purpose16,17.
We now come to the progress on a solution which may prove to be a better answer than any other
for transmission of speech, whether single-channel or time-shared. It is called the "log differential"
method; it is a coded form of delta, that was studied first in the USA.
In principle it is not a very new idea, but only recently has it aroused much
interest22,23. Like delta, it can avoid for speech an unnecessary number of
transmitted levels; but unlike delta the level number that is still needed, e.g. about 32 to 128, is
sent in efficient binary coded form. Further statistical tests on articulation, naturalness, etc. are
essential before we can truly assess the new method. But so far results seem to show that 16
suitably-spaced levels on log differential PCM are about equivalent to the direct PCM use of 32
levels at optimum speech volume, and of perhaps a few more where volume range is the limiting
factor. Such a saving of at least one transmitted digit is quite important in most applications in
reducing the transmission bandwidth. The coder is simplified, moreover, and not, as first
expected, at the expense of the channel equipment.
In theory, the log differential method in its simplest form is equivalent to a network suitably
emphasizing the high-frequency speech components, followed by a normal log-law compandor
and coder. The log differential method has one drawback: it is not suitable for any waveform, only
those of the nature of ordinary speech. If multi-tone signalling were used on it, for example, we
do not yet know that the resulting intermodulation terms would be acceptable, though some early
tests have shown that there is a good chance that they may be in normal cases. The digital speech
systems of the longer-term future, though, will undoubtedly be efficient in both the signalling
paths and the speech paths and so will use digital methods for that purpose as well.
Progress has been made on PCM for at least one other application television. In one case in
point34 the object was to make possible TV transmission on a long-haul waveguide,
which almost unavoidably has phase-distortion characteristics, due to many slight discontinuities,
that make it unsuitable for nearly every other method. In Japan too a PCM coder for TV has been
developed: it used Esaki diodes, with 10 Mc/s sampling and 6-bit coding. The Bell Laboratories
as well have now a PCM TV System in an advanced development state.
No mention has been made so far of decoders. The reason for their scanty coverage in this article
is that they do not have to be decision devices and their design is therefore basically easy.
Particularly to line up with special coding methods, however, for example those of digitally
companding types, the circuit research worker can still find the problems interesting - to cheapen
the design, unconventional circuit methods can sometimes still be justified. One such unusual
design, a revival in new form of an older idea, has been studied by STL24.
There is not much that need be included under this heading, as the present
state of PCM is naturally the sum of its progress since the start, which has already been reviewed
in brief. PCM has been a child with a long infancy; except for certain, military uses not described
here, in application it is still only in the adolescent stage. At the moment [February 1965] the only
PCM systems definitely known to be in regular commercial operation comprise about 3000
24-channel groups of the Bell "TI" type, all in the USA, and about twelve similar groups of Italian
design, used in that country. Quite a few more "TI" trunks, though, are expected to be operating
in the USA in the immediate or near future. Nearly all of them use pairs in existing cables. As a
fair number of such circuits had become full when employed in the usual way and would have
been expensive to duplicate, especially in city areas, there was an immediate demand here for a
cheaper way of extending the inter-office service. It is most likely that the first sales of civil-type
PCM both in Europe and Japan will also be to meet this kind of demand., probably within the next
one or two years.
In the immediate future it is probable that the civilian market for PCM equipment will be confined.
to local area applications, mainly because the longer parts of the routes are in any case becoming
cheaper by conventional methods, and comprise only 15% of the total global investment in
telephone plant. This immediate need for only self contained types of PCM is perhaps not a bad
thing in the end for the future of the new art, as by these first steps any teething troubles can more
easily be cured without causing wide spread inconvenience and consequent prejudice against
it.
In PCM techniques we have now almost a multitude of coder and decoder ideas to choose from.
Much work has been done too on repeater design, including the problem of timing the
regenerative variety. But inter-terminal synchronizing clans, though now being discussed, are by
no means yet finalized.
Many of the national telephone administrations or public utility corporations are showing
increasing interest, and international bodies such as the CCITT are studying PCM closely; but it
is after the limit of about the next five years, a time scan discussed in the next section, that the
real, relatively wide exploitation of the method will begin.
It is the purpose of the present article to describe the civil aspects of PCM rather than the military.
In several countries, however, e.g. the U. S. A., the various defence departments have studied the
subject closely for some years, and have now decided that it will be a major factor in their
communications networks38.
PCM is known to work. It is known that its principles are sound. It is known too that it has many basic advantages, coupled with some limitations. In the world's networks, will it ever be used on a really large scale? Or will it, except for military and other special applications, remain a mere scientific curiosity? In making our informed guesses on these questions let us start by "seeing the woods" not the trees - by seeing in what ways, if any, PCM principles are likely to be needed to meet the probable general trends in the world's telecommunication expansion, not its power of solving specific, detailed problems. And let us take the unusual course of starting backwards, dealing first with the most future date that except in science fiction it is sensible to think of, then the near future, and ending with the middle distance. In that way immediate practical difficulties can be seen in better perspective while at the same time we shall keep our feet sufficiently on the ground. To me the problem is one of boring a tunnel through the next 36-year time span, not of building a road across it - for I see the two ends more clearly than I see the middle. In tunnelling, it is usual to complete the middle section last.
Let us take the year 2000. Consider first what new factors could then make
more efficient methods really necessary in a communications network, not just marginally or even
probably economic to install widely. In a newly developing area such as many in Africa or Asia,
though various new systems will have little or no backlog of older methods to link up with or to
replace and for that reason may well come earlier if they look economic, on no technical grounds
will they be a "must" - a choice will be there. It is in already highly-industrialized parts of the
world, especially where the average man and woman is already very telephone minded both for
social and business purposes, that basically new methods may become almost mandatory if the
daily demand for interconnections should exceed a certain figure.
The obvious example to take, then, is the USA. Extrapolating from the present growth rate and
including saturation effects, there should be about 220 million telephone subscribers in that area
in the year 2000, compared with the figure of 87,299,000 for July, 1964.26,27 This
is not a startling increase. Are unforeseen chain reaction between reduced costs and demand could
no doubt steepen the curve; so would a still further rise in telephone consciousness than we now
expect. But it would be unwise, I think, to assume an extra factor of more than about 50%, on
these two counts combined. There will no doubt be a great increase in data traffic that could react
on the telephone networks; but as some of it will use relatively small bandwidths it is unlikely to
affect the total information capacity needed in a really major way.
By the year 2000 new telephone methods, of which PCM, is an example, will by no means be a
"must" on a truly wide scale anywhere in the world, though economies may well justify a fairly
large mileage by such new and improved means. We shall have to seek other reasons, if they exist,
for PCMs really large-scale introduction into the world's civil networks.
In my view, this "other reason " by that date will be the necessity for widespread closed-loop TV
- a necessity, I repeat, not just the urge for a status symbol that is likely to start this kind of
demand in the nearer future. The first need will arise in the field of information retrieval.
Considering my own case as an example, it would take me even now about 30 hours in each day
of a seven-day week to keep myself thoroughly up-to-date in all the scientific and technological
subjects that I really need to know and to look up in my own sphere of circuitry research alone,
if I were to digest and consider properly all that I read. By AD 2000 it would be even more
impossible, by a large factor.
Though increased specialization and team work will help, there are limits to the usefulness of
"knowing more and more about less and less". The only possible answer will be a very greatly
streamlined way of getting information in the form needed, and at the exact moments that the
needs arise. Every professional man, industrialist, and administrator will require this
service.
The only adequate answer will be for a few information--processing centres to be set up in each
large industrialised area, staffed by top-grade people, the information being available to the public
immediately and automatically when a dialled request is made. An ordinary high-speed data link
may be good enough for the next twenty years or so, but by AD 2000 the only way to pass the
information fast enough to the callers brain will be to use moving pictures, films. By AD 2000 a
service of this general kind must come, for without it no nation will then by modern standards be
able even to survive, as the very life-blood of that survival will be the most efficient use of
knowledge. This need far better information services has already been pointed out by others
notably, for example, by Z.W. Halina in talks at a Study Group meeting of the AIEE, and to
another group in 196328,29. He has allowed me to mention. these talks in this
article.
It is my opinion that a second vital need will arise from the almost impossible transportation
problem in the year 2000. Commuters will refuse to accept the delays and inconveniences that
even a moderate journey to and from their place of work would then entail. Decentralized town
planning will alleviate this nuisance, but it is only in light industries that plants and offices can,
without undue loss, be sufficiently divided. We shall have to transport the brains, the skills of the
staff, not their bodies, to their daily jobs, again involving not merely ordinary data links but a great
many private TV channels as well. This new service will raise the communication demand- in the
area, measured in megacycle-miles, by several orders of magnitude - which would soon justify a
complete modernization of the network to suit it.
But will such a large increase in network bandwidth be technically possible? Yes - but, to make
economic sense at all, only by transmitting on optical beams. Some of the needed techniques are
not yet with us, but helped by the laser a fair start has been made30, 31 and what
we lack now can be available well before the end of the century if the conditions that we must all
meet at that date are realized by enough skilled people, and in time. It will happen; it is only the
shorter-term economics of optical methods that look less promising for public use.
What reaction will this revolution have on improved digital methods such as PCM? We have here
the reason for including optics in this article - for by well known basic physics connected with the
high energy per photon at optical and infrared frequencies the efficiency of optical methods, for
a given signal/noise ratio, is many times greater when digital rather than analog methods are
employed.
PCM with between 16 and about 80 levels would meet many of the TV requirements very well;
and of course on wavelengths from the visual to the near-IR the basic bandwidth available is many
times that needed for a long time to come. The detailed technical problems involved are extremely
interesting and merit wide discussion among the experts in the field; but space does not allow me
to go into them here.
In my view, therefore, by AD 2000 PCM in some form will be the very backbone of the world's
communication systems that are internal to national or still larger units. But except for satellite
routes, the widespread trans-ocean use of PCM may have to wait until later, as the severe
technological problems inherent in an optical submarine cable may not be solved until some years
further ahead.
As to the PCM equipment that will probably be used at that time, it is premature to make any
predictions even now there are many designs and even principles to choose from. A vital point,
though, is the means used to line up the gate-opening timings at the many regenerative repeaters,
terminals, and drop-off equipments, a matter that is now being debated. One school of thought
favors a "start-stop" method at the distribution points, with storage and retiming where necessary;
another sees this answer as one that could mortgage future good planning for the convenience of
only short-term goals. Though the arguments are fairly well balanced, I myself share the second
view. I believe that with the right principles and devices a network truly synchronous in average
frequency, that can also operate as independent local units in emergencies, is not only feasible
technically but economically justified at a fairly early stage32.
We have seen that PCM has had a slow growth. We have examined some of
the salient points in its early stages, and in present-day PCM thinking and application
trends.
Crystal balls de not usefully mix, so I have looked through just one of them, my own, to describe
what I see of the general communication landscape of the year 2000 and of the place in it of PCM
in particular - to my eye a major place in a wider digital background, that scenery as a whole being
one that can be ignored only at the peril of any nation which at that date ever wishes to survive
by modern standards. If this picture is deemed wrong, so be it: but if right in its essentials, let us
do our utmost to avoid at least the major kind of mistake that has so often been made in the past,
lack of adequate foresight in planning, national and international, hanging a millstone round our
necks that has taken decades to remove.
I an indebted to mane of my ITT colleagues for their advice and help in preparing this article: in
particular to David Thomas of Standard Telecommunication Laboratories, London, Maurice
Deloraine of ITT (Europe), and Henri Busignies of ITT (New York) and his staff.
I gratefully acknowledge too the help of Dr. Harashima and Dr. Susuki of Nippon Electric
Company, Tokyo, for their information on Japanese activities in the PCM field.
Principles of Pulse Code Modulation by Kenneth W. Cattermole
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1 | A. H. Reeves | French Patent 852,185 3rd October 1938. Assigned to ITT |
2 | A. H. Reeves | US Patent 2,272,070 3rd February 1942. Assigned to ITT |
3 | W. M. Goodall | Telephony by Pulse Code Modulation BSTJ 26 3951947 |
4 | H. S. Black | Pulse Code Modulation Bell Lab Record 25 265 1947 |
5 | L. A. Meacham and E. Petersen | An Experimental Multichannel Pulse Code Modulation System of Toll Quality BSTJ 27 No. 1 January 1948 1-43 |
6 | R.W. Sears | Electron Beam Deflection Tube far PCM BSTJ Jan. 1948 |
7 | B.M. Oliver, J.R. Pierce and C.E. Shannon | The Philosophy of PCM Proc IRE 36 1324 1948 |
8 | P.A. Reiling | Companding in PCM Bell Lab Record 2012 December 1948 487-490 |
9 | Schanten, Jager, Greetkes, | Delta Modulation: A new Modulation System for Telecomunication Philips Tech. Rev. 13 9 March 1952 |
10 | Jager | Delta Modulation: A Method of PCM Transmission using the 1-unit Code Philips Research Report 7 442-466 1952 |
11 | E.M: Deloraine | Pulse Techniques in Line and Radio Communication Electrical Communication 33 3 September 1956 |
12 | B. D. Smith | Coding by Feedback Methods Proc IRE August 1953 |
13 | J.C.H. Davis | A PCM Logarithmic Encoder for a Multi-channel TDM System Proc IEE 1098 4 8 November 1962 |
14 | A.T. Starr, K.W. Cattermole, J.C. Price | British Patent 806,397 November1957. Magnetic Coder using Materials having a Rectangular Hysteresis Characteristic ITT |
15 | Touraton, Le Corre | British Patent 932.612 ITT October 1958 |
16 | K.W. Cattermole D. R. Barber, J. C. Price, E. J.E. Smith | Experimental Pulse Code Modulation Transmission for Local Area Telephoning Electrical Communication Vol. 38 No. 1 1963 |
17 | C.G. Davis | An experimental Pulse Code Modulation System for Short-Haul Trunks BSTJ January 1962 |
18 | A.H. Reeves, D. R. Barber | Digital Companding Coder British Patent (Provisional) 3250/F3 ITT |
19 | A.H. REEVES | Equilibrium Coder British Patent (Provisional) 2084263 ITT |
20 | A.H. Reeves, R. S. Grant | British Patent - Applied. For ITT |
21 | A. E. Karbowiak, G. Craven | Transmission Aspects of Communications Networks Conference Paper, IEE 24-28 February 1964 "Time Quantisation, a New Approach to the Design of Ultralinear Analogue-to-Digital Conversion Equipment for Application of PCM Communication Systems." |
22 | C . C . Cutler U.S. | Patent 2,605, 361 |
23 | E.A. Feuell, J.M. Gardner B.J . Feenaghty | Log Differential Coder ITT 9637/64 British Patent (Provisional) |
24 | A. H. Reeves, K. W. Cattermole, R. Kitajewski | British Patent Provisional 37714/63 Vibrational Decoder ITT |
25 | B. Derjavitch, M. Deloraine, S. Van Mierlo | French Patent 93,140 August 1946 ITT Also filed in the USA as US Patent 2,629,857 Oct. 1947 |
26 | Telecommunication Journal of Australia, Feb. 193 p 454 | |
27 | ATT "World's Telephones" 1963 | |
28 | AIEE Winter Study Group on Data Communication Systems, New York, April 1963 | |
29 | American University Institute on Data Transmission, Washington, June 1964 | |
30 | C.C. Eaglesfield | Optical Pipeline: ~A Tentative Assessment Proc IEE Vol 109B No. 243 |
31 | G. Goubeau, J.r. Christian | Some Aspects of Beam Waveguidds for Long-Distance Transmission at Optical Frequencies IEE Trans on Microwave Theory and Techniques, March 1964 |
32 | A.H. Reeves | "PCM Synchronising Methods" - article not yet published. |
33 | A.H. Reeves | "Stored Negative Feedback for PCM Coders" - article not yet published. |
34 | W. Neu | Some Techniques of Pulse Code Modulation Senderabdruk aus dem Bulletin des Schweizerischen Electrotechnischen Vereins (SEV) SI Jg Nr 20 vom 8 October 1960 |
35 | A.G. Clavier, P. Panter D.D. Grieg | "Distortion in Pulse Count Modulation System" Trans. AIEE, Vol. 66, Nov. 1947 |
36 | P.F. Panter, W Dite | "Quantization Distortion in PCM with Non-Uniform Spacing of Levels",Proc. IRE , Vol.. 39 January 1951 |
37 | A. G. Clavier, P.Panter, W. Dite " | Signal-to-noise Ratio Improvement in a PCM System", Proc. IRE, Vol. 37, April 1949 |
38 | "Signal", July 1964, No. 11 (Journal of the USA Armed Forces Communication and Electronics Association) |