US2452547A - Pulse modulation system of electric communication - Google Patents

Description

Nov. 2, 1948. P. K. CHATTERJEA EI'AL 2,452,547

I PULSE MODULATION SYSTEM OF ELECTRIC COMMUNICATION Filed July 7, 1945 I N V EN TORS PAAFl/LL/l K. CHATTEIFJEA 1 .ChWkLES 7: scan Y Patented Nov. 2, 1948 t an? PULSE-,MODULATION SYSTEM OF ELECTRIC COMMUNICATION Application July '7, 1945, Serial No. 603,654

In Great Britain June 22, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires June 22, 19.64

2 Claims. (Cl. 179-1715) The present invention relates to pulse modulation systems of electric communication.

In communication systems of this type the rim telligence to be transmitted is carried by a train of electrical pulses which are, from a broad aspect, time modulated in accordance with the instantaneo-us amplitudes of the intelligence signal wave. One of the earliest forms which the time modulation took was the modulation of the duration of the pulses in accordance with the instantaneous amplitudes of the signal wave. It is not necessary to transmit the whole of these duration modulated (sometimes referred to as solid) pulses. Instead, marking pulses of short constant duration may be transmitted to mark the leading and trailing edges of the duration modulated pulses and such systems have been termed double pulse systems and form a particular case of a system in which the intelligence is transmitted as the modulation of a time interval, marked in this case by pairs of marking puses,

but in other cases by a change in an electrical characteristic of the system, such as the phase of the carrier wave. The occurrences of one or both of the marking pulses may vary in accordance with the instantaneous amplitudes of the signal wave and in the latter case both pulses of a pair must be transmitted, but when the occurrences of one of the pulses, either the lead ng or trailing marking pulse are at constant intervals of time, that is, they are unmodulated, the unmodulated pulses need not be transmitted and may be pro-- vided at the receiver for demodulation purposes. In such a case when only the variably occurring pulses are transmitted, the system is known as a single pulse modulation system and the interval between the pulses may be occupied by other pulse trains forming a multi-channel pulse system. If desired. the single pulse train may be demodulated without the insertion of a constantly occurring train of pulses and at the same time eliminating the reconstruction of the solid pulses rep sented by the pairs of constantly and variably occurring pulses with the aid of build back" circuits.

In such a single pulse system, the pulses are time phased in accordance with the instantaneous amplitudes of the intelligence signal Wave.

British patent specification No, 551,282, dated February 16, 1943, discloses arrangements for producing a phase modulated carrier frequency in accordance with signal voltages by producing a train of time phased peak pulses at the desired carrier frequency or at'a sub-harmonic frequency thereof. Such pulses are rich in harmonics of the pulse repetition or base frequency and the desired phase-modulated carrier frequency is obtained by applying the time phased pulses to a suitable pass filter from the output of which is obtained the desired phase modulated carrier Wave. The arrangements for obtaining the peak pulses comprise a device and associated electrical circuits which on the application thereto of an alternating voltage produces in one stage a peak voltage pulse during one or both half cycles of the said alternating voltage. By a device and associated operational circuits is meant a single stage, for example a high vacuum tube stage and its associated circuits, for example, for applying thereto the necessary operational voltages or for making the device function a desired manner so that when analternating currentis applied to the input terminals, peak volt ages are obtained in the output, in contradis tinction to a differentiating c'puit which is in effect a high pass filter, forwhich the wave applied to the input must be distorted from a sinusoidal waveform to, for example, rectangular wave form. in order to produce peak pulses in the output thereof.

Among the apparatus described in the afore mentioned British specification No. 551,282 for producing the time phased peak pulses is the magnetic harmonic generator of the kind described in British patent specification No. 477,875, dated May 5, 1937. This generator is fact an inductance coil having a suitable magnetic core and Whose output varies non-linearly with the input and when an alternating current sinusoidal or other form is applied to the coil to energise the magnetic core, peak pulses are produced when the current passes through zero amplitude, a positive pulse in one half cycle and a negative pulse in the other half cycle. When a variable current as from an intelligence signal wave is used to energise the magnetic core as well as the alternating current the timing of the generated peaks is advanced or retarded by an amount propor tional to the instantaneous amplitude of the intelligence signal wave. In other words, time phased pulses are produced.

'The present invention resides tion of devices and their operational circuits of the type specified to transmitters of pul munication systems and in particular to the ap plication of the non-linear magnetic inductance coil to transmitters of pulse communication systerns.

The invention will further be explained in connection-with the accompanying drawings which form show an embodiment of the invention, utilising by way of example the non-linear magnetic inductance coil for producing the modulatedpulses.

Figure 1 shows a circuit arrangement for producing pairs of pulses marking the beginning and end of a time interval or a double pulse system, and

Figure 2 shows several explanatory curves.

Figure 1 shows a simple circuit for producing the desired pulses. In this figure P is the source of pulse repetition frequency, L is the non-linear magnetic inductance or harmonic generator as described in said patent No. 477,875. Its operation for producing time phased pulses is fully described in said patent specification No. 551,282, and need not be repeated here. Q is the source of signal wave. The output of P mustbe such as to take the coil L well beyond its saturation point. The condenser CI and inductance Ll in series with P and L are *so adjusted that L1 is in general large compared with Land so that Cl and Li in combination with L tune the circuit to the frequency of P. Such tuning serves, among other things, the purpose of filtering the current wave from the source -P. Further assistance in this direction may be obtained by the introduction of the shunt anti-resonant circuit C2L2.

Condenser C3 and resistance R3 serve as a load circuit bridged across the coil L. As fully explained in the aforesaid Specifications sharp voltage peaks are generated in the coil L when this latter is energised well into the saturation region. There will be a positive pulse and a negative pulse for each complete cycle of the wave from P, these occurring near to the time when the magnetising force on the coil passes through zero, that is when the current in the winding of the coil passes through zero. Such pulses are shown graphically in a Figure 2, in full line.

When the signal wave from the source Q is also impressed on L,'the times of occurrence of the pulses is altered, the instant of the peak pulse occurrence being shifted by an amount corre sponding to the shift of the zero current of the combined energies from source Q, and the positive pulses are shifted in the opposite direction to the negative pulses as shown in broken line in a Figure 2. Either the positive pulsesor the negative pulses constitute a single pulse train timephase modulated in accordance with the instantaneous value of the signal wave amplitude. The rectifier RF is therefore provided to eliminate either the positive or negative pulses. In I) Figure 2 the positive pulses alone are represented and are obtained across resistance R3 (Fig. l.)

A low pass filter LPF is inserted between the source Q of signal waves and thecoil L and source P to prevent the high frequency currents'from P flowing to the source Q.

In Figure 1 the signal wave from Q is shown as fed in parallel with'the waves from P, but this is not essential. Several alternative ways are disclosed in said British specification No. 551,282, but it is not necessary that the sources P and Q should energise the same winding.

It has been stated hereinb'efore and shown in a Figure 2 that the times of occurrence of the positive and negative pulses produced in the coil L during each cycle of the base frequency source P, are varied in opposite directions. The reversal of one of the pulse series for example the negative pulses, will thus provide a double pulse system in which both the leading and trailing pu1ses are phase modulated inaccordance'with the instantaneous amplitude of the signal wave. This may be accomplished by providing the inductance L withtwooutput circuits in parallel having rectifiers polarised in opposite senses so that the output of one circuit provides the positive pulses only and the output of the other circuit provides the negative pulses only. The pulses from one of these circuits are then reversed by means of a thermionic tube or a suitable transformer or any other reversing device, the two trains of pulses of the same sign are then applied to respective valve amplifiers having a common anode resistance from which the train of pairs of pulses is obtained. A duration modulated solid pulse may be obtained from the pairs of pulses by means of any known build-back circuit for example those describedin the British patent specification No. 583,511 dated June 9, 1947 either for transmission or for demodulation purpo:es at the receiver The width of the unmodulated solid phase may be varied as desired by introducing into one of the parallel paths a suitable delay device.

Figure 1 shows a circuit arrangement .ior pro ducing pairs cf pulses in which one pulse of each pair occurs at constant intervals of time. The arrangement consists of two like circuits, the upper one to produce a train of time phased pulses, and the lower one which is similar except that the signal wave is not applied to th non linear magnetic inductance L and the source P of base frequency is applied through a delay device D to the non-linear inductance L. The rectifiers RF and RF in the respective circuits are arranged to pass pulses of the same sign. The lower circuit thus produces fixed pulses, that is, pulses occurring at constant intervals of time. Curve 0 Figure 2 represents the pulses generated. The negative pulses are assumed to be eliminated by the rectifier Rl. Curve b Figure 2 represents the time phased pulses produced in the upper circuit and cl Figure 2 represents the pairs of pulses, A the time phased pulse and B the fixed pulse, obtained by adding the outputs from the upper and lower circuits of Fi ure 1, together. This is efiected in the arrangement shown in Figure l by applying the output from the upper circuit to a valve amplifier Vi and the output from the lower circuit to a valve amplifier V2, these valves having a common anode circuit resistance RC from which the combined train of pulses is obtained at the terminals OIO2.

Curve e Figure 2 shows the reconstituted solid pulses from th pairs of pulses AB, CD in curve at Figure 2. In this curve e it will be observed that the mean pulse width W is 25% of the pulse repetition time T, and that it can be duration modulated from 0 to 50% for maximum speech or signal modulation, that is, only 50% of the transmission duration is used for speech modulation. By using diiierent phasing or delay produced in D Figure 1 of the fixed pulses'any shorter duration mean pulses may be used, e. g. 25% mean pulse width as illustrated in curves 1, g and 71. Figure 2, which correspond respectively to curves 0, d and c Figure 2. Only 10% of the total time duration is used for 100% speech modulation.

It will be observed by those skilled in the art that if the voltage applied from P to the lower circuit Figure l weregiven a suitable phase lead or lag in D of over 270 on the voltage applied to the upper circuit a train of pulses would result in which the first pulse of a pair is fixed in time. Instead of using the positive fixed pulses from the lower circuit of Figure 1, the negative fixed pulses may be used and reversed and the delay produced in D Figure 1 suitably adjusted to pro vide a fixed loading or training pulse of a pair.

In the foregoing description the rectifiers RF and RF may be any one of a number of different forms, such as a vacuum tube diode or a copper oxide rectifier, the only requirement being that for the transmission in the one direction the impedance shall be quite low and that it shall be able to Withstand the back voltage during the period of no substantial transmission through the rectifier. Alternatively, the valves VI and V2 Figure 1 may be biassed to cut-ofi in the absence of a positive pulse and the rectifiers RF, RF may then be omitted.

It is shown in said British specification No. 551,282 that when the amplitude of the signal wave is small compared to the amplitude of the wave from source P, that is, also when the time displacements of the pulses are small, the displacement is substantially directly proportional to the biassing current or signal wave amplitude. For effective modulation, however, it is desirable that the time modulation or displacement of the pulses shall be relatively large compared to the pulse repetition period. The most favourable condition for this is to utilise a saw-tooth or triangular Wave form generator for P, for example any well known time base circuit. Certain other distortion effects appear in the output of the non-linear inductance. One of these is due to the hysteresis which is characteristic of the iron forming the core of the inductance and several ways are described in said specification No. 551,282 for reducing this distortion. A second distortion effect is due to the timing of the pulses with respect to the exciting current and may be corrected by an additional direct biassing current as described in said specification No. 551,282.

The single pulse trains produced by the nonlinear magnetic inductance are such that only a fraction of the total of the available timing,

Number namely the pulse repetition period, is taken up by speech or other signal modulation and they are therefore suitable for multichannel communication systems in which the channels are discriminated by the times of occurrence of the respective pulses.

What is claimed is:

1. A transmitter for producing trains of pairs of pulses, in which one pulse of each pair occurs at constant time intervals while the second of the pulses of each pair is time displaced according to the intelligence to be conveyed, comprising a source of alternating voltage, a pair of magnetic impulse coils, means for applying said voltage to one of said coils, means including a delay device introducing a time delay for applying said voltage to the other of said coils, a signal source, means for applying the signals from said signal source to only said one of said coils, and means for combining pulses produced in said coils into a common train.

2. A transmitter according to claim 1 including a pair of rectifiers each associated with one of said coils and arranged to suppress pulses of a given polarity produced in said coil.

PRAFULLA KUMAR CHATTERJEA. CHARLES THOMAS SCULLY.

REFERENCES CITED The following references are of record in the

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