US2298922A - Electric signaling system - Google Patents

Description

Filed Jan. 6, 1940 Patented Oct. 13, 1942 ELECTRIC SIGNALING SYSTEM William Arnold Beatty, Sidcup, England, assigner to International Standard Electric Corporation,

New York, N. Y.

Application January 6,1940, Serial No. 312,645 l In Great Britain February 7, 1939 Z Claims.

This invention relates to methods of generating, transmitting and receiving pulses which have a time function of the amplitude of a signal corresponding to intelligence.

In the co-pending application No. 212,731 filed June 9, 1938, by Alec Harley Reeves, .Patent No. 2,266,401, December 16, 1941, and assigned to the present assignee, International Standard Electric Corp., various types of pulse modulation systems are described wherein the width of transmitted pulses of rectangular` wave form is varied so that the width of a pulse is a function of the instantaneous amplitude of a continuous signal wave representing the` signal intelligence it is desired to transmit. In accordance with a second feature of the disclosure of co-pending application No. 212,731 means are provided whereby sharp pulses are transmitted for the purpose of delineating the times of the beginnings and ends of the rectangular pulses, i. e. two sharp pulses are transmitted in place of one rectangular pulse. Methods of generating the various types of pulses are also described in my co-pending application No. 313,041 led January 9, 1940, Patent No. 2,256,336, September 16, 1941. In this latter application, two types of pulses are coded RT and S/RT, these being respectively a solid type of rectangular pulse with a variable width in which the trailing edges of the pulses occur at fixed time intervals, and a double type of sharp pulse o1 the second type described above in which the trailing edge pulse is. suppressed, only the variable leading edge pulse being generated.

The RT type of pulse modulation requires a band width much greater than that required for ordinary amplitude modulation conveying the same intelligence, while the S/RT type of pulse requires a still greater band width. The former type of pulse can b e intelligentlyr received on an ordinary amplitude modulation type of receiver, if the band width of the receiver is suiiiciently great, while the latter type of pulse can only be intelligently received by receivers specially adapted for this type of pulse modulation.

` `mitter is located in the centre of a densely populated area, as for instance the sound channel of a television transmission.

Short wave transmissions on wave lengths of approximately seven meters are subject to car ignition interference, and on television transmissions this interference is much more objectionable on the sound channel than on the vision channel, therefore any system which tends to reduce interference on short wave sound transmissions would indirectly have the eiect of increasing the service area of a television transmitter.

The average television sound receiver has a comparatively wide band response usually over 250 kc., this being necessary in order to avoid troubles with frequency drift of oscillators. Since a wide band width is available with the average television sound receiver, such receivers are capable of receiving without further modication the solid RT type of pulse modulation.

The various types of pulses, and a manner in which the new type of pulse may be generated and utilised areillustrated on the accompanying drawing. Figure 1 shows RT, S/RT and the new type of pulse coded RT-i-S/RT while Figure 2 illustrates a method of generating the RT-I-S/RT type of pulse.

Referring to Figure 1, curve I I illustrates an alternating current, points E, D and F being maximum, mean and minimum values of the current, curve I2 shows an RT type of pulse having a time function of the amplitude of curve I2 and curve I3 shows an S/RT type of pulse which indicates the leading edges ofthe RT type of pulse shown in curve II. These two types of pulses are similar to those shown in curves I'I and I9, Figure 1, of the above-mentioned application No. 313,041. Curve I4 shows a pulse modulation which is a combination of the types of modulation achieved by the RT and S/RT type of pulse. A method of generating this type of pulse will be described later.

The combined RT and S/RT pulse coded as RT-l-S/RT comprises an RT type of pulse mmediately preceded by an S/RT pulse having a greater amplitude than the RT pulse. The pulse v illustrated assumes that the `RT pulse has a peak value which is 20% of the peak value of the S/RT pulse.

The RT, portion of the combined pulse can be received on an existing type of short wave` amratio can be obtained as compared with ordinary amplitude modulation.

It will therefore be seen that the RT portion ofthe pulse makes it possible for existing amplitude-modulated types of receiversto be used for the reception of the pulse modulation intelligence, and if the receivers are modified in a simple manner, an improvement in signal to noise is possible. There are, however, conditions under which the RT type of pulse will not give suicient freedom from interference, and under such cirsynchronising pulses for a pulse insertion generator which is utilised to establish a short pulse corresponding to the trailing edges of the RT pulses. Any known type of relaxation oscillator can be used as a pulse inserter, the fly back stroke of the generated signal being utilised as a short pulse marking the trailing edge of the corresponding RT pulse. Alternatively a short pulse can be obtained by utilising the RT pulse ink such a manner that a condenser is rapidly charged or discharged.

The combined RT-l-S/RT pulse can be passed through an amplitude lter which cuts off the RT pulse leaving the S/RT pulse only. The S/RT pulse can now in any known manner be combined with the short pulse generated by the pulse insertion generator, with the result that a D/RT pulse similar to that illustrated in curve I5, Figure 1, is achieved.

The D/RT pulses are similar to those illustrated in the said application No. 313,041, Fig. 1, curve I8, and in application No. 212,731. These pulses can now be utilised in any known manner and intelligence can be received vwith a very great measure of freedom from interference.

One method of generating .the RT-I-S/RT pulses is as follows. In application No. 313,041, there is described a method of generating various types of pulses, It is proposed to use similar methods for the generation of the RT+S/RT pulses. In the above case, Figures 9 and 11 show collector plate arrangements suitable for the generation RT-l-S/RT pulses respectively. Figure 2 of the accompanying drawing shows a plate arrangement which is a combination of the plate arrangement mentioned above, and Fig. 3 shows a device including the plate arrangement of Fig. 2. A plate I6 is utilised as described in the said application for the generation of RT pulses while a strip-like plate I1 insulated from and located partially behind the plate I6 is used for the generation of the S/RT pulses. The plate I'l is placed along the edge of the plate I6 corresponding to the leading edge of the RT pulses, the active portion of the plate I'I projectingfrom the plate I6 by an equal amount along its length so that immediately prior to the electron beam striking the plate I6, it strikes the plate I1 for equal short intervals. The combined projected area of plates I6 and I1 is equal to the projected area of plate 42, Figure 9, of the said' application. This dimensioning of the plates ensures that the .tical to the RT pulses already mentioned. The

voltages applied to plates I6 and I'I can be such that the required differences in amplitude between the pulses is automatically achieved, or alternatively the voltages applied to the plates can be identical and by suitable adjustment of the output impedances in series with the plates different amplitudes for the RT and S/RT pulses can be obtained. Fig. 3 shows a pulse generating device in which different voltages are applied to the plates I6 and II,v the plate I6 being supplied by the voltage source Ba, while the plate I'I has applied to it the additional voltage Bb. It will be seen that by this arrangement the electron stream directed at the platesis deflected horizontally by the frequency source and vertically by the amplitude o1' the signal source.

The `,proposed system offers'the following advantages:

(a) Existing broad band short wave receivers without modiiication can receive the intelligence conveyed by the RT pulses.

(b). If the above type of receivers are modified in a simple manner, an improved signal to noise ratio will be obtained (c) Outside the normalservice area of the RT pulseftransmission, the combined RT+S/RT pulses make possible a further service area with reduced interference as compared with amplitude modulation.

It is obvious that there are other methods of combining pulses of different amplitude, in which one short pulse marks the leading or trailing edge of the longer duration pulses, as for instance the marking pulse could be achieved by reducing the amplitude of the main pulse for a short period.

What is claimed is:

1. The method of modulation which comprises generating a train of pulses so that the individual pulses of the train all have the same constant amplitude and have trailing edges occurring at xed time intervals, varying the duration of said individual pulses in accordance with the instantaneous amplitude of an intelligence wave, generating at the leading edge of each of said individual pulses, a pulse of shorter duration and of greater amplitudev'than any in said train of pulses, and combining the said pulses of shorter duration with the corresponding ones of the firstmentioned train of pulses.

2. The method of modulation which comprises generating Aa train of pulses so that the individual pulses thereof all have the same constant amplitude and all have corresponding edges occurring at xed time intervals, varying the duration of the individual pulses in accordance with the instantaneous amplitude of an intelligence wave, generating at the edge of each pulse other than the edge occurring at the xed time interval, a pulse of shorter duration and of different amplitude than any in said train of pulses, and combining the said pulses of shorter duration with the corresponding ones of the first-mentioned train of pulses.

WILLIAM ARNOLDk BEA'I'I'Y.

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