US2569970A - Control system fob eletronic - Google Patents

Description

Oct. 2, 1951 c. E. G. BAILEY 2,569,970

CONTROL SYSTEM FOR ELECTRONIC DECODERS Filed Nov. 5, 1949 v 2 SheetsSheet 1 *ZOOV +ZOOV 100V IT I N VEN TOR.

AQE/V T Oct. 2, 1951 c. E. G. BAILEY CONTROL SYSTEM FOR ELECTRONIC DECODERS 2 Sheets-Sheet Filed Nov. 5, 1949 W l lllxll..- I

By fiwlr y' V AGENT Patented Oct. 2, 1951 CONTROL SYSTEM FOR ELETRONIC DECODERS Christopher Edmund Gervase Bailey, London, England, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application November 3, 1949, Serial No. 125,317 In Great Britain November 26, 1948 9 Claims. (Cl. 178-435) This invention relates to telephonic pulsecode-modulation systems and more particularly to improvements in decoding devices therefor.

In such a system the instantaneous value of an audio-frequency phenomenon (hereinafter referred to, by way of example, as the speech voltage) at the sending station is sampled at regularly recurrent intervals and is expressed in terms of a code combination of signal elements such as marks and spaces. The combination is decoded at the receiving station and is used for reconstructing an approximate reproduction of the original speech voltage. If each code combination consists of n units the number of dif-- ferent combinations is 2 the all-space combination being included. The value of the speech voltage, on the other hand, varies continuously from to, say, 80 v., so that it has in theory an infinite number of possible values and can be only approximately represented by the values, finite in number, denoted by the code; nevertheless it is found that, with quite a small number of units per combination, a sufiiciently undistorted reproduction can be obtained. The system has the advantages that, since the decoder has merely to determine presence or absence of a signal, the signal noise ratio can be made high and perfect signals can be regenerated by repeaters.

Moreover, since a very brief period (say 1.5 as.) sufiices for each signal element, a large number of speech channels can be multiplexed on a timedivision or distributor basis with a single carrier frequency. An ultra-high-frequency carrier is preferably employed.

The code may be simply a binary system of numeration. Thus, in what may be called the straight code, if a mark is denoted by -l and a space by 0, the first unit 1 or 0 in a three-element combination stands for the value 4 or 0, as the case may be, the second for 2 or 0 and the third for 1 or 0. Other codes may be preferred for various purposes, and one such is shown together with the straight code in Table I.

Table I.-PCM codes Value de- Value dc Code Comnoted, noted,

bination straight alternative code code With a three-element code seven difierent instantaneous values of the voice-voltage can be signalled, in addition to zero. By way of example the present description will be restricted to system in which such three-element combinations are. used, but it will be obvious that by increase in the number of elements per combination any desired degree of approximation to a continuous series. of voice-voltage values can be obtained.

In the following description and claims the expression element period will denote the time allocated to one element (mark or space) in a code combination, and. the expression combination period will denote the time allocated to one complete combination of such elements. Thus. in the systems hereinafter more particularly described by way of example, a combination period will be three times as long as an element period.

The present invention relates more particularly to improvements in the means for decoding the signals. Suitable transmission and multiplexing equipment and other components in a pulse-code-modulation system will be known to those skilled in the art, and have been described in the Bell System Technical Journal for July 1947, (page 395) and for January 1948 (pages 1, 44) and elsewhere.

It is sometimes desirable to adapt a decoding device for use at will with a straight binary code or with some other code. This cannot easily be done with known decoding devices, but a feature of the invention comprises a decoding device for a pulse-code-modulation system wherein the reconstituted values of the voice-voltage or other audio-frequency phenomenon are derived from the received signals by means of a cathode-ray tube. Such an arrangement readily permits of a change of code.

In order that the nature of the invention may be better understood certain embodiments thereof will now be described with reference to the accompanying drawings, in which:

Figure 1 showsa decoding device according to the invention;

Fig. 2 shows a circuit fed by the device illusttated in Figure 1, and

Figures 3 and 4 are diagrams for use in explaining the operation of the decoding device shown in Figure 1.

A decoding device in accordance with the invention will now be described by way of example with reference to Figures 1, 2, 3 and 4 of the accompanying drawings. In this arrangement a cathode-ray tube is employed, not for encoding, as has hitherto been usual, but for decoding. Such an arrangement is particularly advantageous when a code other than the straight binary code is used, but for the sake of simplicity it will in the first place be described in connection with a straight code using threeelement signal combinations.

Figure 1 shows conductive target plates S1, S2, S3, OP of a cathode ray tube, these plates being swept by a cathode beam under the control of timing and multiplexing devices of a kind whichwill be familiar to those skilled in the art and need not be further referred tohere. The cathode beam completes a circuit through any plate on which it impinges, so that an impulse of negative voltage flows from that plate through a conductor connected to it. The cathode beam sweeps across three strip-shaped plates S1, S2, S3, shown 'in the same ratio. On the right of each of the graphs a and b are shown the fly-back pulses,

which have no effect on the ultimate output of the decoding device.

as vertical in Figure 1, in succession from left to right in the sense of that figure, and finally sweeps across one or other of the output plates OP, arranged in a vertical column, according to the height to which the beam is being raised at that instant by a deflecting voltage set up on the Y plates. This deflecting voltage is determined by the received signal in a manner which will be described later with reference to Figure 2.

When the cathode beam impinges on one of the output plates OP, electrons from the beam flow through one or more sections of the attenuator A2, the output terminal OT, and a voicevoltage-reconstituting equipment to a source of positive voltage (not shown). The strength of the outgoing impulse thus set up will be the less according as the number of sections of the attenuator A: through which it passes is the greater,

and will therefore depend on the height at which the beam crosses the row 01 of output plates OP.

The output impulses from the terminal OT can therefore be used in known manner for reconstructing the speech voltage in correspondence with the final height, in the sense of Figure 1, of the electron beam. The manner in which this height is determined by a received impulse combination will now be described.

Signals of the kind shown in the first column of Table 1 are received from a radio receiver and applied through terminals IT as gating pulses to one grid of a multigrid grating valve V3, Figure 2. One such signal, comprising the combination 101 and denoting the value 5, is indicated at c, Figure 3. The valve V3 is biased to be class C in respect of these signal pulses, so that the marking pulses permit the output of valve V2 to be passed on through the gating valve V3 and condenser C5 when they occur, while the spaces permit the gating valve V3 to remain closed to the output of valve V2.

The strips S1, S2, S; are connected to an attenuator A1 which is so dimensioned that the strength of the impulse outgoing from terminal BV, Figure 1, when the beam sweeps over strip S3 is one half of the strength of the impulse outgoing when the beam sweeps over strip S2, and one quarter of the similar impulse from strip S1. Thus the strengths of these impulses, which are predetermined and independent of the signals, form a binary scale and the impulses constitute what may be termed a binary voltage appearing at terminals BV. At each sweep of the beam, therefore, the binary voltage at BV follows the graph a, Figure 3.

The terminal BV is also shown at the left of Figure 2 as the input voltage of a phase-inverting valve V2, the ouput of which is shown at b, Figure 3. It will be seen that the three impulses shown at the left of graph a, Figure 3, are negative and are successively smaller in the ratio 2: 1, while the three corresponding impulses shown in graph b are positive, and successively smaller In the combination of gating or signal impulses graphed at c the first and third elements are marks and impulses representing them are therefore shown as present at 01, 03', while the second or middle element is a space and no impulse is present for it. The gating valve V3 is biased to be in Class A in respect of that one of its grids which receives pulses from the inverting valve V2; it follows therefore from the action of the gating or signal pulses .c, that the first impulse hr of positive binary voltage can pass through the gating valve V3 and appears as a full-strength negative pulse d1, graph (1, passing through capacitor C5; the second binary impulse In is suppressed; and the third binary impulse b3 appears as a quarter-strength negative pulse d3, graph d, passing through capacitor C5.

The negative pulse d1 appearing in the output of valve V3 induces a positive impulse on the right-hand plate of capacitor C5 and so permits electrons to flow momentarily on to the anode of a diode D3. On the cessation of the pulse d1, therefore, a negative charge appears on that anode, and leaks through a further diode D4 on to the upper plate of the capacitor C6. Thus a negative charge remains for the time being on this plate, as is indicated at e1, Figure 3. On the arrival of the final pulse 113 a quarter-strength charge is added, so that the voltage on capacitor C6 is then as indicated at 63. At the end of the cycle the capacitor C6 is discharged through diodes D5, D6 by means of a synchronized positive pulse applied automatically over the lead DP. The capacitors C5, C6 form a potential divider and are so dimensioned as to make the potentials e1, ea, a convenientsmall fraction of the high-tension supply voltage.

The potential across capacitor C6 is applied through the lead Y]? and an amplifier to the Y plates of the cathode-ray tube, and Figure 4 shows the trace of the cathode beam for each of the eight possible signal combinations. The traces are shown as clear of one another for the sake of clarity, although in practice they will coincide in part. It will be seen that the trace for strength 5 rises to a full deflection under the influence of a deflecting voltage corresponding to en, Figure 3, retains this deflection during the period corresponding to the second signal element, which is a space, and increases by the addition of a quarter-strength deflection in correspondence with the final part 63 of the voltage e, Figure 3.

The code values of the signal combinations can readily be altered by interchanging the wiring of the plates OP in relation to the attenuator A2 Figure 1.

In order to reduce the number of seals required the resistors forming the attenuators may be mounted inside the cathode-ray tube. It will be obvious that the tolerances permissible in the values of these resistances are fairly coarse. The voltage derived from the terminals BV will be large, so that the large negative feedback can be applied to valves V1 and V2. The system can be made independent of fluctuations in the supply voltage by stabilizing the high-tension voltages of the valves in terms of those of the cathode-ray tube, or vice versa. Alternatively additional tabs may be fitted at the top and at the bottom of the column OP and employed to correct the decoder gain and mean Y shift respectively, through circuits having long time constants.

Volume expansion can be effected, in a compander system, by suitable dimensioning of the attenuator A2.

What I claim is:

1. In a communication system wherein pulse code modulation signals are transmitted which are representative of the instantaneous ampli tude of a message wave and are constituted by code groups of individual elements each having any of a plurality of values, a receiver for said signals provided with a decoder for reconstituting the message wave, said decoder comprising a cathode ray tube having a plurality of separate target electrodes, an electron beam source and means to deflect said beam to impinge on a selected target in accordance with an applied control potential, means responsive to the received signal to develop a control potential for said deflection means by adding together increments of voltage depending on the respective elements in the code combination, and means to derive from the target electrodes the reconstituted message Wave.

2. In a communication system wherein pulse code modulation signals are transmitted which are representative of the instantaneous amplitude of a message wave and are constituted by code groups of individual elements each having any of a plurality of values, a receiver for said signals provided with a decoder for reconstituting the message wave, said decoder comprising a cathode ray tube having a plurality of separate target electrodes, an electron beam source and means to deflect said beam to impinge on a selected target in accordance with an applied control potential, means responsive to the received signal to develop a control potential for said deflection means by adding together increments of voltage depending on the respective elements in the code combination, and means including an attenuator to derive from the target electrodes the reconstituted message wave, said target electrodes being connected to different points along said attenuator whereby the voltages derived 6 from the respective electrodes through the attenuator each have a value depending on the point of connection thereto.

3. A decoder, as set forth in claim 1, wherein said means to develop a control potential comprises an electron discharge device, means to apply the received signal elements jointly with locally produced voltages of different predetermined values as an input to said electron discharge device, and means to derive said increments from the output of said device.

4. A decoder, as set forth in claim 3, wherein said differing predetermined values form a binary scale.

5. A decoder, as set forth in claim 4, wherein said cathode ray tube includes a plurality of plates which are impinged upon by said beam, an auxiliary attenuator connecting said plates and means for deriving said locally produced voltages from the output of said auxiliary attenuator.

6. A decoder, as set forth in claim 5, wherein at least one of the two attenuators is constituted by resistors which are enclosed within said cathode ray tube.

'7. A decoder, as set forth in claim 1, wherein the parts are so connected as to conform to a straight binary code.

8. A decoder, as set forth in claim 1, wherein the parts are so connected as to conform to a binary code other than a straight binary code.

9. A decoder, as set forth in claim 2, wherein said attenuator is included in a volume expression device for use in a compander system.

CHRISTOPHER EDMUND GERVASE BAILEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,441,296 Snyder May 11, 1948 2,466,705 Hoeppner Apr. 12, 1949 2,482,782 Lenny Sept. 27, 1949 2,489,883 Hecht Nov. 29, 1949 2,496,633 Llewellyn Feb. 7, 1950

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