US2647238A - Telephonic pulse-code-modulation system - Google Patents

Telephonic pulse-code-modulation system Download PDF

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Publication number
US2647238A
US2647238A US129226A US12922649A US2647238A US 2647238 A US2647238 A US 2647238A US 129226 A US129226 A US 129226A US 12922649 A US12922649 A US 12922649A US 2647238 A US2647238 A US 2647238A
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pulse
code
voltage
value
combination
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English (en)
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Bailey Christopher Edm Gervase
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Hartford National Bank and Trust Co
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Hartford National Bank and Trust Co
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/10—Calibration or testing
    • H03M1/1009—Calibration

Definitions

  • This invention relates to telephonic pulse-codemodulation systems and more particularly to improvements in encoding devices therefore.
  • the instantaneous value of an audiofrequency 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 diiierent combinations is 2
  • the value of the speech voltage on the other hand, varies continuously from 0 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.
  • the system has the advantage that, since the decoder has merely to determine presence or absence of a signal, the signal to noise ratio can be made high and perfect signals can be regenerated by repeaters.
  • a large number of speech channels can be multiplexed on a timediyision 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.
  • the Straight code if a marl; is denoted by 1 and a spaceby 0, the first unit 1 or 0 in a three-elementcombination 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.
  • leinent period will denote the time allocated to one element (mark space) in a code combination
  • combination period will denote the time allocated to one complete combination of such elements.
  • the present invention relates more particularly to improvements in the means for encoding the signals.
  • Suitable transmission and multiplexing equipment andother 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, 4e) and elsewhere.
  • a known encoding device for impulse-codemodulation telephony cathode-ray tube provided with a coded stencil plate is employed.
  • Such a device is of value when combination signals containing relatively large numbers of signal elements are used, so that the instantaneous values of the speech voltage can be finely graded into a relatively large number of steps; for instance when the combinations cach comprise 7 elements so that 128 steps are provided for.
  • a much coarser.for instance an 8-step or lG-step-grading is adequate, and 3-element or Jr-element combinations will accordingly give a sufficient degree of fidelity.
  • an economy can be effected by discarding the cathode-ray tube and employing a circuit composed of ectifiers and other passive corn.- ponents, and one object of the invention is to provide an encoding device of this kind.
  • One feature of the invention accordingly com prises an encoding device for a pulse-code-modulation system wherein the characters of all code elements in each combination are determined by a control voltage whose initial value corresponds to the instantaneous value of the speech voltage or other audioefrequency phenomenon to be signalled, while its subsequent value decays during the combination period.
  • a pulse-code-modulation system in accordance with a preferred embodiment ⁇ Of the invention is constituted by a sampling device and an encoding device.
  • the audio si nal to be coded is fed to the sampling device which is arranged to derive therefrom periodically a control pulse whose initial value is a function of the instantaneous value of the signal, the control pulse thereafter decaying at a predetermined rate.
  • the resultant sawtooth shape of the control pulse is determined by instantaneous value of the signal being sampled, the height of the leading edge of the pulse being a function of the instantaneous yalue 3 of the signal, and the trailing edge having a fixed slope depending on said predetermined rate of decay.
  • the control pulse produced by the sampling device is then fed to the encoding device which contains a plurality of paralleled unidirectional paths, each of which includes two serially connected rectifiers.
  • the paths have common input and output circuits, periodic code element pulses bein supplied to the common input circuit at a rate determined by the number of elements in the code combination. Thus if three elements constitute the code combination, for each sampling control pulse, three element pulses are applied to the input circuit.
  • the control pulse is imposed on the parallel paths and determines whether or not any paths will be rendered conductive when an element pulse is supplied thereto, and in that sense, the control pulse preconditions said paths for conduction.
  • the presence or absence of conductive paths determines whether a mark or a space is developed in the output circuit of the encoding device when an element pulse is fed therethrough. In this fashion, each instantaneous value of the signal is converted by the sampling device into a control pulse having a wave shape representative of said value, which control pulse, in turn, by its form determines the code combination produced by the encoding device.
  • FIG. 1 shows diagrammatically an encoding device in accordance with the invention.
  • Figure 2 shows a circuit for modifying input Signals for use in the device shown in Figure 1.
  • the speech or modulation voltage is applied at the input terminals M, Figure 2, of a circuit, to be described later, which is controlled by synchronising or channel-timing pulses so that it samples the instantaneous value of the speech voltage once at the beginning of each combination period.
  • the circuit is such that its output voltage E, beginning at a level proportional to the value of the sample, falls off exponentially during the combination period.
  • each combination is to comprise three code elements and the rate at which E decays is arranged to be such that the voltage E is halved during the time allocated to one element.
  • the output pulse voltage E yielded at the output terminals NW of the sampling circuit shown in Fig. 2 is applied asa control pulse to the similarly labelled input terminals MV of the encoding system shown in Fig. 1, which encoding system yields a code combination depending on the form of the applied control pulse.
  • the voltage E ranges from to 80 volts. Then its instantaneous values at the instant of sampling can be classified in eight steps, numbered from 7 to 0 in column A in the table, the first step comprising all those values which lie between 80 and '70 volts, the second step all those between '70 and 60 volts, and so on, as shown in column B of the table.
  • the value of E at the arrival of the first of the synchronizing pulses by which the emission of the three code elements is timed (hereinafter called element pulses) will lie in one or other of the steps shown in column B. As a result of the exponential decay, however, the value of E will have fallen by one half by the time that the second element pulse arrives.
  • first, second and third elements in the combinations representing the grades shown in column A will be those denoted by 1 for a mark and 0 for a space in columns E, F and G respectively, and the combination itself will be as shown in column H.
  • These results are F earth.
  • Three equal resistors R1, shunted by capacitors C1 are connected between terminals P1 and P2 so that potentials above earth of 80, 60, 40
  • resistors Rs shunted by capacitors C3, are so arranged that potentials above earth of '70, 50, 30 and 10 volts respectively are applied to the cathodes of the second set of diodes (is to (is.
  • Resistors R2 and capacitor C2 are provided for preventing the establishment of an eiiective path for impulses through pairs of diodes which are not adjacent to one another. For instance a value 65 of the voltage E opens a path through the diodes (Z1 and do, but this path is not effectively available for the brief impulses of small amplitude by which the coding is effected, provided that'appropriate values be given to the components C2. R2. If it is found that the capacities of the diodes rovide a sufficient path to impair the functioning of the device, they can be balanced. out by means of a preset capacitor The manner in which the instantaneous speech voltage is sampled and in which the requisite decay rate is imparted to the resulting voltage E will now be described with reference to Figure 2.
  • the modulation or speech voltage is applied at the input terminal M of a cathode-follower valve Vi, so that a voltage proportional to the instantaneous value of the speech voltage appears across the cathode resistance Rs.
  • Peaked positive synchronising pulses are applied to the terminal CP in such a way as to make the diodes D1, D2 momentarily conductive at the beginning of each combination period.
  • a resistor Rais connected in series with the diode D2 and its resistance is chosen to be much smaller than that of the time-constant resistor shown at R4 but much larger than that of the cathode resistor R6.
  • the pulses applied to terminal CP occur once at the beginning of each combination period, that is to say once for each three of the element pulses applied at terminals EP, Figure 1.
  • the diodes D1, D2 become blocked and the initial charge on the condenser G4 has thus been determined.
  • This is the initial value of the control voltage E applied to terminals MV, Figure 1, which are shown also in Figure 2, and it controls the character (mark or space) of the first signal element in the combination in the manner already described.
  • the value 01 the voltage E across capacitor C4 then falls off exponentially, however, owing to the discharge of the condenser through the resistance R4, and the latter is chosen to be of such magnitude that the value of E is halved during a single element period.
  • the second and third signal elements are thus controlled by it in the manner already described, and hence the code shown in column H is signalled.
  • apparatus for encodin -audio frequency signals comprising a sampling system to derive periodically from said audio signals a control voltage pulse whose initial value is a function of the instantaneous value of the signals and whose value thereafter decays at a given rate, and an encoding system coupled to said sampling system and responsive to each control voltage pulse to generate a code combination having a predetermin d number of code elements, the character of said elements being determined by the form of said control voltage pulse.
  • Apparatus as set forth in claim 1, which includes compander means for regulating the magnitude of said audio signals in a predetermined fashion in order to produce audio signals with a predetermined signal-to-noise ratio.
  • control voltage pulse decays at a rate at which said voltage halves itself over the time interval in which a code element is generated.
  • said sampling system includes a network for determining the rate of decay and having a capacitance and a resistance, said resistance shunt ing said capacitance.
  • apparatus for encoding audio frequency signals comprising a sampling system provided with a oathode follower amplifier responsive to said audio signals and means to derive periodically from said amplifier a control voltage pulse whose initial value is a function of the instantaneous value of the signals and whose value thereafter decays at a given rate, an encoding system including a plurality of unidirectional current paths, means to supply each of said control pulses to said paths to precondition the conductivity of said paths in accordance with the control pulse values, and means to apply periodic timing pulses to said paths in the interval during which said paths are conditioned by a control pulse for rendering said paths conductive and non conductive in a sequence depending on said control pulse values thereby producing a code combination having a predetermined number of elements.
  • apparatus for encoding audio frequency signals comprising a sampling system provide with a cathode follower amplifier responsive to said audio signals, and means to derive periodically from said amplifier a control voltage pulse whose initial value is a function of the instantaneous value of the signals and whose value thereafter decays at a given rate, an encoding system including a plurality of parallel unidirectional current paths intercoupling the common input and output circuits, each of said paths including at least two serially-connected rectifiers, to supply each of said control pulses to said parallel paths to condition the conductivity of said paths in accordance with the control pulse values, and means to apply periodic timing pulses to said common input circuit in the interval during which said paths are preconditioned by a control pulse for rendering said paths conductive and non-conductive in a sequence depending on said control pulse values thereby yielding in the output circuit a code combination having a predetermined number of elements.

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  • Engineering & Computer Science (AREA)
  • Theoretical Computer Science (AREA)
  • Electrotherapy Devices (AREA)
  • Amplitude Modulation (AREA)
  • Dc Digital Transmission (AREA)
US129226A 1948-11-26 1949-11-25 Telephonic pulse-code-modulation system Expired - Lifetime US2647238A (en)

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BE (1) BE492403A (is")
CH (1) CH277505A (is")
DE (1) DE812441C (is")
FR (1) FR1002679A (is")
GB (1) GB648587A (is")

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2883529A (en) * 1954-03-15 1959-04-21 Boeing Co Voltage monitoring circuits
US2903604A (en) * 1955-01-03 1959-09-08 Ibm Multistable circuit
US2965767A (en) * 1955-07-15 1960-12-20 Thompson Ramo Wooldridge Inc Input circuits and matrices employing zener diodes as voltage breakdown gating elements
US3312903A (en) * 1959-03-04 1967-04-04 Itt Jitter compensating circuit for angle encoding apparatus
US4522060A (en) * 1982-03-24 1985-06-11 Murata Manufacturing Co., Ltd. Dry/dew/frost sensor

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
SE171634C1 (sv) * 1951-05-10 1960-06-07 Siemens Ag Anrodning foer att medelst ampitudkvantisering uttrycka en elektrisk maetstorhet som en kod
US2759998A (en) * 1951-10-26 1956-08-21 Itt Pulse communication system
FR1062271A (fr) * 1952-08-28 1954-04-21 Labo Cent Telecommunicat Circuit de codage
DE1225235B (de) * 1963-11-07 1966-09-22 Standard Elektrik Lorenz Ag Schaltungsanordnung zur pulsfoermigen Abtastung von Wechselstromsignalen, insbesondere fuer Kanalverteiler fuer die PCM

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2357652A (en) * 1942-05-25 1944-09-05 Rca Corp Rectifier timing circuit
US2449467A (en) * 1944-09-16 1948-09-14 Bell Telephone Labor Inc Communication system employing pulse code modulation
US2521733A (en) * 1948-02-03 1950-09-12 Fed Telecomm Lab Inc Pulse code modulator
US2541039A (en) * 1948-03-06 1951-02-13 Fed Telecomm Lab Inc Amplitude channelizer
US2556200A (en) * 1948-02-26 1951-06-12 Int Standard Electric Corp Electrical translation system

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2357652A (en) * 1942-05-25 1944-09-05 Rca Corp Rectifier timing circuit
US2449467A (en) * 1944-09-16 1948-09-14 Bell Telephone Labor Inc Communication system employing pulse code modulation
US2521733A (en) * 1948-02-03 1950-09-12 Fed Telecomm Lab Inc Pulse code modulator
US2556200A (en) * 1948-02-26 1951-06-12 Int Standard Electric Corp Electrical translation system
US2541039A (en) * 1948-03-06 1951-02-13 Fed Telecomm Lab Inc Amplitude channelizer

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2883529A (en) * 1954-03-15 1959-04-21 Boeing Co Voltage monitoring circuits
US2903604A (en) * 1955-01-03 1959-09-08 Ibm Multistable circuit
US2965767A (en) * 1955-07-15 1960-12-20 Thompson Ramo Wooldridge Inc Input circuits and matrices employing zener diodes as voltage breakdown gating elements
US3312903A (en) * 1959-03-04 1967-04-04 Itt Jitter compensating circuit for angle encoding apparatus
US4522060A (en) * 1982-03-24 1985-06-11 Murata Manufacturing Co., Ltd. Dry/dew/frost sensor

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CH277505A (de) 1951-08-31
BE492403A (is")
FR1002679A (fr) 1952-03-10
DE812441C (de) 1951-08-30
GB648587A (en) 1951-01-10

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