US2303909A - Transmission of electrical signals - Google Patents

Transmission of electrical signals Download PDF

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Publication number
US2303909A
US2303909A US275169A US27516939A US2303909A US 2303909 A US2303909 A US 2303909A US 275169 A US275169 A US 275169A US 27516939 A US27516939 A US 27516939A US 2303909 A US2303909 A US 2303909A
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United States
Prior art keywords
potential
signal
valve
signals
datum
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US275169A
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English (en)
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Blumlein Alan Dower
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EMI Ltd
Electrical and Musical Industries Ltd
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EMI Ltd
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00—Details of television systems
    • H04N5/14—Picture signal circuitry for video frequency region
    • H04N5/16—Circuitry for reinsertion of DC and slowly varying components of signal; Circuitry for preservation of black or white level
    • H04N5/18—Circuitry for reinsertion of DC and slowly varying components of signal; Circuitry for preservation of black or white level by means of "clamp" circuit operated by switching circuit
    • H04N5/185—Circuitry for reinsertion of DC and slowly varying components of signal; Circuitry for preservation of black or white level by means of "clamp" circuit operated by switching circuit for the black level
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03G—CONTROL OF AMPLIFICATION
    • H03G1/00—Details of arrangements for controlling amplification
    • H03G1/04—Modifications of control circuit to reduce distortion caused by control

Definitions

  • the present invention relates to systems for the transmission of electrical signals and is concerned with the correction of variations in the effective amplitude of signals representative of intelligence, such as may arise in the transmission of said signals as the result of the complete or partial loss of the D. C. component of said signals, the incorrect representation of that component, or varying attenuation of the signals.
  • the negative synchronising pulses cause the diode valve to conduct when the potential of the cathode becomes equal to the potential of the anode of the diode and the condenser is given a certain charge which biases the control electrode of the cathode ray tube.
  • the peaks of the synchronising pulses become more negative due to the loss of the D. C. component, the charge on the condenser will increase. Should, however, the peaks now become less negative owing to a change in the D. 0. level of the signal then the charge on the condenser will not be altered because the valvescwill be insulating and will thus be unable to harge the condenser to the new peak values.
  • the leak resistance is connected across the diode valve so that between synchronising peaks the charge slowly leaks away, thus making it possible for the diode to charge the condenser during those synchronising pulses, the amplitudes of which are less than the amplitude of the maximum negative pulse.
  • the level during the picture signals due to the super-imposition of the saw-tooth shaped discharge potential is such that the sense of the saw-tooth potential tends to make the amplitude of the picture sig nals increase towards the end of the discharge period owing to .the leak resistance being taken to a positive source of potential.
  • apparatus for substantially correcting unwanted variations of arecurrent datum level in electrical signals such as may arise as a result of varying attenuation of said signals or the partial or complete loss of their D. C. and/or low upon the correction of the variation of said datum level is reduced.
  • the observing devices are situated at successive points in the signal channel through which said signals pass.
  • a balanced, or push-pull signal channel is provided and an observing device is associated with each side of said balanced signal channel.
  • FIG. 1 which illustrates one embodiment according to the invention of a method of D. C. re-insertion
  • television signals with, for example, a waveform of the Marconi/E. M. I. type such as is illustrated in the Wireless World of October 4, 1935, page 373, are
  • valve i2 The anode and screen grid of valve i2 are taken to suitable positive potentials and the cathode of valve I2 is taken via resistance l5 to a source of negative potential.
  • the output from the cathode follower valve I2 is taken via coupling condenser l9 to the control electrode of an amplifier valve 2
  • the sense of the television signals applied to terminal II are such that the synchronising pulses are in a negative sense.
  • the purpose of the valve 5, which has its anode connected to the control grid of valve I2 is to re-insert the D. C.
  • positive synchronising pulses (which may with advantage be slightly shortened in duration) are applied to the control grid of valve I6 through the coupling condenser I 8 and leak resistance I! in such a manner that the valve IE only conducts during the synchronising pulse periods.
  • the condenser I3 is thus charged during the synchronising pulses to a datum level determined by the cathode potential of valve l6 so that the D. C. is re-inserted on the grid of valve l2.
  • the resistance H which is connected between the grid of valve l2 and a source of positive potential, tends to make the control grid of the valve more positive so that a saw-tooth shaped potential is super-imposed on the picture signals.
  • the D. C. is again re-inserted at the grid of valve 2i by means of the diode 23 which operates in the manner described in the specification of U. S. Patent No. 2,252,746 supra.
  • the battery 25 provides a suitable bias potential for the grid of the valve 2
  • Patent No. 2,252,746 and subsequently re-inserted by an inverted method of re-establishment such that the rate of drift of the D. C. potential during the picture signal of the latter method of D. C. re-establishment neutralises the sum of the potential drifts on the first two re-establishments
  • the grid leak resistances for example, resistances l4 and 20, are preferably of high value and are fed from sources of high potential so that the rate of drift is substantially independent of the picture signal amplitude.
  • the leak resistance may be taken to a positive potential of say 50 volts, the value of the leak resistance being related to the value of the coupling condenser so as to produce a rate of drift of the charge on the condenser greater than any probable drift of the picture signals.
  • the method according to the invention may be applied to a push-pull or balanced transmission system.
  • Figure 2 shows such an arrangement.
  • the ampli- .fier 26, which may be of the type described in the specification of U. S. Patent to Blumlein No. 2,185,367, issued June 2, 1940, provides two outputs which are fed into similar condensers 21 and 28.
  • the D. C. is re-established with regard to the recurrent datum level, such as the black datum level or the synchronising signal of a television wave, by means of the uni-directionally conductive devices 29 and 30 shown as pentode valves. These pentodes act as diodes as regards their anode-cathode circuits when the control grids are switched on.
  • valves 29 and 30 are switched on by positive pulses which are applied at terminal 42 to the control grids of valves 29 and 30, via coupling condensers 31 and 38 and leak resistances 35 and 36 respectively.
  • the positive pulses applied to the grids of these valves cause grid current to flow, thus charging the grid condensers 31 and 38 and producing a negative bias on the grids of these valves in the absence' of switching pulses from 42, which bias is sufficient to cut oil the currents in valves 29 and 30.
  • the production of such pulses is described, for
  • the pentode re-inserting valve which has the syn-v chronising pulses applied to it in a positive sense may bereplaced by an un-switched diode valve, but this is not preferred as the different characteristics of the diode and th longer negative period (especially during long frame pulses) spoil the symmetry of the arrangement.
  • and 32 are connected to the output side of each of the coupling condensers 21 and 23, that is, to the leads33 and 34. These resistances 3
  • valves 29 and 30 are made conductive and charge condensers 21 and 28 negatively so as to make the potential of lines 33 and'34 approximately earth potential during datum periods.
  • both lines 33 and 34 drift in potential, due to the leak resistances 3
  • the lines 33 and 34 are followed by an amplifier or transmission link 39' which is so arranged as to modify substantially only the push-pull input potentials, that is, the difierence of the potentials between lines 33 and 34, to the exclusion of the absolute average potentials of lines 3
  • a suitable amplifier arrangement for stage 39 is described, for example, in the specification of U. S. Patent No. 2,185,367 supra.
  • and 32 operating from a common large positive potential on similar condensers 21 and 28 will therefore cancel as regards the output of stage 39 which output may be balanced or unbalanced as-desired, the output being derived from terminals 40 and 4
  • valves 29 and 30 The tendency of the valves 29 and 30 to charge condensers 21 and 28 to a slightly different potential during the frame pulse (there are twice as many datum periods per line with the pulse generating methods described) due to the finite resistance of the valves, is also neutralised by the push-pull stage 39.
  • the condensers 21 and 23 should be large compared with any other stray capacities associated with this part of the circuit in order that a symmetrical arrangement be preserved. For this reason it is preferable that the output from amplifier stage 23 has a low impedance.
  • capacity of these condensers should be small enough, however, as to be substantially charged through the resistance of the valves 29- and 30 and the output circuit of condensers 21 and 23 in series during one datum period.
  • the resistance of a pentode 29 plus the output impedance of line 33 say, 1,500 ohms and the datum period at 4 microseconds
  • the time constant, that is, capacity of the condenser 21 multiplied by 1,500 ohms should be less than, say 2 microseconds, that is. the capacity of condenser 21 may be 0.001 microfarad.
  • must be greater than that required to charge a 6011-.
  • the datum period used in the above method was repetitive once per line at least.
  • a once per frame datum period may be used although of cource changes as rapid as that described above, that is, 0.01 second, cannot then be corrected.
  • Figure 3 shows an arrangement for observing the black level by charging two condensers which are provided with leak resistances, the purpose of the observation being either for D. C. re-insertion or for A. V. C.
  • the two condensers were connected in the signal leads so that their charge was added directly to the signal.
  • the condensers are situated in a separate path from the signal path and the potentials across them may be used either to add to the signal afterwards, to feed back into the input circuits, or to control the gain of an amplifier.
  • 45 is a push-pull, or balanced, transmission link such as an amplifier supplying a push-pull output signal to leads 46 and 41, which are connected to output terminals 63 and 64.
  • the lead 46 is connected to the control grid of a valve 5
  • and 53 are connected also to the cathode of a diode valve 43, the anode of which is connected via resistance 59 to a condenser 51 and its associated discharging resistance 6
  • the lead 41 is connected to the grid of valve 50, the cathode of which is connected to the cathode of valve 52 which is provided with a suitable cathode impedance 54 and is also connected to the cathode of a diode valve 48.
  • the anode of this diode 48 is connected to another charging condenser 56 via resistance 58, the condenser 56 being connected to the discharging resistance 60 which is similar to resistance'tl.
  • are taken to a source of positive potential as are the anodes of the valves 50, 5
  • , 52 and 53 act as two uni-directional conducting switches as described in co-pending U. S.
  • condensers 56 and 51 are preferably of equal value and are charged positively by the equal leak resistances 60 and lil from a source of positive potential. If. the condensers are not equal in value the time constants of the condensers with their associated leak resistances divided by the effective voltages associated with the leak resistances should be made equal in order to give an equal rate of drift to the two condensers.
  • the potentials on the two condensers 56 and 51 will be set during each datum period to a value depending on the datum potentials on lines 41 and 46, and the diflerence of potential will be equal to the difference of potential of the lines 46 and 41 during datum periods. After the datum period the potentials on the condensers will drift positively but since the rates of drift are equal the potential difference between the lines 56a and 51a will not be affected.
  • the difference of potential across the condensers 56 and 51 which is obtained from the terminals 56a and 5111 may be added to the potentials of lines 46 and 41 in reverse phase so producing in effect a D. C. re-establishment.
  • the difference of potential on the condensers is th same as if the clamp system of D. C.
  • a clamp system is meant a system, for example, as described in the specification of U. S. Patent No. 2,190,753 supra, wherein a 'bi-directionally conductive switch is employed so that the charging condenser is not discharged between datum periods.
  • the potentials may be fed back in a negative sense into the push-pull input of stage 45, thus serving to re-establish D. C. on lines 46 and 41.
  • stage 45 must be a D. C. coupled push-pull amplifier and is preferably of the type described in the specification of U. S. Patent No.
  • the amplifier 45 is provided with two special input valves for the introduction of the feedback potentials. These input valves may have their anodes connected to the anodes of the input valves receiving the incoming signals.
  • the resistances 58 and 59 may be necessary in order to prevent oscillation around the loop circuit as described in the patent application Ser. No. 275,168 supra.
  • the arrangement described above may with advantage be used to re-establish D. C. as described in the U. S'. patent application of Blumlein, Ser. No. 275,367, filed May 24, 1939, wherein the blacked out signals from a cathode ray type of television transmitting tube are used for the re-cstablishment of the D. C. component of the picture signals.
  • the terminal 62 of Figure 3 will have negative shortened blackout pulses applied to it.
  • the potential difference across terminals 56a and 51a may be used to control the gain of an amplifier stage preceding the stage 45 and may thus control the potential difference between thclines 46 and 41.
  • carrier frequency signals may be amplified in a suitable amplifier and the output applied to two detector valves which may apply push-pull signals to the input of stage 45.
  • Alternatively 45 may be an amplifier of the type described in the specification of U. S. Patent No. 2,185,367, supra which will convert the output from a single detector into a push-pull signal.
  • the gain of the carrier frequency amplifier is thus adjusted with reference to the difference of potentials on terminals 56a and 51a, thus operating as an automatic volume control, the sense of control being arranged so that any difference of potentialon lines 45 and 41 during the datum periods tends to alter the amplifier gain so as to suppress such potential differences.
  • the sense of control being arranged so that any difference of potentialon lines 45 and 41 during the datum periods tends to alter the amplifier gain so as to suppress such potential differences.
  • Figure 4 shows a circuit arrangement for controlling the gain of an amplifier stage in accordtentials as derived from terminals 56a and 51a of Figure 3.
  • Carrier frequency signals are applied to the primary of transformer 10, the secondary of which is connected to the grid of valve 1
  • An output transformer 16 is connected in the anode circuit of amplifier valve 1
  • cathode of valve 1! is connected to the cathode of another screen grid valve 12, the anode of which is connected to a suitable positive potential.
  • a cathode resistance 13 which is taken to a source of negative potential.
  • the cathodes are lay-passed to earth for radio frequencies by means cf a tuned circuit comprising inductance 14 and condenser 15.
  • the control potentials from termnals 56a and 51a are applied to the terminals 11 and 18 which are connected to the control electrodes of the valves 1
  • the reference potential is always earth potential, for example, in Figure 2 where the cathodes of valves 29 and 30 are connected .to earth. Any other reference potentials may of course be used by inserting suitable sources of potential such as batteries at the appropriate places.
  • the invention has been described above as applied to television signals although it will be understood that this invention is not limited in its use to television systems.
  • the datum period may be inherent in the signals when generated or may be inserted therein after the signals are generated or by passing the signals through a valve and switching ofi the valve at intervals so that pulses of datum level are interspersed through the signals.
  • Apparatus for developing a substantially undistorted corrective signal from a received signal having a recurring datum value, said corrective signal being proportional to the value of the recurring signal comprising a first thermionic tube having anode, cathode and at least one control electrode, a second thermionic tube having anode, cathode and at least one control electrode.
  • the received signal comprises a composite television signal, and wherein said recurring datum portion bears a definite relationship to the background level of the image transmitted, and wherein said combined stored energies represent the corrective signal for're-establishing the direct current component.
  • Apparatus for developing a substantially undistorted corrective signal from a received signal having a recurring datum value, said corrective signal being proportional to the value of the recurring signal comprising a first thermionic tube having anode, cathode and at least one control electrode, a second thermionic tube having anode, cathode and at least one control electrode, means for impressing the recurring datum portion of said received signal onto the control electrode-cathode paths of both of said thermionic tubes in balanced relationship, first and second uni-directional conductors, means for connecting each of said uni-directional conductors into the anode-cathode path of one of the thermionic tubes, a first capacitive means, a second capacitive means, means for storing energy in said first and second capacitive means during the occurrence of the recurring signal having a datum value, a leakage path connected substantially in parallel with each of said capacitive means, and output terminal means for removing a corrective signal substantially free
  • the received signal comprises a, composite television signal, and wherein said recurring datum portion bears a definite relationship to the background level of the image transmitted, and wherein said combined stored energies represent the corrective signal for re-establishing the direct current component.
  • a corrective signal developing means wherein recurring signals are received having predetermined voltage levels as compared to a fixed value, and wherein the developed corrective signal must bear a definite relationship to the value of said recurring voltage level signals during intervals between occurrences of said signals
  • the method of developing said corrective signal which comprises the steps oi storing electrical energy of a value proportions to the value of the received signal during its time of occurrence, the stored electrical energy being subject to change in a predetermined polarity direction between recurring signal intervals, combining a signal indicative of the stored energy with the composite signal, storing a second quantity of energy dependent upon the same signal interval, the stored electrical energy being subject to change in the opposite polarity direction between received signal intervals, and combining a signal indicative of the value of said latter stored energy with the composite signal whereby a corrective signal will be produced that does not vary between voltage level signal intervals.

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Networks Using Active Elements (AREA)
  • Picture Signal Circuits (AREA)
US275169A 1938-05-30 1939-05-23 Transmission of electrical signals Expired - Lifetime US2303909A (en)

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GB16084/38A GB515364A (en) 1938-05-30 1938-05-30 Improvements in or relating to the transmission of electrical signals

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US (1) US2303909A (enrdf_load_stackoverflow)
FR (1) FR855462A (enrdf_load_stackoverflow)
GB (1) GB515364A (enrdf_load_stackoverflow)
NL (1) NL69792C (enrdf_load_stackoverflow)

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2525106A (en) * 1946-11-21 1950-10-10 Rca Corp Electronic keyer for direct current restoration
US2529428A (en) * 1947-03-01 1950-11-07 Philco Corp Automatic gain control system
US2547648A (en) * 1946-01-25 1951-04-03 Hazeltine Corp Automatic contrast control system for television apparatus
US2550178A (en) * 1946-11-21 1951-04-24 Rca Corp Direct current reinsertion circuit for television systems
US2559038A (en) * 1949-08-01 1951-07-03 Avco Mfg Corp Line pulse keyed automatic gain control circuit with control voltage delay
US2586193A (en) * 1948-08-19 1952-02-19 Rca Corp Keyed automatic gain control system
US2589927A (en) * 1949-02-25 1952-03-18 Gen Precision Lab Inc Keyed automatic gain control and sync separator
US2615089A (en) * 1948-12-18 1952-10-21 Rca Corp Keyed automatic gain control system
US2620393A (en) * 1948-07-15 1952-12-02 Emi Ltd Circuits for reinserting direct current and low-frequency components
US2654799A (en) * 1948-08-26 1953-10-06 Rca Corp Keyed automatic gain control with keying pulse limiter

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2547648A (en) * 1946-01-25 1951-04-03 Hazeltine Corp Automatic contrast control system for television apparatus
US2525106A (en) * 1946-11-21 1950-10-10 Rca Corp Electronic keyer for direct current restoration
US2550178A (en) * 1946-11-21 1951-04-24 Rca Corp Direct current reinsertion circuit for television systems
US2529428A (en) * 1947-03-01 1950-11-07 Philco Corp Automatic gain control system
US2620393A (en) * 1948-07-15 1952-12-02 Emi Ltd Circuits for reinserting direct current and low-frequency components
US2586193A (en) * 1948-08-19 1952-02-19 Rca Corp Keyed automatic gain control system
US2654799A (en) * 1948-08-26 1953-10-06 Rca Corp Keyed automatic gain control with keying pulse limiter
US2615089A (en) * 1948-12-18 1952-10-21 Rca Corp Keyed automatic gain control system
US2589927A (en) * 1949-02-25 1952-03-18 Gen Precision Lab Inc Keyed automatic gain control and sync separator
US2559038A (en) * 1949-08-01 1951-07-03 Avco Mfg Corp Line pulse keyed automatic gain control circuit with control voltage delay

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FR855462A (fr) 1940-05-11
GB515364A (en) 1939-12-04
NL69792C (enrdf_load_stackoverflow)

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