US2018489A - Signal transmission system - Google Patents

Description

Oct. 22, 193.5. ,L S'JDQBAY JR I 2,018,489

SIGNAL TRANSMISSION SYSTEM Filed Aug. 12, 1952 f ir Patented Oct. 22, 1935 UNITED STATES ATENT OFFICE Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application August 12,

20 Claims.

v'Ihis invention relates to signal transmission systems and particularly to the control of the volume range of the signals on transmission systems.

One object of the invention is to provide a signal system that shall in an improved manner compress the signals a fraction of the original volume range before transmission over a line and expand the signals at the receiving end of the transmission line to the original volume range.

lAnother object of the invention is to provide a signal system that shall compress the signals a fraction of the original volume range at the transmitting end of a line under the control of a backward acting circuit and expand the signals to the original volume range at the receiving end of the line.

A further object of the invention is to provide a signal system that shall compress the volume range of signals a fraction of the original volume range before transmission over a line by threeelement space discharge devices connected across the line under the control of a backward acting circuit and that shall expand the volume range of .the `signals to the original range at the receiving end of the line by three-element space discharge devices connected effectively in series with the line under the control of a forward acting circuit.

The signals received for transmission over a line in many instances have a volume range larger than can be ef-Iiciently transmitted. Signal systems are limited as to the volume range which can be satisfactorily transmitted. Satisfactory transmission of signals has an upper limit to avoid overloading of line apparatus such as repeaters and interference with other circuits and has a lower limit by reason of interference from line noises.

The present invention discloses a system wherein the volume range of the transmitted signals is held within the upper and lower limits of satisfactory transmission by changing the shape of the signal Waves-at the transmitting end of the line and restoring the signal waves to the original shape at the receiving end of the line. The invention is an improvement over the system disclosed in the S. Doba, Jr. Patent No. 1,931,063, dated October 17, 1933 and the patent to S. Doba No. 1,854,828 dated April 19, 1932.

In the present invention, the compressor for reducing the volume range of the signals a fraction of the original volume range comprises two three-element space discharge devices connected across the transmission line and a backward act- 1932, Serial N0. 628,505

ing circuit for controlling the grid bias on the devices. The backward acting control circuit includes a linear operating rectifier. The characteristic of the rectier vand the characteristics of the two space discharge devices have a deiinite` volume range. e

The expander at the receiving end of the line for expanding the volume range of the signals to the original volume range is very similar in congo struction and operation to the expander disclosed in the above mentioned patent to S. Doba, Jr., No. 1,931,063. The impedance means employed by the expander comprises two three-element space discharge tubes which are effectively connected in $5` series with the line. 'I'he grid bias on the space discharge tubes is controlled by a forward acting circuit having a linear acting rectifier. The characteristics of the space discharge tubes and the rectier are adjusted in the manner disclosed in 30 the S. Doba, Jr. Patent No. 1,931,063.

In a compressor of the type employed in this invention a fixed relation is maintained between the characteristics of the space discharge devices comprising the impedance element and the charu acteristic of the rectifier in the backward acting control circuit for the impedance element. The relation between the characteristics of the impedance element and the rectier is set forth be1ow. 0

If the current voltage characteristic of the lmpedance element is I KE' then a 1. 25 R`DE the impedance, is

50 d V1 R0* nKE-1 E is the potential impressed on the grids of the impedance element and K is a constant.

In a shunt type oi' variable repeater which is 55 10g E=20 log Ka-l-'rLiVo (3) VCombining Equations (2) and (3) G:2o1ogK2- n-o 2010g1f3+mv0;

" K4-(7L-1ML1V0 (4) Y employed in the compressor, the amplification is directly proportional to the impedance of the variable impedance element.

K1 K2 A nK n-l-En-l A is the voltage amplication, K1 and K2 are constants and the gain in decibels is:

If E is obtained from a rectiiier circuit with a characteristic Y E=K360TL1 where E is the output voltage of the rectiiier circuit and e0 is the input voltage of the rectiiier circuit and the output Voltage of the variable repeater, Vthen 20 log E=2O log KS4-m20 logr eo (20 log en) `may be called the output volume Vo of the variable repeater, hence The output volume of the Variable repeater is the input volume plus the gain or Where V1 is the input volume to the variable repeater in decibels.

lSubstituting the value of G Equation (4) in Equation (5) The constant Ki'may be arbitrarily given a value of Zero in Equation (6) and If the output volume range of the variable repeater is to vary as a fraction F of input Volume range,

Y The valueV of n may be varied by the means disclosed in the patent to S. Doba, Jr., No. 1,931,063 or by using more than one stage of im- Y pedance elements in tandem.` The Value of m may beV controlled by means of rooter devices or squarer devices as disclosed in the patent to G. Crisson No. 1,737,830 dated' December 3, 1929.

' YIvhe amount of rooting or squaring may be controlled by the means disclosed in the above mentionedS. Deba, Jr., Patent No. 1,931,063.

v In the present application of the invention, the

value of F is assumed to be 1/2 and the Variable impedance element to be adjusted so that n has a value of 2. The value 11,1 derived from Equaf tion (8) is 1.

If it were desired to have a value for F of 1/3,Y when n is equal to 2, then Equation (8) gives a Value for n1 of 2.

In considering the characteristics of the impedance element and therectifier circuit of the expander at the receiving end of the line it should `be, noted the. rectier circuit is forward acting and the space discharge tubes of the impedance element are connected effectively in series with the vline5 fstart the percussive consonants.

pressors and expanderinthe present invention amplification is inversely proportional to the 1mpedance of the variable impedance element.

A=K17tKEn`1:KzEL-l (9) The gain in decibels is:

G=20 log A=20 log K2+(n-1)20 log E (10) If E is obtained from a rectier circuit with a. characteristic, i

E=K3ein1 Where E is the output voltage of theV rectiiier circuit and ei 1s the input voltage of the rectifier circuit and also the input voltage to the Variable Y impedance element, then 20 log E=20 log E34-m20 logV ei The value (20 log ei) may be called the input volume V1 to the variable impedance element. Substituting V1 for 20 log e1, y f

20 log E=20 log K3-l-V7L1Vi (11) Substituting for 2O log Ev Equation (l1) in Equation (1 0) v The output volume from the variable repeater of the expander is the input volume plus the gain.

Vo=VilGY Y (13) The output volume V0 is given in decibels.Y Substituting the value of G Equation (12) in Equation (13) The constant K4 may be arbitrarily given a value of zero in Equation (14) if the output volume from the variable re peater of the expander varies inversely according to a fraction F The values oi n and n1 may be varied in the same manner as the values of n and ln1 ink the compressor. The expander in operation is complementary to the operatien of the compressor.

In a compressor having a backward-acting control circuit as employed in the present invention there is no over-accentuation of the initial part of the syllables which start with percussive consonants.V Ina compressor' having a; forward- Vactingccntrol circuit there is a tendency for the compressor and expander in tandem to overaccentuate the initial part of the syllables that The comhave no such tendency by reasonjo-f the compressor having a backward-acting control circuit and the expander having a forward-acting control circuit. Signals having identically the same variations in volume range are applied'to the two control circuits. The term signal when used in the specification and claim is intended to refer not only to the communication of intelligence but also to the transfer of music and other programs. f

In the accompanying drawing,

Fig. l is a diagrammatic view of a compressor-- expander circuit constructed in accordance with the invention;

Fig. 2 is a diagrammatic View of a modified circuit for controlling the compressor.

Referring to 1Fig. l of the drawing a compressor I and an expander 2 are shown connected to a transmission line having input conductors 3 and 4 and output conductors 5 and 6. 'I'he `compressor i comprises a variable repeater 1 under the Vcontrol of a backward-acting rectifier circuit 8. IIhe expander 2 comprises a variable re- Y peater 5 under the control of a forward-acting rectifier circuit I9.

The variable repeater 1 in the compressor` I comprises an impedance element I I and two ampliers I2 and I3. The amplifiers I2 and I3 are preferably of the pure electron discharge type. The impedance element II comprises two threeelement space discharge devices I4 and I5 which are symmetrically connected across the line circuit between resistance elements I6, I1, I8 and I9. The resistance elements I1 and I9, serve to effect a rough adjustment of the characteristics of the space discharge devices I4 and I5 in the manner disclosed in the above mentioned patent to S. Doba, Jr., No. 1,931,063.

The device I4 comprises a grid 2l, a cathode 22 and a plate 23. The device I5 comprises a grid 24, a cathode 25 and a plate 28. A battery 21 is provided for supplying plate current to the devices I4 and I5 and a battery 28 is provided for supplying negative bias to the grids 2| and 24 of the devices I4 and I5. A variable grid bias 'for the devices I4 and I5 is supplied by the control or rectifier circuit 9. Resistance elements 29 are provided in the grid-filament circuits of the devices I4 and I5 to effect a fine adjustment of the characateristics of the devices I4 and I5.

The impedance element is connected to the input conductors 3 and 4 by means of a transformer 38 and is connected to the amplifier I2 by means of a transformer 3l. The amplifier I2 is connected to the amplifier I3 by a transformer 32. The amplifier I3 is connected to a high-pass filter 33 by means of a transformer 34. rPrefera ly, the filter 33 will pass waves only in the speech band to prevent the operation of the rectifier circuit 8 by the transients due to the unbalance of the devicesM and I5 or the unbalance of primary windings of the transformer 3l. The amplifier I2 is provided with a grid biasing battery 35 and plate battery 35. The

lamplier I3 is provided with a grid biasing battery 31 and aplate battery 38.

The rectifier circuit comprises a rectifier 48 of the space discharge type and an amplifier 4I of the space discharge type. The input circuit of the amplifier 4! is connected by a transformer 42 to the line circuit beyond the high-pass filter The output circuit of the amplifier 4I is connected by a transformer 43 through the rectifier and a resistance condenser network 44 to the 'grids 2l and of the space discharge devices Ita'nd I5. The resistance condenser network 44 lpreferably comprises a condenser 45 and a resistance element 45. rIhis rnetwork serves to smooth out the ripples in the rectified current supplied to the grids oi the space discharge devices I4 and I5. A low-pass filter as shown in the patent to S. Doba, Jr., No. 1,931,063 may also be employed.

The variable repeater 9 in the expander 2 comprises an impedance element 41 and an amplifien 5 48. The impedance element 41 comprises two space discharge tubes 49 and 59 which are Aconnected effectively in series with the line circuit. In the compressor circuit it will be noted the two space discharge devices I4 and I5 are con-'10 ing a rough adjustment of the characteristics of 20 the space discharge tubes 49 and 50. A condenser 51 is provided as an alternating current bypass across the resistance elements 55 and 55. Resistance elements 58 are provided in the gridfilament circuits of the space discharge tubes 49725 and for effecting a fine adjustment of ythe characteristics of the tubes. The tubes'49 and 50 are provided with a grid biasing battery 65 and a plate battery 6I Variable grid bias for the tubes 49 and 50 is provided by the forward-acting recti-A 30 fier or control circuit I0.

The rectifier circuit I0 comprises a rectifier 83 of the space discharge type and an amplifier 64 preferably of the space discharge type. The input circuit of the amplifier 64 is connected to the linei'35 ahead of the transformer 5I by means of a transformer 95. The output circuit of the amplifier is connected by transformer 66 through theY rectifier 63 and a condenser resistance network 61 to the grids of the space discharge tubes 49 and 5I).v The 40 resistance condenser network 81 is the same type as the network 44 in the rectifier circuit 8 and comprises a condenser 68 and the resistance element (i9. By making the networks at the trans-v mitting and receiving ends of the line identical, 45

any delay in compressing action at the transmitting end of the line is balanced by an equal delay at the receiving end of the line in expanding. In

'this way over accentuation of percussive con-A sonants is avoided. A low-pass filter may also be' 50 used here when one is used in the compressor for the same reason.

In the compressor-expander circuit above described the compressor I compresses the volume range of the signals received from the conductors 3 and 4 by one-half. The characteristics of the impedance element I I and the rectifier circuit 8 are adjusted together in o-rder to obtain the correct compression. Inasmuchas the rectifier circuit is backward-acting, itis unnecessary to"00 provide rooter devices of the type disclosed in the above mentioned Crisson patent in order to obtain an output from the variable repeater 1 which has an output in decibels which is half the The expander input to the variable repeater. 2 operates in the same manner as the expander disclosed in patent to S. Doba, Jr., No. 1,931,063. In Fig. 2 of the drawing is disclosed a modified rectifier or control circuit for the variable re-m.

peater of the compressor. The same circuit may To also be used for the expander. Similar parts in the control circuit shown in Fig. 2 to those shown in Fig. l will be indicated by like reference characters. In the control circuit shown in Fig, 21s

of the drawing a network comprising condensersv u V'aus Y drawing.

times. drawing the ccndensers i and il are charged up :10, 14|, inductance element 12 and rectifier 13 is substituted for the resistance capacity network 44 shown in the control circuit 8 in Fig. 1 of the The resistance capacity network 44 shown in Fig. 1 of the drawing is quick-acting so Vthat the control followsthe envelope of the speech Waves without causing any appreciable delay. However, in such a circuit the gain does not remain constant at any time but is continuously changing. The network shown in Fig. 2 of the drawing has the advantage of the quick action of the network 44 shown in Fig. lof the drawing and does not have the disadvantage of the network 44 in that the gain is changing at all In the network shown in Fig. 2 of the practically simultaneously upon the application of a signal to the input of the rectier circuit due to the Yshort circulting of the inductance 'I2 by the cathode-anode impedance of the rectifier tube 13. When the input to the rectifier circuit is removed the flow of current through the network is reversed and the impedance of rectifier tube 'i3 becomes infinite. The discharge of the condensers Hi and 'Il must then take place through Y element connected to said path, a rectifier circuit connected to said path beyond the connection of the impedance element to the path for controlling the impedance element, and means for effecting a definite relation between the characteristics of the impedance element and the rectifying circuit to obtain a variation in volume output on the path beyond the impedance element which is a fixed fraction of the variation in volume input on the path before the impedance element.

2. In combination, a signal line, impedance means comprising three-element spacer discharge devices connected to said line, means comprising a'control circuit having a rectifier circuit connected to said line beyond said impedance vmeans for controlling the impedance means, and means for effecting a definite relation between the operating characteristics of said spaceV discharge devices and the operating characteristic of said rectifier circuit to obtain a variable output volume on the line beyond said impedance means which is a fixed fraction of the range of the input signal volume expressed in decibels on the line before the impedanceV means.

' 31 InV combination,YV a signal line, impedance means comprising three-element space discharge devices connected across the line, means comprising a backward acting control circuit connected to the line beyond said impedance means for controlling the bias on the grids of said devices, and means for maintaining a fixedY relation between the operating characteristic of the impedance-'means and the operating characteristic of said control circuit to obtain a variable output volume on the line beyond said impedance means which is a fixed fraction of the range of the input signal Yvolume expressed in decibels on the line before the impedance means.

4. In combination, a signal line, impedanceV .means comprising three-element space discharge controlling the bias on the grids of said devices, 5

the operating characteristics of said space discharge devices and the operating characteristic of said rectifier having a definite relation to obtain an output on the line beyond said impedance means which is half the range of signal k volume expressed in decibels on the line before the impedance means. l

5. In combination, a signal line, an impedance element connected across saidy line, and means comprising a linear acting rectifier element connected to said Vline beyond the connection of the impedance element to the line and havingV a definite relation between its operating characteristic and the operating characteristic of said impedance element for controlling the impedance element to obtain an output on the line beyond said impedance element which is half the range of signal volume expressed in n decibels :on the line before the impedance element.

6. In combination, a signal line, impedance means comprising two three-element space discharge devices -connected across said line, a back- Ward-acting control circuit connected to said line beyond said impedance means, and having a rectifier for controlling the bias on the grids of said devices and means for maintaining a fixed relation between the characteristics of said space discharge devices and said rectifier to obtain a variation in volume output on the line beyond said impedance means which is av flxed fraction of the variation in volume input on the line before the impedance means.

7. In a signal system, a transmission line, two

space discharge devices connected across saidY line at the transmitting end thereof, means com-,w prising a backward-acting control circuit con-`A` nected to said line beyond said discharge devices for controlling the devices to obtain an output on the line beyond said devices which is a fractional part of the range of signal volume expressed in decibels on the line before the devices, two space discharge tubes connected effectively in series with the line at the receiving end thereof, Vand means comprising a for-k ward-acting control circuit connected to said.50 line before said space discharge tubes for controlling the tubes to obtain anV output on the line beyond the tubes having the same volume range as the volume inputv to saidV devices at the transmitting end of the line.

8. In a signal system, a transmission line, threeelement space discharge devices connected across said line at the transmitting end thereof for controlling the volume range of the transmitted signais, means comprising a backward-acting control circuit having a rectifier connected to said line beyond said discharge devices for controlling the devices, the operating characteristics of said devices and the Voperating characteristic of said control circuit having a definite relation to ,65 obtain a variable volume output on the line beyond said devices which isa lxed fraction of the variations in the input signal volume expressed in decibels on the line before the devices, threeelement space discharge tubes connected, eflectively in series with the line at the receiving end thereof, and means comprising a forward-acting control circuit connected to said line before said tubes for controlling the tubes to obtain an output onV the line beyond the tubes 7| mitting end thereof for reducing the range of the transmitted signals, impedance means at the receiving end of the line for expanding the Volume range of the transmitted signals, control means governed according to the volume range on the line beyond the impedance means at the transmitting end of the line for controlling the impedance means at the transmitting end of the line to obtain a fractional reduction in the volume range of the transmitted signals, and oo nftrol means at the receiving end of the line governed according to the same volume range governing the control means at the transmitting end of the line for controlling the impedance means at the receiving end of the line to obtain an out- .'L-put beyond the impedance means at the receiving end of the line having the same volume range as the input to the impedance means at the transmitting end of the line.

10. In a signal system, a transmission lin --'space discharge devices connected across the line at the transmitting end thereof for reducing the range of the transmitted signals, a backward-acting control circuit for controlling said device to obtain an output on the line beyond said devices Which is a fractional part of the range of signal volume expressed in decibels on the line before the devices, space discharge tubes connected effectively in series with the line at tlie receiving end thereof for expanding the range of the transmitted signals, and control means at the receiving end oi' the line governed according to the same volume range supplied to said backward-acting circuit for controlling said discharge tubes to obtain an output beyond the tubes having the same volume range as the input to the devices at the transmitting end of the line.

ll. In combination, a signal line, space discharge impedance means in said line for controlling the impedance on said line, means comprising a rectifier circuit connected to said line beyond the impedance means and having an output proportional to the output on the line beyond the impedance means for controlling the impedance means, and means for eiecting a definite relation between the characteristics of the impedance means and the rectier circuit to obtain a variation in volume output on thev linel beyond the impedance. means which is a xed fraction of the variation in Volume on the line before the impedance means.

12. In combination, a signal line, space discharge impedance means in said line for controlling the impedance on the line, a control circuit connected to the line beyond said impedance means and comprising a rectifier and a filter for controlling said impedance means, and means for effecting a definite relation between the characteristics of the impedance means and the control circuit to obtain variation in volume output on the line beyond the impedance means which is a fixed fraction of the variation in volunie input on the line before the impedance means.

13. In combination, a signal line, space discharge impedance means in said line for controlling the impedance on the line, means comprising a control circuit having a rectifier and a iilter and connected to the line beyond said impedance for controlling the impedance means,

the output from said rectiiier being proportional to the output on the line beyond the impedance means, and means for effecting a definite relation between the operating characteristics of said impedance means and the operating char- 5 acteristics of said control circuit to obtain a variable output volume on the line beyond said impedance means which is a fixed fraction of the range of the input signal volume expressed in decibels on the line before the impedance means.

lli. In combination, a signal line, impedance means in said line, means comprising a rectifier circuit connected to said line, beyond the impedance means for controlling the impedance means, 15 a network in said rectifier circuit comprising a capacity and an inductance shunted by a rectifier for varying the time of charging the capacity as compared with the time of discharging the capacity, and means lfor effecting a definite relation between the characteristics of the impedance element and the rectifier circuit to obtain a variation in volume output on the line beyond the impedance means Which is a fixed fraction of the variation in volume input on the line 25 before the impedance means.

l5. Inl combination, a control circuit, an inductance element connected in series with said circuit, a capacity element connected across said circuit beyond said inductance element, and a 30 one-Way rectiiier connected across said inductance element to insure charging and discharging of the capacity element at different rates.

i6. In combination, a control circuit, means for applying direct current impulses to said cir- 35 cuit, an inductance element connected in series with said circuit, capacity means connected across said circuit adjacent to said inductance element, and a rectiiier connected across said inductance element for transmitting said direct 40 current impulses in one direction to insure charging and discharging of the capacity means at different rates.

17. In combination, a signal line, impedance means connected to the signal line, control means 45 governed according to the volume range on the lsignal line beyond said impedance means for controlling said impedance means, means for relating the characteristics of said impedance means and the control means to obtain a signal 50 volume range beyond said impedance means Which has a fixed ratio with respect to the signal volume range before said impedance means, an expander line liaving signals supplied thereto that have the same volume range as the output 55 beyond the rst mentioned impedance means, a second impedance means connected to the eX- pandor line, a second control means governed by the same wave shape as said first mentioned control means for controlling said second impedance means, and means for relating the characteristics of said second impedance means and the second control circuit to obtain an output beyond the second impedance means having the same volume range as the input to the first mentioned impedance means.

18. The method of controlling the volume range of signals on a signal line having an impedance connected thereto and a control circuit therefor and an expandor line having a second impedance conected thereto and a second control circuit therefor which consists in governing said signalline control circuit according to the signal output beyond said signal line impedance, in controlling the signal-line impedance by the signal-line control circuit, in relating the characteristic of the signal-line impedance to the characteristic o'I the signal-line control circuit to obtain a signal volume beyond said signal-line impedance Which has a xed ratio to the signal volume range before said signal-line impedance, in governing the eXpandor-line control circuit by the same wave shape as controls said signal-line control circuit, in supplying said expandor with signals corre spendingtothe output from the signal-line impedance, in controlling the eXpandor-line impedance by the eXpandor-line control circuit, and in relating the characteristic of the expandorline impedance to the expander-line control cirtrolling said impedance by the control circuit,

and in relating the characteristic of the impedance to the characteristic of the control circuit to obtain a signal volume beyond said impedance which has a xed ratio to the signal volume range before said impedance.

V20. In combination, a linerl means connected to the signal line, a controll cir-A cuit connected to the signal line beyond said impedance means to be governedv according to the signals beyond the impedance means for control"-l ling the impedance means, means formrelating the characteristic of the impedance means with respect to the characteristic of the control cir cuit to obtain a signal volume range beyond said impedance means which has a fixed ratio with respect to the signal volume range before said impedance means, an expandor line havingv sig'- nals supplied thereto that have the same volume range as the output beyond the rst mentioned impedance means, a second impedance means connected to the expander line, a second control circuit connected to the expandor line before said second impedance means and having the same characteristic as the rst mentioned control circuit for controlling said second impedance means, and means for relating the characteristic of Second impedance means to the characteristic or the sec-' ond control circuit to obtain an output beyond the second impedance means havingihe same volume range as the input to the rst mentioned impedance means. t

STEPHEN DOBA, JR.

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