US2342238A - Variable attenuation circuits - Google Patents

Description

Feb. 22, 1944; H BARNEY 2,342,238

I VARIABLE ATTENUATION CIRCUIT Filed D80. 31, 1941 2 Sheets-Sheet 1 FIG. R,

T k 2 FIG. 2 31' g FIG. 4 Q 3 4 1o gum 5 \l w 2 505/0/52025a035' 100a,, /NPUT(2'b REE To .00/ marr) i h '5 50.1 a 8 FIG. 3 g 0 5,: 10 a0 so so k 2 1000 INPUT (mus yous) b. it h j Q A 10ao- INPUT (/ws voLrs) INVNTOR HL. BAR/V5) ATTORNEY Fb. 22,1944. L B EY 2,342,238

VARIABLE ATTENUATION CIRCUIT Filed Dec. 31,1941 2 Sheets-Sheet 2 FIG. 5

1 T, 7; A i 1 AMF? o I PEAK INPUT SINE WAVE T0 PEAK CHOPPER F IG. 6

OUTPUT 0F PEAK CHOPPER mrnour M714 VARIABLE PEAK CHOPPER SENSITIVITY PEAK CHOPPER lNl ENTOR H L. BA RNEV BY MM ATTORNEY Patented Feb. 22, 1944 Harold L. Barney,

Madison, is. 1;, minor to Bell Telephone Laboratories, York, N. Y., a corporation oi Incorporated, New

New York Application December 31, 1941, Serial No. 425,099 4 Claims. (01. 178-44) The invention relates to variable attenuation networks and their pplications to electrical signal wave control circuits.

Objects of the invention are to obtain various types of non-linear action, such as instantaneous compression and expansion, peak chopping, etc, on speech or other signal waves with relatively simple and economical apparatus and circuits.

These objects are attained in accordance with the invention by the use of non-linear impedance elements in combination with linear impedance elements in a particular arrangement in a Wheatstone bridge circuit, with the selection of suitable values for the impedance elements and the application of suitable direct current biasing voltages to the non-linear elements to provide the desired output-input characteristics. i

A feature of the invention is the application of the circuit of the invention as a peak chopper with variable sensitivity for use with a variable gain device to prevent output amplitude overshoots tending to produce crosstalk, overload ing or disturbing noise in the listeners receiver, when the variable gain device is subjected to sudden large increases in input.

The various objects and features of the invention will be better understood from the following complete description when read in conjunction with the accompanying drawings, in which:

Fig. 1 shows schematically a variable attenuation network embodying the invention;

Figs. 2 and 3 show curves illustrating two types of compressor output-input characteristics which may be obtained with the arrangement of Fig. i;

Fig. 4 shows a curve illustrating a type of expander output-input characteristic which may be obtained with the arrangement oi Fig. i;

Fig. 5 shows schematically a circuit employing the variable attenuation network of Fig. l to provide a peak chopper of variable sensitivity; and

Figs. 6 and 7 show curves used to explain the operation of the circuit of Fig. 5.

As shown in Fig. 1, the variable attenuation circuit of the invention is in the form of a Wheatstone bridge. One arm of the bridge comprises a linear resistor R1; 2. second adjacent arm includes a network comprising a series linear resistor R2 and two branches in shunt therewith comprising linear resistor R4 and copper-oxide varistor V1 in series and the linear resistor Rs and copper-oxide varistor V2 in series, respectively, the two varistors V1 and V2 being equivalent and oppositely poled in that arm and biased by the voltage drop through the resistors R4 and R5 which are connected through resistances Rs and R6 to the battery B as shown; and the third and fourth arms of the bridge comprise respectively equal portions P1 and P2 of the primary winding of the output transformer T2- An inon the other side of the circuit.

put circuit I is connected across one diagonal of the bridge between the point of connection of the first two bridge arms and the point of connection of the third and fourth arms, by input transformer T1. The output circuit 2 is effectively connected across the other diagonal of the bridge by connection to the secondary winding S of the output transformer T2, symmetrl cally inductively coupled to the two equal pri mary winding portions P1 and P2 of that trans former in adjacent arms of the bridge.

In the circuit of Fig. 1, when alternating cur rent signals are applied from input circuit i through input transformer T1, current flows through the primary winding P1 of output transformer and resistance R1, and through the other primary winding P2 of that transformer and the network comprisingresistances R4 and R5 and the biased copper-oxide varistors V1 and V2, shunted by resistance R2. The output-input characteristic of the circuit is dependent on the balance obtained between resistor E1 on one side and the network of resistors and varistors At very low signal inputs, the impedance balanced against R1 is essentially only that of R2. Thus if Rs is larger than R1, the loss through the circuit is a minimum. Resistor R2 is used in this circuit to give a small improvement in the sharpness with which the output-input curve breaks at the point where compressing starts. At higher signal in-' puts the impedances of the varistors V1 and V2 are decreased due to the applied higher amplttude currents and the balance with R1 improves, thus increasing the attenuation between input circuit 8 and the output circuit 2 of the system.

The amplitude at which output compressing starts is determined by the bias on the varistors V1 and V2, the impedance ratios of the transformers T1 and T2, and the magnitudes of it; and R2. Varying the value of R1 changes the shape of the characteristic in the non-linear range. If the resistance value of R1 is smaller, the output of the system rises more at higher inputs, and if it is larger, the output decreases with increasing input, and then rises again with phase reversed as the input is further increased.

With the constants of the impedance elements of the circuit as shown in Fig. 1 when measured between GOO-ohm circuits, an output-input characteristic l ke given in Fig. 2 is obtained, which is that of an instantaneous compressor of very high compressing ratio or peak chopper. will be noted that this peak chopper is extremely simple; uses no vacuum tubes or gas-filled tubes:

does not have a time lag due to ionization or deionization time such as is present in gas tube limiters; and gives a sharp break in the outputinput characteristic at the limiting point. The

eflect oi the peak chopping action on a sine wave is illustrated inFig. 6. I v

If the biasing potential is removed from the varistors V1 and V2 in the circuit of Fig. 1, and the value of resistance R1 is made about 200 ohms, a compressing characteristic of the logarithmic type may be secured. Fig. 3shows" the output-input characteristic. of such an arrange? ment. As shown, over about a 45-decibel range of inputs from .09 to l5 volts, the output voltage is proportional to the logarithm of th input voltage.

The circuit of Fig. 1 may readily be made into an instantaneous expander by changing R1 from a low to a high resistance so that a good balance is obtained at low signal input. In that case, at high inputs the impedance of the varistors V1 and V2 decreases, unbalancing the arrangement and causing the signals to be transmitted from circuit I to circuit 2 with less attenuation.

By removing the direct current bias on the varistors V1 and V2 and using for the input transformer T1 and the output transformer T2, transformers which provide a low impedance level at the varistors, the output-input characteristic may be made to approximate an antilogarithnric expander, that is, one in which the logarithm of the output voltage is proportional to the input voltage, as shown in Fig. 4.

In addition to the charactersitics described above obtainable, with the circuit of Fig. .1, a number of others combining compressing and expanding actions may be produced by adjustments of R1 and R2 and the bias used on the varistors. An application of such circuit would be for shaping the control versus input amplitude. characteristics of control circuits for voiceoperated switching devices.

When variable gain devices such as vogads (volume-operated gain adjusting devices), compressors and volume limiters are subjected to suddent large increases in input, the finite length of the time interval required by the devices to make a gain reduction results in a momentary overshoot of the output amplitude. Though this overhoot may last only a few thousandths of a second, it may be objectionable because of overloading or crosstalk it produces, or because of the click or pop it causes in the listener's receiver which follows the output of the variable gain device. Fig. 5 shows a peak chopper of the type illustrated in Fig. 1 combined with a volume range compressor of known type to provide a peak chopper of variable sensitivity to prevent such momentary overshoots and thus their objectionable effects.

In Fig. 5 the compressor shown comprises the one-way amplifier A preceded by a vario-losser consisting of an input transformer T3, an out-- put transformer T4 and a copper-oxide rectifier bridge circuit VR having one diagonal connected in shunt to the circuit between the two transformers, and a backward-acting control circuit connected across the output of the amplifier A, including in order the one-way amplifier A1, the copper-oxide bridge rectifier R and the timing circuit comprising shunt condenser C and series resistance R7 connected across the other diagonal of the copper-oxide rectifier bridge circuit of the vario-losser. The peak chopper comprises a variable attenuation network in the form of a Wheatstone bridge, like that of Fig. 1, including in one arm the resistance R1, in a second adjacent arm a resistance R2 shunted by a circuit comprising varistor Vi and resistance R4 in series, and by a circuit comprising varistor V2 and resistance Rs in series, the varistors V1 and V2 being oppositely poled in the latter arm; the third and fourth arms comprising equal primary windings P1 and P2, re-

spectively, of the output transformer T2, the secondary winding S of which is connected to i put of the amplifier A in the compressor.

The sensitivity of the peak chopper in the combination of Fig. 5 iscontrolled by the plate current of a three-electrode amplifying vacuum tube A2 the anode-cathode circuit of which includes the resistors Riland R5 of the variable attenuation network in series with the .plate battery B1 of that tube, which in turn depends on the voltage across the output terminals of the rectifier R of the compressor because of the connection of the control grid-cathode circuit of tube A2 across those terminals. The greater the voltage produced across the output terminals of the rectifier R in the compressor control circuit the higher will be the just-operate point of the peak chopper, so that. for a steady state high signal input to the compressor, the peak chopper will not be operated by the signals in the output of the compressor. However, on suddenly applied strong signals from the output of the compressor, the initial sensitivity of the peak chopper is sufficient to attenuate these output signals for the first few millliseconds until the control circuit rectifier R of the compressor charges up condenser C and thus causes a decrease of peak chopper sensitivity, along with a reduction of the signal amplitudes in the output of the compressor.

The operation of the combination of Fig. 5 is illustrated by the curves of Figs. 6 and '7. As in the case ofthe peak chopper circuit of Fig. l, the effect of the peak chopper of Fig. 5 on a sine wave during the initial adjustment period of the compressor is as shown in Fig. 6.

Fig. 7 shows on the left an oscillogram of the output wave such as would be obtained by the use of a compressor only in the system of Fig. 5 when a strong tone is suddenly applied to the input. To the right in Fig. 'I is shown an oscillogramsuch as would be obtained by combining this compressor with the peak chopper in the invention shown in Fig. 5 to provide for the combination a variable sensitivity peak chopper. As shown in Fig. 7, an advantage of this system is that the peak chopper introduces no distortion on the steady state output, yet can limit the initial transient voltages to values lower than the steady state. r

The scheme of Fig. 5 could be applied to other variable gain devices such as volume limiters and vogads by employing an auxiliary rectifier circuit connected across the output of such a device and arranged to decrease the sensitivity of the following peak chopper in a manner similar to that which has been described for the system of Fig. 5. With the latter circuit, the lag in charging the condenser from the rectifier, and the range of sensitivity of the peak chopper would be adjusted so as to introducea minimum of distor-.

' tion on speech which was not varying in volume at the vogad or volume limiter input. Onapplication of a stronger signal which would require a large reduction of gain, the output amplitude would be initially reduced by the peak chopper to less than the steady state value for a few milliseconds, thus minimizing the effect of overshoots in overloading the following line and in produc- 'overaperiodsofat leastseverali'vocalicycles. If a: fixed'sensitivity peakichopper'is'iusedgithe first iewvocalrcycleszjat'l the output may have' amplitudsiasrgreatras. thosein the middlelpart of the syllable;which. would:result.:in -a veryunnatural speechsound; .With the variable sensitivity peak choppenhowever; theinitialfewwocal cycles are attenuated to amplitudes more nearly approximating their relative importance in an unaltered syllable. This is done at the expense of introducing more distortion for these few vocal cycles,

but the listener's ear' would probably be less likely to discern this type of distortion than it would the greatly increased amplitudes permitted by the plain peak chopper or with no peak chopper at all.

As described above, the arrangement of Fig. does not provide a definite limit point beyond which the output voltage may not rise. On steady state input, presumably the peak chopper described would not limit regardless of amplitude of the signal passing through the system. If it were desirable to have a given maximum amplitude not exceeded, the circuit of Fig. 5 could be modified by adjusting the resistances R4 and R5 so that when the vacuum. tube A: is overloaded and the maximum amount'of plate current through it was reached, the sensitivity of the peak chopper could not be reduced further. This would set the upper limit of amplitudes the system could put out, regardless of transient characteristics.

Variousv modifications oi. the circuits which have been illustrated and described which are within the spirit and scope of the invention will occur to persons skilled in the art. It is understood that the values of the impedance elements in the circuits of the invention given above are for illustrative purposes only and should not be construed as limiting the invention.

What is claimed is:

1. A system for controlling the amplitude range of alternating current signals of varying amplitudes, comprising a Wheatstone bridge having a linear resistor in series with one arm, a network having three parallel branches in series with a second adjacent arm, two of said branches being equivalent each comprising a copper oxide varistor in series with a linear resistor with the two varistors oppositely poled with respect to said second arm, and the third branch comprising a relatively large series resistor, and equal impedances in the other two circuit supplied with said ignals. inductively coupled to one diagonal of said bridge and a signal output circuit efi'ectiveLv inductively coupled to the other bridge diagonal, the resistance values of the copper oxide varistors and the linof alternating current signals of varying amplitudes including a Wheatstone bridge having a linear resistor in series with one arm, a network comprising two equivalent parallel branches in series with a second adjacent arm, each branch comprising a copper oxide varistor biased ,by a direct current voltage, in series with a linear resistor, the two varistors being oppositely poled in said second arm, and equal impedances in the other two adjacent arms, an input circuit supplied with said signals, inductively coupled to one diagonal of said bridge and a signal output circuit effectively inductively coupled to the other bridge diagonal, the values of the linear resistors and the copper oxide varistors in said bridge and of the biasingvoltages applied to said varistors being proportioned so that the balance oi the bridge varies with the amplitude of the input signals to provide an outpu -input characteristic which is that of an instantaneous compressor of very high compressing ratio, a large loss being produced between said input and output circuits for high amplitude signal inputs and a low loss adjacent arms, an input car resistors in said bridge being proportioned so that it is balanced to provide a high loss between said input circuit and said outputcircuit for signal inputs of certain amplitudes and is unbalanced to provide a low loss between those circuits for input signals of other amplitudes.

2. 'A system for controlling the amplitude range therebetween for very low signal inputs.

3. The system of claim 2, in which said amplitude range controlling system includes an amplifier in said input circuit, a rectifier in circuit with said amplifier for rectifying a portion of the input signals, control means having an appreciable time constant, responsive to the rectified signals for varying the gain of said amplifier in accordance with variations in the signal level and an electron discharge device having an input circuit connected across the output of said rectifier, and an output circuit including said network in said second arm of said bridge so that the amount of bias on the copper oxide varistors in said network, and thus the sensitivity of said bridge, is a function of the amplitude of the output current of said electron discharge device, which in turn depends on the voltage in the output of said rectifier, the combination providing a signal peak chopper the sensitivity of which varies in such manner as to compensate for momentary overshoots in the amplitude of output signals when said amplifier is subjected to sudden large increases in signal input, due to the time interval required for the adjustment of gain of said amplifier.

4. The system of claim 2, including a volumeoperated gain adjusting device comprising an amplifier, a variolosser in the input thereof, a rectifier for rectifying a portion of the signal output of said amplifier and control means having an appreciable time constant, responsive to the rectified signals to vary the loss value of said variolosser in accordance with the level of the signal output of said amplifier, connected in said input circuit in front of said bridge, and an electron discharge device having a control grid-oath ode circuit and an anode-cathode circuit. said control grid-cathode circuit being connected across the output of said rectifier, said anodecathode circuit including said network in said second arm of said bridge so that the biasing direct current voltage applied to the copper oxide varistors therein and thus the sensitivity of said bridge is controlled by the space current in said electron discharge device which in turn depends on the voltage across the output of rectifier, the combination providing a peak chopper whose sensitivity varies in such manner that it is greatest during the initial part of an applied signal and decreases thereafte HAROLD L. BARNEY.

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