US2200073A - Electrical transmission system - Google Patents

Description

May 7, 1940. A. v.l BEDFORD ELECTRICAL TRANSMISSION SYSTEM Filed Sept. 30, 1937 2 Sheets-Sheet 1 m l Sou n Snventor Alda yV Bedford Gttorneg May 7 1940- A. v. BEDFORD 2,200,073

ELECTRICAL `TRANSMISSION SYSTEM Filed Sept. 30. 1937 2 Sheets-Sheet 2 1.o 1o 10o 10aa 14000 FREQUENCY /A/ /r/Lo'cYcLL's Pan cs/v'r saro/vs o l l I I l F'HEQUENGYJN KLOCYCLE 'nventor Hmaledford Patented May 7, 1940 PATENT OFFHCE ELECTRICAL 'rnANsi/nssroN SYSTEM Alda V. Bedford, Collingswood, N. J., assigner to Radio Corporationof America, a corporation of Delaware Application September' 30, 1937, Serial No. 166,627

17 Ciairns.

My invention relates to apparatusfor conveying or transmitting signals covering a wide frequency band, and particularly to transducers such as transmission lines or ampliers for trans- 'mitting or amplifying picture signals in tele- Vision systems.

In a picture signal or video amplifier, because of the extremely Wide range of frequencies which must be amplified, yresistance coupled amplifier stages are employed in which theA plate resistance is low. As a result, the gain per stage is low and a rather large number of stages is required to obtain the required gain.

If the input signal is weak, as is the case when a cathode ray transmitter tube supplies the signal, the first few stages of the amplier operate at such 10W level that trouble arises from pickup in the amplifier of low frequency disturbances occurring in the plate and screen grid voltage supplies. High frequency disturbances give less trouble since they are more easily removed by filters in the voltage supply.

`Low frequency disturbances may originate from other sources. For example, an imperfect ground system will introduce such disturbances, especially if signals are being 'transmitted for a considerablev distance over a cable. In this particular case, the ground connections at the two ends of the cable are widely separated and it has been found in practice that there is a slowly varying potential difference between the .two ground connections.

Also, in the case of a long coupling conductor (such as the conductor of a cable) between amplier stages, low frequency disturbances may be picked up by this wire.

It is, accordingly, an object of my invention to provide an improved means for and method of reducing low frequency disturbances in a wide frequency band transducer.

A further object of my invention is to provide an improved wide frequency band amplifier.

More specifically, one object of my invention is to provide a wide band picture signal amplifier so designedthat trouble caused by low vfrequency disturbances is reduced.

A vfurther 'object of my invention is to provide an improved picture signal amplierfo-r 'amplifying the signal output of a cathode ray transmitter tube in a television transmitter.

l In one embodiment of my invention, I so design one of the earlier stages in an amplifier, preferably the first stage, that it vhas considerably more gain at the extreme low frequency end of the Yfrequency range than at the higher .fre-

quencies. Thus, the signals in the frequency region wherevoltage supply disturbances occur, are transmitted through the succeeding stages at a level high enough to give a good signal to noise ratio. In one of the later amplifier stages 5- the low frequency signals are attenuated as compared with the higher frequency signals to bring the amplification of all frequency components to the same level. In other words, this last-mentioned amplifier stage is given a frequency charlo acteristic which is complementary to the frequency characteristic of the rst amplifier stage.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which: lo

Figure l is 'a circuit diagram of an amplifier embodying my invention, and

Figures 2, 3 and 4 are groups of curves which are referred to in explaining my invention. Referring to Fig. 1, there is shown a portion of a television transmitter which includes a cathode ray transmitter tube i of a well known type. The tube i comprises a highly evacuated envelope h'avingtherein an electron gun which includes a cathode 2, a control grid 3, and a first anode 4. A second anode 6 accelerates the electrons and aids in electrostatically focusing them upon a photo-electric screen 1. The screen 1 consists of a mosaic of photo-electric globules la, microscopic in size, the globules being insulated from' each other and from a metallic backplate 1b.

The image of the scene to be transmitted is projected upon the .screen 'l and is scanned by the cathode by means of suitable deflecting devices not shown. n

The resulting picture signal is supplied to an output resistor R1 which is connected between the back-plate 'lb and ground when a switch 5 is closed, the connection to ground being through ,40 a biasing source, such as a battery 8. Resistor R1 is also included in the input circuit of a ysuitable amplifler tube!l T1, the battery 8 supplying a suitable biasing potential to the control grid of tube Ii. The output capacity of the tube I 45 is indicated at Hl.

The amplifier tube T1, as Well as the amplifier tubes which Will be referred to hereinafter, may be of any suitable type, such as the screen grid tube illustrated. 50

The resistor R1 may be given a comparatively high value for thepurpose of increasing the signal to noise ratio and a high peaker stage ineluded at a 'later point in the amplier vto compensate for the resulting non-linearity, as described and claimed in my application Serial No. 18,136, led April 25, 1935, U. S. Patent No. 2,151,- 072, granted March 21, 1939 and assigned to the Radio Corporation of America. My invention will be described in connection with an amplifier embodying a resistor R1 of high Value and a high peaker stage but, as will be apparent from the description which follows, this feature of the amplifier design may be omitted in some cases.

As explained in my application Serial No. 18,- 136, when the high peaker stage is employed the resistor R1 preferably should have a resistance at least three times the reactance of the capacity It in shunt thereto at the highest frequency to be amplified. Also, the high peaker or compensating stage should have an amplification which is substantially proportional to Where R1 is the resistance of resistor R1, C is the capacity of the distributed capacity Il),

j=1jl and f equals frequency in cycles per second.

In accordance with one embodiment of my present invention, I so design the first amplifier stage that it amplies the signals at the extreme low frequency end of the frequency range more than the signals at the higher frequencies, this` being done by including in series in the plate circuit of tube T1 a plate resistor R3 and a lter condenser C1 in series, the condenser C3 having such small capacity that the impedance of the plate circuit is comparatively high at low frequencies. The commonly employed peaking coil 9 may be included in series with R3 and Ca to hold up the frequency response of the amplifier at the extrem-e high frequency end of the amplifier characteristic. However, the use of the conventional peaking coil 9 is no part of the present invention and it may be omitted, if desired. A suitable operating voltage is supplied to t'he plate of tube T1 through a high impedance choke coil L1 and a filter resistor R1 of high resistance. Either the resistor R4 or the coil L1 may be omitted or short circuited, as indicated by the switches li and l2. The impedance of the circuit Ril-L1 is so high that it has negligible admittance as compared With C3, this portion of the plate circuit serving only to supply the D. C. Voltage to the plate of tube T1.

Since tube T1 is a high impedance tube, its amplification is substantially proportional to its load impedance. Therefore, since the capacity of C3 is so small that the load impedance increases greatly at the low frequencies, say frequencies of the order of 60 cycles, the amplification of such frequencies may be many times the amplification of the higher frequencies.

It is well known in the art that an excessive low frequency response due to a filter condenser such as C3 being too small can be corrected, that is, the amplifier response made linear, by making the coupling condenser between amplifier stages small enough to attenuate the low frequencies. In this way, according to past practice, each amplifier stage has been made to have a linear frequency response in both phase*l shift and amplitude. Specifically, in the circuit shown where amplifier tubes T1 and T2 are coupled by a condenser C2 and where tube T2 has a grid leak resistor R5, the rst amplifier stage would be made linear by making R5C2=R3Cs if the usual practice were followed. In this equation, and in equations which follow, the reference It may also be noted that in the conventional amplifier the gain for the low frequencies as compared with the gain for the higher frequencies is usually less than 2 to 1.

In accordance with my invention, the excessive gain at low frequencies caused by the small capacity of C3 is not equalized in the first stage but, instead, is equalized several stages later in the amplifier. For example, the signal with the accentuated low frequencies may be amplified by three amplifier stages, including tubes T2, Ta and T4 and the excessive gain equalized by giving the coupling condenser C4 located between tube T1 and the succeeding tube T5 and the grid resistor R7 of tube T5 the correct values. These values are such that' R3 C3=R7 C4- In this case switch 35 would be open.

Preferably, the three stages located between the elements CsRs and CiRff are made to have an over-all linear frequency response. Thus, in the plate circuit of tube T2, Where voltage is supplied to the plate of T2 through a filter resistor It, a

peaking coil Il' and a plate resistor I8, the filter condenser i9 accentuates the low frequencies a certain amount which is compensated for by giving the coupling condenser 2l and grid resistor 22 the proper Values.

Likewise, in the next stage, including tube T3 which is supplied with plate voltage through filter resistor 23, peaking coil 24 and plate resistor 26, the low frequency accentuation caused by filter condenser 2l is compensated for by giving coupling condenser 28 and grid resistor 29 the proper Values.

It Will be understood that the peaking coils I1 and 24, like peaking coi1 9, may be omitted if desired.

As previously stated, the accentuated low frequency response due to C3 is reduced by giving the coupling condenser C4 and the grid resistor Rv the proper values rather than by following the usual practice of utilizing elements C2 and R5 for this purpose. Therefore, the condenser Cz is given such large capacity, l 'microfaracL for example, that the attenuation of low frequencies ligble.

An analysis of the effect of the small condenser C3 shows that at low frequencies, in addition to accentuating the amplitude response, the phase is shifted such as to cause the output of amplifier T1 to lag the input. Analysis also shows that the effect of the compensating circuit CrR': is to restore both the phase and the amplitude.

In the specific amplifier illustratedthe amplifier stage which includes tube T4 is the high peaker stage, the plate circuit of tube T4 including an induct'ance coil 3i which accentuates the frequencies at the high frequency end of the amplifier to compensate for the falling off of high frequency response caused by the high resistance of resistor R1 and the shunting distributed .ca-

pacitance I0.

As explained in my above-mentioned pending application,` the plate resistor of the high peaker` which the coil 3| is connected has substantially zero impedance to ground at all frequencies.

The operation of my amplifier willbe better understood by referring to the curves shown in Figs. 2 and 3.-' v

In Fig. 2 the curve @I shows how the frequency response at the extreme low frequency end of the amplifier range is accentuated. It is apparent that the very low frequency picture signals `pass through the succeeding amplifier stages at a level high enough to override the low frequency power disturbances. At the input circuit of tube T5, the elements C4 and Rv produce an attenuation of the low frequency signals and disturbances which, as shown by the curve l2 in Fig. 3, is complementary in multiplication, to the earlier low frequency attenuation. It will be apparent that by means of this amplifier design I have increased the signal to noise ratio in the frequency region where the power supply causes disturbances, and have obtained a net uniform response.

The action of the high peaker is represented by the curves 43 and 44 in Figs. 2 and 3, respectively. As shown by curve 43, the low frequency and middle or intermediate frequency picture signals are amplified to a fairly high level because of the high resistance of R1, but the gain at the higher frequencies falls off. The gain of the high peaker stage, as determined by coil 3l, is made complementary to the gain shown by curve 43, the gain of this stage being shown by curve 44.

On the drawings, the values of certain resistors and condensers have been indicated, by way of example, in ohms and microfarads. With the values indicated, the gain of a 60 cycle signal in the first amplifier stage is about` twelve times the gain of the higher frequency signals, that is, of signals from 1000 cycles up. It will be understood that the falling off in gain of the high frequencies as indicated by curve 43 does not occur in the first stage proper but, rather, in the output circuit of the cathode ray tube l. From theforegoing description, it will be apparent that in the example illustrated, any disturbances occurring at 60 cycles are attenuated to ,-l the value they would have as compared with the picture signals in a conventional amplifier. Lower frequency disturbances will be attenuated still more.

It may be noted that power line disturbances introduced into the amplifier through the tube T1 are not reduced by utilizing the small condenser Cs. Because of this, it may be desirable in some cases to supply the screen grid and plate electrodes of tube T1 with voltage from a separate voltage supply such as a battery while supplying the D. C. voltages to the succeeding tubes from the usual rectified and filtered 60 cycle supply.

The attenuation of the low frequencies accentuated by condenser C3 may be accomplished by other means than by the use of a small coupling condenser such as C4. For example, by closing a switch 35 the condenser C4 is shunted by a condenser 30 which has large enough capacity to prevent any substantial attenuation of the low frequencies.

The desired attenuation of low frequencies is then obtained by properly designing the plate circuit of tube T5. This plate circuit includes the usual filter resistor 45, filter condenser 4S, peakingcoil 4'! and plate resistor Rs. As described thus far, the plate circuit of tube T5 -is the same as the plate circuits of tubes T2 and T3;

Up to this point my invention has been def scribed on the assumption that the desired accentuation of low frequencies is accomplished by the small condenser C3. Instead of accentuating low frequencies by means of condenser C3, they may be accentuated when the switch 5 is open by means of a condenser C1 which is connected' in series with R1. A resistor R2 is connected across condenser C1 whereby a suitable bias is applied to tube T1. The resistance of R2 is much greater than the resistance of R1. For example, R2 may have a resistance of 1 megohm. It will be understood that the output circuit of tube I may be coupled to the input circuit of tube T1 through a coupling condenser, bias being applied to the grid of tube T1 through the usual grid leak resistor.

At first glance it might appear that where my high peaker invention is used the low frequencies can be accentuated as much as is desired by increasing the value of R1. 'Howeven Athe amplifiers may be working at such level that an increase in the resistance of R1 will overload them, whereas the use of condenser C1 of the proper value will accentuate only the very low frequencies and will not overload the amplifiers. The following description of a variation of the design where C2 accentuates the'low frequencies will make this clear. In certain television amplilfiers in which the lowest useful frequency to be transmitted is 60 cycles per second (this being the framing frequency), there are troublesome power supply variations which may have an average frequency of about 6 cycles per second. Also, these amplifiers may be working at such level that any very large accentuation of the G0 cycle per second response would overload them. It has been found impracticable to remove the harmful d cycle signal from the amplifier channel by the introduction of filters which cut-off the extreme low frequency response because of the harmful phase shift which such a filter introduces at the useful 60 cycle frequency.

It should be noted that the amplifier design first described, the condenser Cahad such small capacity that it caused a large accentuation of frequencies in the region of 60 cycles per second. In the case of the television amplifiers just mentioned, ther harmful signals in the region of 6 cycles per second may be greatly reduced as follows:

The condenser C3 is given a value such that the accentuation of a signal at 60 cycles per second is small, say 1.41 to l, whereby the amplifiers arel not overloaded. In this specific case the reactance of C3 at 6G cycles per second equals the resistance of Rz, Cs being 1.21 mfd. where Rs=2200 ohms. Then the accentuation of a signal at 6 cycles per second is about l0 to l. With the switches 35 and 40 open, the complementary frequency response characteristic is obtained at amplifier T5 by proportioning C4 and Rv in the manner previously described. Or switches 35 and 4t may be closed and Ra and La given the proper values determined by the equation previously given.

Referring now to the luse of condenser C1 to accentuate lowA frequencies, it will be assumed that ,C3 is given. the usual. capacity of a filter condenser whereby it causes very little accentuation of low frequencies. C1, however, is given such capacity as to accentuate signals of 6 cycles per second in the ratio of about 1G to l. Signals at 60 cycles will not be accentuated enough in this case to overload the amplier. Ina late stage in the amplifier the accentuated 6 cycle signals are attenuated 10 times as much as the high and middle frequency signals, this late stage and the output circuit of cathode ray tube I being complementary. For example, with switches 35 and 40 open, condenser C4 provides the desired attenuation` The several values of the complementary networks are determined by the equation C1 R1=C4XR7.

Or the desired attenuation may be provided by closing switches 35 and 45] and giving the several elements the values determined by the equation LszRiXRaXCiX 106.

The curves in Fig. l show the frequency characteristics of an amplifier designed as just described. Curve 5I shows the frequency response characteristic of the output circuit of the cathode ray tube I. It will be apparent that the high frequency response falls olf whereby the high peaker stage is required and that the very low frequencies are greatly accentuated whereby low frequency attenuation in a late stage is required. The curve 52 is the over-all characteristic of the high peaker stage T4 and the late stage T5. The low frequency end of the characteristic curve 52 represents the characteristic of either network Ci--Rv or network RaLs depending upon which one is being utilized for low frequency attenuation.

In some amplifiers it may be desirable to accentuate low frequency signals by 'both C3 and C1. In that case the network C4-Rv may be used to correct for the boosting of lows by C3 while the network Ra-Ls is used to correct for the boosting of lows by C1.

In the foregoing description of Fig. 1, it has been assumed that all the amplifier stages are located near each other. In some instances the amplifier stages may be separated into two groups spaced apart several hundred feet or even several miles. For example, as indicated by the dotted conductor I5, amplifier stage T2 may be a considerable distance from stage T3. The conductor I5 may be a transmission line such as a cable several miles long. In that case it will be understood that the resistor I8 will be reduced in value to make it match the characteristic impedance of the cable, reduced to the order of 500 ohms, for example.

Disturbances due to inductive pick-up by conductor I5, or due to difference in ground potential at the two ends of the conductor are therefore reduced by the invention.

I claim as my invention:

l. In a signal transducing system, a signal generating means for producing a useful signal covering a wide band of frequencies, said signal consisting of low frequency components, intermediate frequency components and high frequency components, transducing means having an input circuit and an output circuit, said input circuit being coupled to said generating means, a source of extraneous undesired signal which combines with said useful signal to produce a mixed signal in said transducing means, a first frequency selective means for accentuating said 10W frequency components impressed upon said input circuit, and a second frequency selective means in said output circuit for selectively attenuating the low frequency components of said mixed signal in a manner substantially complementary to the accentuation by said first frequency selective means, said rst frequency selective means having a frequency response curve which is concave upward where frequency rel line, said transducer having an input circuit and an output circuit, said input circuit being coupled to said source, a first frequency selective means for accentuating the low frequency components of said signals impressed upon said input circuit, and a second frequency selective means at said output circuit for selectively attenuating said low frequency components in a manner substantially complementary to the accentuation by said rst frequency selective means whereby low frequency disturbances introduced by the presence of said transmission line are reduced in amplitude, said first frequency selective means having a frequency response curve which is concave upward where frequency response is plotted against the logarithm of frequency.

3. An amplifier for amplifying signals covering a wide frequency band, said signals comprising low frequency components, intermediate frequency components and high frequency components, said amplifier comprising a plurality of amplifier stages connected in cascade by resistance coupling,y one of the first of said stages comprising an amplifier tube having a plate circuit so designed that its impedance is several times as high for said low frequency components as for said intermediate frequency components whereby its frequency response curve at said low frequencies is concave upward where frequency response is plotted against the logarithm of frequency,

one of the later stages of said amplifier being so designed that its frequency response characteristic is substantially complementary to the frequency response characteristic of said first-mentioned stage, and the over-al1 frequency response characteristic of the amplifier stages coupling said first-mentioned stage and said secondmentioned stage being substantially fiat for said low frequency and intermediate frequency components. 4. In combination, a plurality of resistanc coupled amplifier stages, one of the rst of said stages comprising an amplifier tube having a plate circuit so designed that its impedance increases rapidly at 10W frequencies whereby sig- `nals at said low frequencies are amplified more istics of said first-mentioned stage and of saidY second-mentioned stage being substantially complementary and the amplier stages coupling said first-mentioned stage and said second-mentioned stage having a frequency response such that they amplify said low frequency signals at least as much as said higher frequency signals whereby said low frequency signals are at a higherlevel T IT than Athe said higher frequency 'signals' when they reach said second-mentioned stage.

5. The invention according to claim 4 characterized in that the plate circuit of said first mentioned stage includes a plate resistor and a filter resistor in series, a filter condenser connected in shunt relation to said filter resistor, the capacity of said lter condenser being so small that the impedanceof said platev circuit at 60 cycles per second is at leastfive times its impedance at`frequencies of 5000 cycles per second and higher.

6. The invention according to claim, 4 charac- '7.An amplifier comprising a plurality of r r`ev sistance coupled amplifier stages which, with the exception of two stages, have a substantially flat frequency response characteristic from below 60 cycles per second to approximately 10,000 cycles per second or alcove, one of said `two stages being one of the first stages in said amplifier and hav ing a frequency response characteristic represented by a curve which is concave upward where frequency response is plotted against the logarithm of frequency and which is such that signals having a frequency of 100 cycles per second or lower are amplified more than signals at the higher frequencies, and the other of said two stages being located near the output endof said amplifier and having a frequency response characteristic which is substantially complementary to said first-mentioned frequency response characteristic whereby low frequency signals originating between said two stages are attenuated.

8. An amplifier for amplifying signals covering a wide` frequency band, said amplifier comprising a plurality of amplifier stages connected in cascade by resistance coupling, one of the first of said stages comprising an amplifier tube having a plate circuit which includes a plate resistor and a filter resistor in series, a lter condenser connected in shunt relation to said filter resistor, the capacity of said filter condenser being so small that the impedance of said plate circuit increases rapidly at frequencies of the order of 60 cycles per second and lower whereby the frequency response curve of said one stage is represented by a curve which is concave upward at said frequencies where frequency response is plotted against the logarithm of frequency, one of the later stages of said amplifier being so designed that its frequency response characteristic is substantially complementary to the frequency ree sponse characteristic of said first-mentioned stage, and the over-all frequency respnse characteristic of the amplifier stages coupling said first-mentioned stage and said second-mentioned stage being substantially flat.

9. A picture signal amplifier comprising a plurality of cascade coupled amplifier stages designed to pass signals having a frequency from below 60 cycles per second toabove 100,000 cycles per second. the first of said stages being designed to accentuate signals having a frequency of 60 cycles per second and below, and the last of said stages being designed to have a frequency response characterist-ic substantially complementary to that of said first stage, there being at least one stage intermediate said-.first and last stages which is supplied with voltage from a power line whereby undesired low frequency disturbances are introduced and later attenuated in said last stage.

10. A television amplifier for transmitting signals representing pictures occurring at a certain frame frequency, said amplifier comprising a plurality of amplifier stages connected in cascade, an early one of said stages being so designed that as compared with signals at 5000 cycles per sec-E ond and higher it accentuates signals at said frame frequency not substantially more than 2 to l but accentuates signals at 6 cycles per second at least 4 to l, Aa late one of said stages, having a frequency response characteristic which is substantia-lly complementary to said early stage, and the remaining stages of said amplifier having a. substantially fiat over-all frequency response characteristic for the frequency range between 6 cycles per second and 5000 cycles per second.

ll. The method of amplifying picture signals originating in a photoelectric cell device which consists of low frequency components, intermediate frequency components and high frequency components, said signals being produced in groups each representative of a complete picture, said groups occurring at a certain frame frequency, said method comprising accentuating in the output circuit of said device certain of said low frequency components below said frame frequency with respect to said intermediate frequency components, amplifying said low frequency components and said intermediate frequency components after said low frequency components have been accentuated, and next attenuating said low frequency components with respect to said intermediate frequency components, said attenuation being substantially complementary to said accentuation.

l2. The method of amplifying picture signals originating in a photoelectric cell device and which are produced in groups each representative of a complete picture, said groups occurring at a certain frame frequency, said method comprising .accentuating the low frequency signal components below said frame frequency with respect to the intermediate frequency components in the output circuit of said device, holding up the intermediate frequency response in said output circuit with respect to the high frequency components of said signals and permitting the high frequency response to fall off, amplifying said picture signals and next attenuating said low frequency components with respect to said intermediate frequency components and accentuating said high frequency response, said attenuation being complementary to said low frequency accentuation and said high frequency accentuation being complementary to said falling high frequency response.

13. In combination, a signal generating device having high internal impedance and having output terminals, an electric discharge tube having an input circuit and an output circuit, an output resistor and a condenser connected in series and connected between said output terminals, said input circuit being connected across said series combination of output resistor and con-4 in series in the order named and connected bef tween said output terminals, said input circuit being connected across said output resistor and said condenser whereby accentuated low freyquency signals appear in said output circuit, and

a resistor connected across said condenser whereby bias is supplied to said electric discharge tube, said resistor having high resistance compared with the resistance of said output resistor.

15. In combination, a signal generating device having high internal impedance and having output terminals, an electric discharge tube having an input circuit and an output circuit, an output resistor, a condenser, and a biasing source connected in series in the order named and connected between said output terminals, said input circuit being connected across said output resistor and said condenser whereby accentuated low frequency signals appear in said output circuit, a resistor connected across said condenser, said resistor having high resistance compared with the resistance of said output resistor, means for amplifying the signals appearing in said output circuit, and means for next attenuating said low frequency signals in a manner complementary to their accentuation.

16. In combination, a signal output device for supplying picture signals occurring in groups at a certain frame frequency, said device having high internal impedance and having output terminals, an amplifier tube having input electrodes, a loading network which couples said output terminals to said input electrodes, said network comprising a resistor and a condenser in series, said series combination of said resistor and said condenser being connected between said output terminals and having a certain capacity C in shunt thereto, said network having an effective resistance R which is at least three times the capacity reactance of said network at the highest frequency to be amplified, the capacity of said series condenser beingsuch that frequency components of said signals below said frame frequency .but not signals. at said frame frequency are greatly accentuated with respect to the intermediate frequency components of said signals,

means for amplifying the signals supplied by said amplifier tube, a compensating amplifier stage following said amplifying means and having an amplification which is substantially propor-` tional to Where and f equals frequency in cycles per second, and

being substantially complementary to said accentuation.

l7. A -signal transducing system comprising '.ilO

a signal supply circuit for supplying signals coV ering a wide band of frequencies, said signal consisting of low frequency components, interme-.\ f

diate frequency components and high frequency components, said signal supply circuit including a rst amplifier stage, said system further comprising at least one intermediate frequency amplifier Astage and at least one late amplifier stage,. v.

said signal supply circuit having a frequency 1response characteristic such that its frequency response curve is concave upward at Said low frequencies and concave downward at said high frequencies where frequency response is plotted,

against lthe logarithm of frequency, the frequency response curve of said late stage being concave downward at said low frequencies and being concave upward at said high frequencies and being substantially complementary to said rst fre-7 quency response curve, each of said frequency response curves being substantially flat at said intermediate frequencies, and the over-all frequency response of said system being substantially fiat over the complete range of said wide band of frequencies.

ALDA V. BEDFORD.A

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