US2315249A - Pseudo-extension of frequency bands - Google Patents

Description

vvvvvvvvv March 39, 194-3. L. A. DE ROSA 2,315,249

PSEUDQ-EXTENSION OF FREQU-FNUY BANDS Filed Oct. 8, 1941 5 Sheets-Sheet l M MWMMM v A p/ Nude [men for pf Fig.7

' March 30, 1943 DE ROSA 2,315,249

P SEUDO-EXTENSION OF FREQUENCY BANDS Filed Oct. 8, 1941 5 Sheets-Sheet 2 March 30,1943; L, A, DE FmA 2,315,249

PSEUDO-EXTENSION OF FREQUENCY BANDS Filed 001;.- 8, 1941 5 Sheets-SheeiS March 3'0, 1943. DE ROSA 2,315,249

PSEUDO-EXTENSION OF FREQUENCY BANDS Filed Oct. 8, 1941 5 Sheets-Sheet 4 K INVENTOR March 30, 1943. 1 5 RQSA 2,315,249

PSEUDO-EXTENSION OF FREQUENCY BANDS Filed Oct. 8, 1941 5 Sheets-Sheet 5 INVENTOR Patented Mar. 30, 19 43 2,315,249 7 n PSEUDO-EXTENSION OFVFREQUENCY BANDS Louis A. De Rosa, Dayton; out, Application October 8 1941, Serial No. 414, 137

13 Claims. (01. 179-4) v This invention relates to communication systems and more precisely to an improved system of pseudo-extension. Pseudo-extension" inv cludes, for the purposes of this invention, the methods and means wherein an audio signal, at 6 some point or at some time in its transmission either directly or indirectly to the ear, is modified so that, while all the composite frequencies present in the original. audio signal are not present in the signal ultimately transmitted to the .10

ear, the auditory perception is of a sound which has substantially all the sonant characteristics of the original signal,

A pseudo-extension circuit is defined as an electrical circuit'for the transmission of the lntelligence contained in a wide band of audio frequencies through a narrow band transmission channel; wherein the signal transmitted through the narrow band transmission channel is distorted by the addition to it of extraneous modulation and inter-modulation frequencies lying within the frequency range of said narrow band transmission channel.

One object of this invention is to provide a method and indicate means whereby signals which are modified and transmitted by a pseudo-extension" circuit are improved in quality and rendered more pleasing to the listener.

Another object of this invention is to lessen unwanted and extraneous percussive and rasping sounds introduced in a signal by the "pseudoextension" process of the present art.

Another object of this invention is to reduce the apparent auditory distortion resulting when 5 a transmitted'signal of transient nature is modified by theaction of a non-linear transducer.

Another object of this invention is to reduce the apparent auditory distortionresulting from the inter-modulation of at least two signals at least one of which is transient in nature.

Further objects will be apparent from the description to be given.

Figs. 1 to.'7 show a single frequency signal of a transient nature and the approximate frequency spectrums existent at various stages of the signal.

Fig. 8 shows a diagrammatic circuit of one embodiment of the invention as incorporated in a pseudo-extension non-linear circuit. v

Fig. 9 shows, diagrammatically,- an alternate circuit which may be employed to utilize the advantages of the invention.

Fig. 10 shows, schematically, a simplified ciracter.

cuit which may also be used to-produce distor tionless pseudo-extension." a t Fig. 11 illustrates a circuit including a frequency selective feed-back circuit-used to accomplish the object of this invention.

Fig. 12 illustrates an embodiment of this in-' vention 'to retain or even accentuate the percussive quality of'a signal. 0

Fig; 13 shows, in'block diagram form, the use] of the circuits of Figs. 8 to '12;.with associated a apparatus in an improved pseudo-extension" system.

Throughout the drawings,- parts performing;

like functions have been given the same reference characters and numbers. 1 1

In order to clarifyand facilitate the understanding of this invention, it is believed'that an analysis ofan elementary form of a signal wave of transient character would be serviceable. Accordingly, Fig. 1 shows such a wave. 3

In Fig. 1 is shown an arbitrary single frequencysignal wave. having a transient char- A single frequency is shown for con I venience since by super-position of a plurality of frequencies',"signals of the typ generally encountered infthe transmission; of audible sounds can'be obtained. r

This signal may be regarded as a single frequency only insofar as frequency is measured by the reciprocal of the time between adjacent peaks of the same, polarity. Actually every change in amplitude hasassociated with it an infinite number of side bandaprovided that the intervals between amplitude changes are infinitely long. While this is the idealizedcase, nevertheless the side-bands encountered in practical cases may extend to the tenth harmonic of the fundamental frequency particularly in sharplyintroduced low frequency waves.

If the wave had aninflnite interval of steadystate condition between amplitude variations then the side-bands would contain all frequencies or would be a continuous spectrum; In other words, as the steady state interval approached infinity, the prime factor ofthe sideband frequencies would approach zero as a limit. An analysis of a practical case may be made by assuming. that the single frequency is amplitude modulated by a frequency modulated signal, wherethe frequency modulation is the variations in the intervals between successivechanges in amplitude. Therefore, in the practicalgcase the prime factor of the side-band'frequencies-is the reciprocal of the time betweenthe longest in- 2 Y l I 2,315,249

tervals during which there is no amplitude change.

As the rate of increase or decrease of the amplitude of a wave such as that shown in Fig. 1 becomes slower and slower; the mean energy of the associated transients is shifted towards the lower frequencies. Simultaneously, the amplitude of the. side-bands becomes less and less. until for steady-state conditions, the side-bands become zero.

Returningto Fig. 1, the wave in starting from a zero reference time in its build-up would, at some time t1 have a frequency spectrum associated with it similar to that shown in Fig. 2.

The side-bands are. grouped about the steady state frequency Qf'the wave and their amplitudes are an inverse function of their displacement from frequency f.

At t: of Fig. 1, when the amplitude of the wave has almost reached a steady state value, the

component frequencies have a spectrum similar to Fig, 3. It is to be noted that the side-bands have diminished in amplitude so that only those side-band frequencies in the immediate neighborhood of the steady-state frequency I have important amplitudes. r

At time t: the illustrative wave is again changed in amplitude. The frequency spectrum consequently, due to this transient, becomes more complex and theifrequencies more distantly removed from-the steady state frequency 1- again become prominent. The spectrum assumes therefore, a complexity such as is shown in Fig. 4.

At time t4 when the rate of increase of the envelope of the amplitudes has almost reached a steady-state condition, the magnitude of the side-bands has again'diminished and, as before, only the frequencies in the neighborhood of the steady-state frequency f have important amplitudes. The frequency spectrum at this time is shown in Fig. 5.

At time is after a relatively long interval the signal wave decreases suddenly in amplitude and immediately the side-band frequencies again appear. The spectrum of the side-band frequencies for this condition is shown in Fig. 6.

At time is the amplitude of the'arbitrary wave of Fig. 1 has diminished considerablyand is approaching zero. The side-bands are therefore localizing in the immediate vicinity of thesteadystate frequency 'f and as they localize, the amplitude of the frequency 1 drops off so that, when the wave of Fig.1 has reached a practical zero emanating from a cello or bass viol having a split or cracked body.

When the high frequencies are pseudo-extended this deleterious effect is particularly noticeable in speech when the fricative consonants are reproduced.

This invention describes definite methods and means for reducing the distortion encountered in prior art pseudo-extension processes. The circuits describedhereinafter may also, with resulting improvement, be used in place of the nonlinear circuit elements of the pseudo-extension system described in the applicant's co-pending U. S. application No. 348,359, filed July 30, 1940.

In order to explain the operation of this invention, an analysis of what happens when the arbitrary functionof Fig. 1 is applied on a nonlinear transducer, is advisable. Returning then,

to a study of Fig. 1, let it be assumed that the wave represented thereby is applied to a modu-' lator or non-linear element such as a square law detectonand, further that the transmission band of this latter device is also uniform between the frequencies 11 and In.

The resulting frequency spectrum at time h which appears in'the output of the non-linear device is similar to that shown in Fig. 2 except that, due to the resulting inter-modulation products, the magnitudes of the side-bands are greatly increased; in other words, the spectrum is similar to that obtained if the original wave had been building up more rapidly at that time. The effect on the listener, hence, is that of a signal building up more rapidly and sharply than the original signal of Fig. 1 warranted.

Proceeding to a later time t: it is noted that at :this time the signal has practically reached a steady state value, hence the frequency components at this time, as shown by Fig. 3 are grouped closely about the steady-state frequency f. The output ofthe non-linear device however, due to the inter-modulation frequencies introduced by its action,will produce a spectrum wherein the side-bands are greater in amplitude than those present in Fig. 3. That is, the spectrum resulting from the action of the non-linear device will at time t2 actually be more nearly that shown by Fig. 2. Consequently, the ear, which if listening to the original wave would hear pracmagnitude, the side-bands have become merged into a position occupied by I which has in the meantime diminished and reached a zero magnitude.

Fig. 7 shows the spectrum analysis at time t1 when the wave amplitude is approaching a practical zero value. 4 l

In general, prior art pseudo-extension" circuits include as an essential element, a non-linear transducer. This non-linear transducer may be a square law detector, a class "0 amplifier tube. a combination of both, or any circuit having a non-linear input vs. outputcharacteristics.

These prior art circuits, while producing a measure of fpseudo-extension have associated with transients occurring in the input signal wave, an augmented percussive quality and a 'raspy harsh effect. f

When the bas's'frequencies are "pseudo-extended the effect noticeable on transients may tically no transient effect, due to the action of the modulator, would detect a sizeable transient effect.

Thus it is evident that the introduction of a.

non-linear device will produce an exaggerated transient quality which persistsafter the transient quality of the original signal has been removed.

The methods and means for producing a pseudo-extended signal free from this abnormal duration of transient quality, which to the ear results in the presence of extraneous harshness and raspy effects, is the essence of this invention,

linear device will contain higher amplitude sidebands than those shown by Fig.4, so that the resulting spectrum will be more nearly similar to that ofFig. 2.

At time t4; the spectrum of the modulator output will be more nearly similar to that shown in .and is leveling out in amplitude this invention.

I off so that it operates as cuit load impedance R4 andC4.

1 C4 has charged. There is measurable from the time plied thru the battery E01,

Flg. 4; the amplitudes of the side-bands again having been increased by the non-linear char- I acteristi'c.

At tlmett the output er the non-linear device a will have a frequency spectrum in which the sidebands are of much greater amplitude than indicated by Fig. 6.

At time is the transient V V gerated by the non-linear device, so that the transient quality is still detectable even when the signal has approached a' steady-state value as it approaches a zero value.

Fig. 8 shows, diagrammatically, one 'circuit which may be'used to accomplish the object of The input signal is impressed across the terminal marked-Elna This signal, if the pseudoextension isto be applied mainly to the bass frequencies; is preferably applied to the thru a low pass filter F. A low' pass filter of one section prototype constant K type having a cutoff frequency of aboutlOO cycles has been found satisfactory. The output of the filter is divided into two paths, the one to the left and consisting of C1; R1; R2; T1; R and C5 being the non-- linear circuit; andthe, lower path, consisting of CajRa; R1; C4; R5; andTz is the control circuit. The purpose of this control circuit is to. change the bias of the tube T1 so that T1 operates as a non-linear device only when the signal wave applied to its grid has .attained a relatively steady-state condition. A portion of the signalfrom the output of the filter F'is impressed, by means of the capacitor C3 and the variable potentiometer R3, between gridof tube T2. Tube the battery E02 to cuta class C amplifier.

the cathode and control T2 isbiased by means of indicatedon the dia- Thistime lag is a function ofithe product of R and C, for increasing signals, and a function of the product R4 and C4 for decreasing input signals.

The point where, on the increasing amplitude envelope of theinput signal the tube T2 begins to conduct can be adjusted by the setting of the potentiometer R3. The rate at which the rectified signal across Ri decays when the input sigcircuit is supplied by the bat-' eifect is again exaginput capacity of tube T1 may be made small in comparison to the time constantso; R5 C4 and R4 C4 so that these latter consta'ntswill'detern'iinev the build-up and decay respectively of the voltage applied tothe gridci tube T1 from the plate circuit of tube T2. For a transient associated with an increase in amplitude, the opera tion of the circuit of Fig. 8 is such thatthe tube T1 operates as a linear device until the input signal hasreached a relatively steady-state conditionwhere after the tube ,Troperatesas a nonlinear device and continues to do so until a dee cay transient occurs inthe input signal at which time the tube T1 again operates as a linear ampliher.

The output Eout of T1. taken across its plate load Re through the blocking condenser C5. is

therefore'fairly free from distortion during the transient intervals of the inputsignal, and the extraneous; unpleasant and foreign auditory effects are eliminated from the pseudo-extended" signal. This output may then be used in place of the uncontrolled output of the non-linear device associated withvarious pseudo-extension circuits. I i

The time constants of the delay circuits must be chosen with a view to the frequencies which are being impressed on the tube T1. Experimentally'it has been found-that satisfactory results are obtainable when the delay between the onset of the signal and the operation of T1 as a nonlinear device is, as a function of the. mean frequency applied to the grid-of ,tube T1, approximately 20 milliseconds for 200 cycles per'second 40 milliseconds for 100 cycles per second;

80. milliseconds for cycles'pe'r second;

a 10 milliseconds for frequencies between 500 and 4000 cycles; 1 and about 40 milliseconds for frequencies higher than 5000 cycles per second. For decay transients, the tube T1 should preferably be returned to its linear operation as soon" 'as the decay transients set in; an approximate time being 8 to l0 milliseconds.

1 Another embodiment of this invention is shown in Fig. 9. This circuit accomplishes the object of this invention in one possible alternate manner. The signalto be pseudo-extended is ap- \plied to the input of thefilter F. This filter may.

if the low audio frequencies are to be'operated upon, have the same characteristics as that described vin connection with the circuit of Fig. 8.

If the high frequencies are to be pseudo-extended then a singlestage high pass constant K pro totype having a cut-off fre'quency of approximately 2500 cycles has been found satisfactory. The output of the filter f is applied to an electronic compressor circuit of the type well known to the art, consisting of tubes T3, T1 and the double diode T5. The operation of this compressor circuit is as follows. The signal from the output nal decreases is adjustable byvarying either the value of C4 or the value of R4. I

The rectified signal appearing across R4 is apof .the tube T1.

absence. of

a voltage across R4. is essentially a non-distorting linear amplifier. A positivevoltage appearing across R4 will destroy the bias of tube Trand cause it to operate as a grid leak detector or square law non-linear amplifier. The delay time of the networks R1 01 and R2 C2 where C2 is the R1 and R2 tothe grid. 4 The battery Em is selected so that the'operation of tube T1 in the of the filter F isapplied by means of the'condenser Cs and the grid resistor R1 to one control grid of a two control grid pentode T3. The amplified signal appearing in the plate circuit of Ta is impressed by means of the condenser C7 to the level of the signal at the grid of T4 being adjustable by means of the potentiometer R3. 'Theamplified signal appearingin theplate circuit of T is applied through the audio transformer Tm to a double diode rectifier T5. A rectifiedsignal appears across the-diode load-resistance R1, the rate at which it builds up being determined by the product of'the series re- 4 sistor Ru and the capacitor C4. This rectified voltage is s'ppliedto the second control grid of Ta and being a negative voltage, decreases the transconductance of ".thetube Ts resulting in a decrease in series, between one terminal of the secondary f the transformer and the control grid of tubeTi.

when the signal applied to the grid of tube T1 is great'erthan its bias potential, grid current will: flow 1 through the resistance R: causing the transconductance of the tube to decrease. The tube will consequently act as a non-linear device. Bvnsing a tube fOrT'a which has a remote cutof! characteristic for the grid to which the diode rectifled'yoltage is applied, the compressor circuit can bemade'tofsuppress transients above a certain amplitude of input signal.

The bias of :tube T1-is adjusted so that tube T1 acts as a non-linear device when the amplitude of the input signal to the compressor circuit has reached an amplitude beyond which further-increases are minimized.

The selection of the time constants associated with'the diode load should be chosen so that the ripple voltage appearing across the diode load and applied toits associated grid of T: is approximately tenper' cent of the signal voltage applied to the other grid of Ta. This condition holds for the charging time of C4 through Rs andthe associated closed circuit and for discharg'etime of 04 as determined by the value of R4, and-the equivalent shunt impedance across C4 due to. the associated shunt meshes.

The output voltage limit is taken from the plate circuit load resistor R0 through the blocking condenser Cs.

The circuits of Fig. 8 and Fig. 9 show twoembodiments of the invention, and in addition they illustrate two difierent solutions to the problem of the elimination of transient distortion in a nonlinear device. f

The first circuit, that of Fig. 8 varies the linearity of the non-linear device so thatit operates essentially as a class A amplifier during the time that a transient is impressed on its 1nput.

The second circuit arrangement, that .of 1"ig. 9, is illustrative of that class of circuits within the scope of this invention wherein the transients are minimized at those times when the non-linear device operates as such.

In the circuit of Fig. 10 is shown another possible alternate arrangement to produce the object of this invention.

If a signal is suddenly applied to a transducer having a narrow acceptance band, such as a tuned resonant circuit, the build-up of the,sig-.

nal in the output of the transducer is very slow. In other words, a sharply increasing or decreasing amplitude will not be effective as such in the output of a transmission network having a narrow frequency acceptance band.

vThis property of a tuned circuit may be used to prevent sharponset and decay transients with their associated side-bands from producing, in the output of a non-linear device, the. characteristic transient distortion oi the prior art pseudo-extendedsignals. a

In Fig.. 10 the signal to'be impressed on the This tube is arranged with a i non-linear device of a "pseudo-extension circuit is designated at E111. This signal is divided'into two channels, the first of these is applied through the volume control. R1 to one control grid of a mixertube-such as the present day 6L7, desig-' The other channel is vapplied to a fitted as T13, tuned circuit Tc tuned to the region of the frequencies which are to be pseudo-extended. When the high frequency spectrum of a signal is to be transmitted over a transmission circuit having a low :pass characteristic extending only to say, 4000 cycles, satisfactory results have been obtained with a resonance circuit for To tuned to about 6000 cycles. of a signal is to be transmitted over a circuit having a high pass characteristic extending to, for example 120 cycles, the tuned circuit Tc should betuned to the neighborhoodof cycles.

The narrow band output of the tuned circuit To is amplified by ,a linear amplifier designated operate, by virtue ofthe initial bias, as a classA amplifier for small signals applied'to its grid. The volume control Rule adjusted so that the signals appearing at the grid of T1 via the first channel, that is. impressed on the first control grid. 01 of Tn, are of sufliciently low magnitude so as to be amplified by T1 without distortion.

The amplifier "A" contained in the second channel. namely that channel impressed on the second control grid G: of tube T13, causes an amplified second channel signal to be impressed, via T1: on the control grid of tube. T1. This signal, by circuit design. is made approximately ten times the magnitude 'of the, first channel signal impressed on the grid of T1. The tube T1 therefore. acts as a non-linear device in the presence of a signal from the second channel.

The signal appearing across the plate load resistor R1: of tube T1 is utilized, through condenser Cs, as the output of the non-linear circuit in prior art pseudoextension" circuits.

The circuit of Fig. 10 described above and the circuitof Fig. 11 to be described operate to prefrom the harsh, rasping, spuriously fricative soundsassociated with prior art pseudo-ex tended signals.

Fig. 11 illustrates a circuit which may be used to realize the utility of this invention. Basically, its operationis similar to that of Fig. 10 but it is shown in order to indicate one of a large number of modifications which are within the scope of If the low frequency spectrum this invention, and whichutilize the method and means described herein. I

Referring to Fig. 11, Ein'is an audio signal containing the frequencies which are to be pseudoextended. This signal Em is applied to one grid G1 of a two control grid pentode T14 of the present day 6 L7 type. R1 is the grid resistance to maintain the bias on grid G1. The tube is biased so as to operate as a linear amplifier by the conventional self-biasing by-passed cathode resistor arrangement as indicated in'the figure.

An amplified signalappears across the plate load resistor R20 of tube T14. Those frequencies appearing in this amplified signal which are to be pseudo-extended are fed-back through the blocking condenser C11 and the network R23, C13, R22, C12 to the second grid G2 of the tube T14. The network 1223,013, R22, 012 is selected so as to produce an approximate phase shift-of 180,degrees for these frequencies.

The potentiometer is used to control the voltage of the fed-back signal so as to prevent oscillation. The phase .of this fedback signal being such that regeneration will occur for frequencies to be pseudo-extended, the tube T14 will therefore selectively amplify these frequencies to a much greater extent thanother frequencies present in the signal Em.' Effectively therefore the tube T1; acts as a tuned amplifier having'a maximum gain in the neighborhood of the frequencies to be pseudo-extended. The principle of operation of the circuit of Fig. and that of Fig. 11, is similar, that is, in Fig. 11, due to the relatively sharp frequency characteristic of the tube T14 and its associated feed-back circuit, the side-bands present during changes in amplitude of the preferentially amplified signal are limited in amplitude. Limiting the number and amplitude of these side-bands produces; after the thug restricted signal is acted upon by the modulator or non-linear amplifier tube T1, a signal wherein the intermodulation products due to these side-band frequencies are. reduced in number and in amplitude. Therefore, the resulting signal produces an apparently undistorted effect upon the listener.

The signal appearing across the plate load resistor R is impressed, through the blocking condenser C11 to the potentiometer R1. The setting of the potentiometer is adjusted so that. in absence of a regenerated signal across its terminals, the ordinarily amplified voltage is insuflicient to drive the tube T1 oil." the linear portion of its grid voltage-plate current characteristic as determined by the grid bias produced by the oath- Fig. 12 shows, diagrammatically an arrangement of a circuit utilizing the novelty of this invention which may be used to retain the or even augment the transient quality of asignal and yet avoid the "harsh and fricative sounds due tothe prolongment of thetransient stateof a signal due to the action of anon-linear trans-' ducer. In Fig. 12, the signal Em to be pseudoextended is impressed on the input of thefilter section This filter section may be a sin-' 1 vgle stage constant K prototype or its'equivalent.

If the circuit is to be used to pseudo-extend the high frequency spectrum of. thegsignal Em,

- the filter F is arranged as a. high pass, a representative cut-off frequency being 3000" cycles. If the circuit is to operate for. base frequencies, the filterF may be a low pass having -a cut-off frequency of about 120 cycles.

The output of the filter Fin either case is impressed on the grid of a .linear amplifier tube Ts, R1 being the conventional grid resistor as-.

, sociated with the grid circuitof T6. The amplifled signal appearing in theplate circuit of tube To is divided into three channels consisting of two control circuits and one main circuit.

The main circuit, by means of the transformer TR; having twosecondary windings, is further divided into two secondary circuits. The first of voltage vs. platecurrent characteristic of the the linear or class. A amplifier'tube T1 appears ode resistor R24 bypassed by the condenser C14.

When a regenerated signal is present however, the tube T1, dueto the much larger voltage of this signaL'will, and aided in this direction by the grid'resistor' Rz, act as a non-linear device and produce inter-modulation and harmonic 7 components in its plate circuit. The signal appearing across the plate load R18 is utilizable in place of the output of the non-linear'device of the related prior art circuits.

In certain audio transmission systems it is often desired to pseudo-extend the audio signal but also transmit, together with the processed signal, the full percussive and transient quality of the original signal. The suppression of the transients associated with the original signal before processing, results in the loss of a certain amount of articulation in speech and the. loss of clangor and percussion generally associated with some sounds.

tube T12.

When the input signal Em is suddenly increased in amplitude, that is, becomes transient in nature, the amplified portion of this signal which is impressed through the blocking condenser and the potentiometer Ra to the grid of as an amplified signal in the plate circuit of T7. This plate circuit signal is coupled by means of the audio transformer 'I'Re to a' full wave diode rectifying tube T8. The rapidity with which the resulting rectified voltage appears across the diode load R9, R1o and C9 is determined by the charging time of the condenser C2 through the series resistor R9. Foran instant, therefore upon the sudden onset of a transient signalno voltage appears across the resistor Rio. The potential of the junctionof Ru and R10 is therefore during this instant, the same as the potential of theground or.common connector. As the condenser C9 charges, the potential at the junction of R9 and R10 becomes increasingly negative with respect to the common connector and therefore also withrespect to the cathode of the tube T12. Y

The tube Tiahaving no grid bias except that available from the junction of R and R10 through the lower secondary of the transformer TRa, will therefore, during thefirst instant of the sudden onset of the input signal Em, operate as a non linear device. The augmented transient sidebands due to the inter-modulation frequencies are introduced by the tube T12 during the interval wherein its grid has a zero bias with respect to its cathode. The next instant after the onset of the signal, the negative voltage [appearing at the junction of R9 and R10 and applied through R13 to the grid of T12 establishes a negative bias on this grid and causes the tube T12 to act as a linear or class "A" amplifier. The signal appearing in the plate circuit of the tube T12 is then applied to the output through the audio transformer TRs.

The upper control circuit consisting of R11, T9, TR4 and T10 controls the grid bias of the tube T11. The tube'Tn is biased by the battery Ect so as to operate, in the absence of a voltage across the load resistor R15 of the rectifier T10, as a linear or class A" amplifier. When the input signal Em is suddenly applied, it is amplified by the tube T6 and that portion of it appearing on the grid of T9 is amplified further, and, by means of the audio transformer TR4, is applied to the diode T10 through the rectifier load impedance R14, C10 and R15. The condenser C10 charges slowly through the limiting resistor R14 and as it charges, the potential of the cathode of the rectifier T10 gradually rises with respect to ground. The cathode is connected through the battery E04, the upper secondary winding of the transformer TR: and R12, to the grid of the tube T11. Consequently as the cathode 01 T10 rises in potential in a positive direction, the voltage of the battery E} is gradually counteracted and the negative bias of the tube T11 is gradually destroyed. As the bias of T11 is destroyed the grid operates with too little bias and finally without bias so that the tube T11 will operate as a nonlinear device similar to a grid leak detector. R12,

a resistance in the grid circuit provides an additional voltage drop across it when, on the positive halves of the main channel signal, grid current flows in the grid circuit or the tube T11. This action biases the grid more heavily at these times and accentuates the non-linearity.

The signal appearing in'the plate circuits of tubes T11 and T12 is impressed across the upper.

Apower amplifier with an associated filter is connected across the secondary winding of TRs. The characteristics of the filter should approximate the frequency characteristic of the limited i'requency acceptance band of the apparatus through which the pseudo-extended signal is to be transmitted. To the output terminals of the power amplifier and filter is connected a loud speaker. This auxiliary apparatus is shown on rug.- 12 by the dotted lines, PA being the power amplifier, Fx beingthe filter and LS being the loud speaker. The vertical plates of a cathoderay oscilloscope are connected between the plate of T11 and ground.

A monotone signal whose frequency lies approximately midway in the frequency band of the signal which is to be pseudo-extended is abruptly applied and removed intermittently to the input Em of the circuit of Fig. 12. The setting of Ru and the value of the variable resistance R14 are adjusted simultaneously to a point where while the cathode ray oscilloscope deflection appears to be unilateral, sharp clicks are not heard in the loud speaker upon the application and removal of the input signal.

After this is done, the shorting lead between the grid of T1: and its cathode is removed and the setting of R8 and R9 adjusted so that a click is evident in the loud speaker upon the application and removal of the input signal. Re and R9 are adjusted so that the clicks thus introduced should be comparable in quality and duration to those which are obtained when the intermittent signal is applied directly to a loud speaker having a wide band response.

As heretofore stated and described, this invention provides methods and means for producing a pseudo-extended signal which is relatively exaggerated and prolonged transient quality apparent as distortion, these unpleasant sounds suddenly applied tothe input (Em) an ampli-' I fied signal containing a plurality of side-bands appears in the plate circuit of T12. (Also, during the first instant, an amplified undistorted signal appears in the plate circuit of T11. These signals are combined in the transformer TR: and are available across the output terminals of TRs. Gradually, after the first instant, the plate circuit signal of T1: becomes undistorted, as a negative potential builds up across R10 and Co; at which time the plate circuit signal of T11 is still relatively undistorted sincethe condenser Cm is still only. partially charged. The signal Emit is therefore relatively undistorted for an interval after the first, after which, when C10 has charged sufficiently to cause the voltage across it to counteract the initial grid bias of T11, the plate circuit signal of T11 becomes distorted producing intermodulation between harmonic components of those. frequencies impressed on its grid.

The plurality of elements involved in the circuit of Fig. 12 make it inadvisable to-fix precise circuit constants for its operation. The time constants of the various complicated and interconnected meshes would render the synthesis of the circuit extremely laborious and difllcult. Accordingly, a simple method of adjustment has been evolved.

being present to a noticeable degree in prior art pseudo-extended signals. In other words, by the use of the circuits herein described, a sound having a wide audible frequency spectrum may be transmitted to a listener over a transmission link having a narrow frequency pass characteristic, and while the sound reaching the listener's ears does not contain a wide band of audible frequencies, nevertheless, the effect is substantially the same as though all the audible frequencies contained in the original sound were available, without distortion, to the listener.

To produce this pseudo-extension effect obviously the representative circuits of Figs. 8 to 12 must be used in conjunction with a sound source, microphone, phonograph record or other primary or secondary sources of audio signals and necessary signal levels produced with the aid of amplifier or attenuators, active and passive transducers, all of which are well known to the art.

In order to indicate more clearly the auxiliary circuits and apparatus necessary in the improved pseudo-extension system of this invention the representative diagram of Fig. 13 is shown.

Fig. 13 shows, in block diagram form, since the elements thus shown are well known to the art, the utilization of this invention to one of many possible and varied applications. The system of Fig. 13 shows the use of themethod and means oithis invention to the transmission of sounds by a radio system. S is a source of audible sound as for example an orchestra. The sounds emanating from this source are intercepted by the microphone and after conversion to an elec- I area 7 m,

lathe to emphasizethepoint that the input filter F-of Figs. 8, 9' and 12 is not necessarily an indis- ;the improved fpseudoextensionil system REC,

subject of this invention. The output of theam. plifier AMP] is considered as the' input Em of the 1 representative circuits. of Figs. 8., to 12,shown herewith. After. being ,modified by, the. circuit means of: this invention; the resulting output signal Eout 'is impressed and'used to modulate a radio frequency carrier; in the usual manner in the radio transmitter'XT. The portion-ofthe transmission link having arestricted frequency pass characteristic maybe anywhere in the entire transmission link beyond theoutputof the circuit PEG, and m'ayeven be dueto a;hearing' deficiency in the ultimate listener.'- If. there are tricahaudio eal are am lified byvthe amplifier =The output of this amplifier isiapplied to,

pensable feature or the circuits shown herewith.

The .class 10f circuits shown: in Figs. 8 to 12 may be used similarly to thoserecogni' zed as prior art circuits, yie lding;in the place of the latter an improved and undistorted result. Obviously, therefore, thesecircuits and the principles gov erning their. operation can be used in conjunction with: the:;consonator {-circuit described in my co-existing application no frequency limitations in the transmission link f but restricted,side-bandradio transmission is desired,ythe,filter Fm of Fig. 13 may be interposed 'in the transmittercircuit.

In applications :where the magnitude of the original soundsource fallingtwithin the trans- I mission-bandis not suflicient to produce a bal- :anced or realistic signal to the listener, apart of v.Rlllof Fig. 13, without requiring circuit modifications in that receiver.

The processor pseudo-extension need not be limited so as toobtain a signal containing as an example frequencies from, 120 cycles tov 10,000 cycles per second andbetween'which limits are included the intermodulation and harmonic-produ g evident to-persons familiar with. the pertinent art, without depart- Other embodimentswill be ing fromthe spirit or scope of my invention. As

an example, an equivalent method. might be used,

and the results be substantiallythe same,- if the circuits were made to operate ona carrier frequencymodulated.bythe audio signal to be ipseudo-extended,in which case whilethe operations would be performed at a difierent range I of frequencies, the method and means would be substantially the same as indicated herein. The

illustrations included herewith and the descripa I tions included herein should serve, in view of the 'WhatIclaimis: i v d l 1. In a fps eudo extension.circuit including a entire disclosure, to indicate rather .than limit the'generaldesign, and to illustrate rather than 0 specify the components and elements ofthe various representative circuits since many modifications may be, made ;without departing from the scope of my invention as set. forthin the appended claims.

non-linear device and; a source of audio fre- I the non-linear device, means for varying the non- ,linearity of said non-linear device during changes ucts .of frequencies lower than 120 cycles; nor

, need it be limited to producing a signal contain ing, as a further example, frequenciesfrom 30 to 4000 cycles per second with the intermodulation products of frequencies, higher than 4000 cycles per second. For, by operating onthe transient portions of the signals as. described above,'a.ddi-

- tional intelligence and a more pleasing signal can be produced by operating on any or all of the frequencies associated with a signal. More specifically, the middle register of a signal, that is, those frequencies in a signal fallingwithin the narrow acceptance band of a transmission link may be operated upon bythe proceduredisclosed herein and a more pleasant and'more quency modulation meansior .impressing said audiofrequency modulations upon the inputof in the amplitude of the impressed signal, means for collecting at leasta part of the-output of said non-linear device,- and means fortransmitting the collected signal. a

2. In a fpseudo-extension circuit including. a:

non-linear device and a source of audio frequency modulations, means for dividing the audio frequency modulations into two channels, means for selecting a part of the frequencies contained in the first of-these two channels, means for realistic signal will result. When these latter frequencies are operated upon, the, resulting signal isgiven more sonority and fullness, similar to'an eifect produceable by increasing the amplitude of a sound to loudinten'sitles.

In addition, the introduction of both the highest'and lowest frequencies simultaneously into the pseudo-extension circuit as described here- I tofore will result in an increase of quality and articulation conveyed by the resulting signal when transmitted through a system whose acceptance band extends only through a middle range of frequencies. Thisis so since the intermodulation of the highest-with the lowest frequencies will result in inter-modulation products outside the-acceptance band ofthe subsequent transmission circuit.

. Since this invention can be usedto operate on.

a wide range of input signal frequencies, the

input filters which determine the range of signal frequencies to be pseudo-extended" have, in

Figs. 10 and 11, been omitted. This has been impressing said selected part of the first channel on the input of the non-linear device, means for varying the non-linearity of said non-linear device during changes in amplitude in' the input signal impressed on" said non-linear device,

fmeans for combining the output signal of said non-linear device with the signal of the second of the two channels, and means for transmitting at least apart of the combined signal.

3. A method of increasing-the apparent frequency range of a transmitted audio signal comprising the steps of selecting at least apart of the audio signal modulations, limiting thechanges in amplitude envelope of the selected signal by the action.of a compressor circuit so as to produce substantially a constant amplitude frequency distortionless signal, impressing the thus limited audio signal on the input of a nonlinear device, and collecting and transmitting at leasta part oithe output of said non-linear device.

4. A method; of increasing the apparent free I quency range of a transmitted audio signal comprising the steps of: selecting at least a part of the audio signal modulations, limiting the' changes in amplitude of the selected signal by the action of a compressor circuit, impressing the together with the collected and transmitted part of the output of the non-linear device. 1

6. Amethod of increasing the apparentfrequency range of a transmitted audio signal according to claim 4, in which an'unmodifled part of the input signal is combined and transmitted together with the collected and transmitted part of the output'of the varied non-linear device.'

7. A method of increasing theapparent frequency range of a transmitted audio signal comprising the steps of selecting at least a part of the audio signal modulations, increasing the relative amplitude of a portion of the frequenciescontained in said selectedpart, impressing the distorted signal thus resulting on the'input 01. a non-linear device, and collecting and transmitting at least a part of the output signal of said non-linear device.

8. A method of increasing the apparent frequency range of a transmitted audio signal comprising the steps of: selecting at least a part of the audio signal modulations, increasing the relative amplitude of a portion of the frequencies contained in said selected part, impressing the resulting distorted signalon the input of a nonlinear device, varying the degree of non-linearity of said non-linear device during changes in amplitude of the original audio signal, and collecting and transmitting at least a part of the output signal of the varied non-linear device.

9.' A method of increasing the apparent Irequency range of a transmitted audio-signal according to claim 7, in which at least a part of the unmodified original input signal is combined and transmitted together with the collected and transmitting part of the non-linear device.

10. A method of increasing the apparent frequency range of a transmitted audio signal comprising the steps of: dividing the audio signal into two channels, impressing the first of these.

two channels on the input of a non-linear device, varying the non-linearity of the non-linear device during'the initial time of a change in amplitudeof the signal of the first channel, impressing the signal of the second of the two channelson the input of a second non-linear device, varying the non-linearity of this second non-linear device. during a change in amplitude of the signal of the second channel, collecting at least a part of the signals appearing in the outputs of both nonlinear devices, and combining and transmitting said collected parts,

11. A method 'of increasing the app rent frequency range of a transmitted audio signal according to claim 8, in which an unmodified part of the original signal is combined and transmitted together with the collected'and transmitted parts of the outputs of the non-linear devices.

12. In a system of the class described, the combination with a non-linear device comprising means for varying the non-linear characteristic of said non-linear device during the times whenthe signal impre'ssed'on the input oi'said non-linear device is undergoing a change in its amplitude envelope.

13. In a system of the class described, the combination with a non-lineardevice comprising means for impressing an audio signal upon the input of said non-linear device, means for vary- LOUIS A. Di: ROSA.

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