US 3560656 A
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United States Patent  lnventor Roswell W. Gilbert New York,.N.Y.  Appl. No. 634,981  Filed May 1, 1967  Patented Feb. 2, 1971  Assignee Dictaphone Corporation Bridgeport, Conn.
 DINAURAL PHASE DIFFERENTIAL SYSTEM 11 Claims, 4 Drawing Figs.
 US. Cl.....' 179/1  H04r 5/00  Field otSearch 179/1.3, 1.3A, 1.3PS, 1E
 References Cited UNITED STATES PATENTS 2,137,032 11/1938 Snow 179/1 2,852,604 9/1958 MacCutcheon 179/1 3,219,757 11/1965 Palladino 179/1 OTHER REFERENCES Tenny Lode, STEREOPHONIC REPRODUCTION; Audio Engineering,.lanuary 1960, pages 15, 46-7.
Proceedings of the IEEE; October 1966 AN lNDUCTOR- LESS ALL-PASS PHASE SHIFIER, pages 1462- 1463.
Primary Examiner-Kathleen H. Claffy Assistant Examiner-Jon Bradford Leaheey Attorney-Curtis, Morris & Safford ABSTRACT: Monaural electrical signals are divided into two separate components flowing in two separate channels, and the phase of the signal in one channel is shifted by means of a Wien bridge which has been offset (unbalanced) so as to transmit signals at all frequencies of interest substantially without attenuation. The signal in each channel is conducted to a separate loud speaker with the transducers either spatially separated or coupled together to form a headset. The phase difference between the signal from one speaker and the signal from the other provides depth and reality in the reproduced sound. Use of this system in dictation recording and reproducing equipment is described.
PATENTED FEB 219" INVENTOR. Pam ELL M Quasi? MMgg BINAURAL PHASE DIFFERENTIAL SYSTEM This application relates generally to sound reproducing apparatus. and relates specifically to apparatus for producing binaural sound reproduction from monaural electrical signals. In an illustrative embodiment disclosed herein, the invention is described as it is used in reproducing dictation.
Sound. especially the spoken word, usually is composed of components which have substantial phase differences caused by reflections off of walls or other objects. The human ear is quite sensitive to such phase differences. Hence, sounds which do not have such phase differences have a flat and unreal quality For example, if monaural sound signals are reproduced in one earphone of a headset, they sound unreal and somewhat unpleasant. Moreover, the quality of the sound is not improved if the monaural signals are reproduced in both earphones of the headset because, as in the single earphone arrangement. the sound is conducted to the ears directly without reflections and without phase shift. This problem is particularly noticeable in dictation reproducing machines used by stenographers toften called transcribers"). It is almost essential that such machines use headphones because, if loudspeakers were used instead of the headphones, the sound would be likely to disturb neighboring office personnel.
In order to solve the foregoing problem, it is an object of the present invention to provide a system for reproducing sound from monaural signals, especially monaural dictation signals, in which system the sound reproduced has phase differentials wh|ch give it realism and a pleasing quality. It is another object oi the present invention to provide such a system which produces high-fidelity sound reproduction, and yet is relatively simple and inexpensive to manufacture.
In accordance with the present invention, monaural electrical signals are divided into two components, and the phase of one of the components is shifted with respect to the phase of the other component. Each of the components then is reproduced by means of a separate sound transducer, with the result that two or more signals shifted in phase with respect to one another are produced for the listener. This gives the sound depth and a realistic quality. An offset (unbalanced) alternatmg-cu rrent frequency-sensitive bridge such as Wien bridge is used as the phase-shifting means. This circuit is simple and provides a relatively large amount of phase shift. Moreover, it is an all pass phase shifter; that is, it produces phase shift without any substantial attenuation of any frequency component of the signal. Furthermore, the phase shift provided by this circuit varies smoothly and continuously with frequency. These features of the invention enhance the fidelity of the sound reproduced, and the simplicity of the circuit enhances the reliability and reduces the cost of the sound reproduction system.
The drawings and description that follow describe the invention and indicate some of the ways in which it can be used. In addition, some of the advantages provided by the invention will be pointed out.
In the drawings:
FIG. I is a schematic circuit diagram of a system constructed in accordance with the present invention;
FIG 2 is a locus diagram for the offset Wien bridge circuit used in the present invention;
FIG. 3 is a schematic circuit diagram of another embodi ment of the present invention; and
FIG. 4 is a schematic circuit diagram of an equivalent circuit of a portion of the circuit shown in FIG. I.
The system shown in FIG. 1 includes a source of monaural electrical signals. The source 10 can be any conventional source of monaural signals such as a magnetic tape or engraved disc record player, but preferably is a dictating machine or dictation transcribing machine for reproducing recorded dictation. The dictating machine can be of any wellknown type, such as those which record and reproduce dictation on a magneticbelt, tape or disc or on a plastic belt or disc with the sound engraved by a stylus. The transcribing machine is of a type adapted to reproduce dictation recorded by the particular dictating machine with which it is sold. The dictating machine may have two spatially separated loudspeakers such as speakers 22 and 24 shown in FIG. I. The transcribing machine preferably has a headset with two earphones l8 and 20 which allow the stenographer to listen to the reproduced dictation without the sound disturbing nearby coworkers.
The monaural electrical signals are conducted from the source 10 over a lead 28 into a phase splitting and shifting circuit indicated generally at I2. The circuit 12 divides the input signal into two components which are conducted through out- 7 put leads l4 and 16 and blocking capacitors 17 and 10 to separate loudspeakers such as the speakers 18 and 20 forming part of a headset, or to physically and spatially separated loudspeakers 22 and 24. The signal flowing through output lead 16 is made to lag behind the signal flowing through output lead 14 by means of a bridge circuit--26, so that the sound reproduced in the speakers 18 and 20 or 22 and 24 contains phase-shifted components and soundsnatural in quality and depth.
The phase-splitting function of the phase splitting and shifting circuit 12 will be explained first. The monaural signal from source 10 is conducted over lead 28 to the base electrode of a transistor 30. Transistor 30 is connected in a common-emitter configuration, with equal resistors 32 and 34 being connected in series with the emitter and collector leads, respectively. A relatively larger resistor 36 is connected between the collector electrode and the resistor 34. The relative values of these re-, sisters and the other circuit impedances will be discussed in detail below. A DC voltage from a low impedance direct current source is applied between ground and the terminal 38 at the upper end of resistor 34. The transistor 30 provides amplification of the monaural AC signal applied to its base lead. The amplified alternating current signal flows through equal resistors 32 and 34. Since the DC source connected between ground and the terminal 38 has essentially zero impedance to alternating current signals, terminal 38 is effectively con- I nected to ground as far as AC signals are concerned.
The output lead 14 is connected to the lower terminal of resistor 34, and the output lead 16 is connected to the upper terminal of resistor 32. Thus. the circuit provides a voltage divider network in which the input signal is divided into two components, each flowing in a separate channel formed by either lead 14 or lead 16. The bridge circuit 26 shifts the phase of the signal in channel 16 with respect to the signal in channel 14, as will be explained next.
The bridge circuit 26 has been redrawn in FIG. 4 in a somewhat more conventional bridge configuration to facilitate explanation of the operation of the circuit. The lower right-hand arm of the bridge contains only the resistor 32. The lower left-hand arm contains resistors 34 and 36. The upper left-hand branch has a resistor 42 and a capacitor 40 connected in series with one another, and the upper right-hand arm has a parallel combination of a resistor 44 and a-capacitor 46. These components also are shown in FIG. 1.
A bridge circuit with the general circuit configuration shown in FIG. 4 is an alternating current bridge sometimes known as a Wien" bridge. Such a bridge circuit can be balanced by varying the frequency of an AC source connected between the left and right comers of the bridge, usually for the purpose of making electrical measurements.
The bridge circuit 26 is a Wien bridge modified to form an all-pass" phase shifter as shown in Proceedings of the IEEE," Oct. 1966, pages I,462-l,463. The modification consists of setting the impedances of the bridge at the relative values shown in FIGS. 1 and 4. That is, if the resistance of resistor 44 is R, and the capacitance of capacitor 46 is C, then the resistor 42 should have the value MR, and the capacitor 40 should have the value of CM, where M is a constant scale fac tor whose value depends upon the particular range of circuit parameters desired. In the lower half of the circuit, if the resistor 32 has the value r, then the total resistance of resistors 34 and 36 should be (4M I )r. By this means the Wien bridge has been offset or unbalanced.
FIG. 2 is a locus diagram for the offset Wien bridge shown in FIGS. I and 4 in which the output voltage V (see FIG. 4) is plotted against signal frequency w 21rf. As is shown by FIG. 2, the voltage vector V .has a constant magnitude at all frequencies. Thus, the circuit has substantially no frequency selectivity and completely avoids that source of distortion. As also shown in the locus diagram in FIG. 2, the output voltage V and the input voltage are in phase with one another at whereas very low and very high frequency signals are approximately 180 out of phase with the input signal. Thus, the total phase shift between the high and the low frequencies theoretically is 360, and actually approaches this value rather closely in practice. Moreover, the amount of phase shift varies smoothly and continuously with frequency, and thus does not have any discontinuities or sharp changes which could produce distortion in the sound output.
FIG. 3 illustrates a three-stage cascaded phase splitter and shifter circuit 48. Many of the components of the FIG. 3 circuit are the same as those in the FIG. 1 circuit, and the same reference numerals are used to indicate corresponding components.
The first stage of the FIG. 3 circuit is substantially identical to the phase splitting and shifting circuit 12 shown in FIG. I. The only exceptions to this are in the addition of a relatively large bleeder resistor 50 between the terminal 38 and the base lead of the transistor 30, and a coupling capacitor 52 in series with the base lead. The unshifted signal is supplied over the lead 14 through the blocking capacitor 17 to the speaker 22. The first, second and third stages are directly coupled together. The second and third stages are substantially identical to the first stage, except that a single resistor 54 having a value r replaces the separate resistors 34 and 36 in the first stage whose total value also is 5r. This simplification has been made in view of the fact that two separate resistors are not needed since an output signal is not taken from the second or third stages of the circuit as it is from the first stage. A final buffer amplifier stage including a transistor 56 with an emitter resistor 58 is provided for coupling output lead 16 through blocking capacitor 19 and to the other speaker 24.
The specific values are set forth below for the circuit elements which actually were used in the circuit shown in FIG. 3:.
10,000 ohms 10,000 ohms 0.047 microfarad The foregoing circuit was tested. The resonant frequency w was found to be 500 Hz., and about 1,000 total phase shift was observed. This phase shift created a sound output which gave the distinct impression of breadth" and reality. The effect was substantially more noticeable than in, the circuit shown in FIG. 1 because of the additional phase shift which was created by cascading three stages together.
It is felt to be desirable to locate the resonant frequency m of the bridge at about the center of the spectrum of frequencies for the particular type of sound being reproduced. However, the value of the resonant frequency can be varied as desired in order to suit the particular requirements of a given application.
The above description of the invention is intended to be illustrative and not limiting. Various changes or modifications in the embodiments described may occur to those skilled in the art and these can be made without departing from the spirit or scope of the invention.
1. Apparatus for reproducing sound binaurally from a monaural electrical signal having a plurality of frequencies in the audio spectrum, said apparatus comprising, in combination, means for supplying a monaural electrical signal, means for dividing said signal into at least two separate components and conducting each of said components into a different sound transducer, means for shifting the phase relationship of said components with respect to one another and maintaining the amplitude of each of said components substantially equal to the amplitude of the other of said components at substantially all frequencies of the sound being reproduced, wherein the maximum phase shift-is greater than 360.
2. Apparatus asin claim 1 in which said phase shifting means is an all-pass phase-shifting network.
3. Apparatus as in claim 2 in which said phase shifting means is an offset Wien bridge circuit.
4. Apparatus as in claim 3 in which said dividing means is a phase splitter utilizing the impedances of said bridge circuit as voltage-divider impedances.
5. Apparatus as in claim 4 in which said impedances are equal resistances in adjacent arms of said bridge.
6. Apparatus for reproducing, in multiple sound transducers, multicomponent electrical signals having a plurality of frequencies in the audio spectrum, said apparatus comprising, in combination, separate means for conducting each of said components to a separate one of said transducers, a Wien bridge forming a part of one of said conducting means, said Wien bridge being offset so as to shift the phase of the signal flowing through it with respect to the signal flowing through another of said conducting means, while maintaining the amplitude of the signal flowing through said Wien bridge substantially constant at substantially all sound frequencies being reproduced, wherein the maximum phase shift is greater than 360.
7. Apparatus as in claim 6 in which said sound transducers are loudspeakers in a headset.
8. Apparatus as in claim 6 in which said sound transducers are spatially and physically separate loudspeakers.
9. Apparatus as in claim 6 including a plurality of cascaded all-pass Wien bridges.
10. In a dictation machine, two separate sound transducers, means for converting monaurally recorded dictation into single-channel electrical signals having a plurality of frequencies in the audio spectrum, a phase splitter for receiving said signals and transmitting them over two separate conduction paths to said sound transducers, means in one of said paths for shifting the phase of the signal flowing in said one path with respect to the signal flowing in the other path while transmitting the signal through said one path with a substantially constant amplitude at substantially all sound frequencies being reproduced, wherein the maximum phase shift is greater than 360.
11. Apparatus as in claim 10 in which said sound transducers are loudspeakers in a headset electrically connected to said dictation machine.