US3055980A

US3055980A - Improvements relating to reproduction arrangements for stereophonic sound systems

Info

Publication number: US3055980A
Application number: US845469A
Authority: US
Inventors: Percival William Spencer
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1958-10-11
Filing date: 1959-10-09
Publication date: 1962-09-25
Anticipated expiration: 1979-09-25

Description

Sept. 25, 1962 w. s. PERcIvAL 3,055,930

IMPROVEMENTS RELATING TO REPRODUCTION ARRANGEMENTS FOR STEREOPHONIC SOUND SYSTEMS Filed Oct. 9, 1959 1 AMPLIFYING AND VOLTAGE- TO-CURRENT glgfggggLATlNG AMPLIFIER AUDIO AMPLIFIER HALL AMPLIFIERS (PHASE INVERTING) CRYSTAL FIG 1 RELAY AMPLIFIER (PHASE INVERTING) IMPROVEMENTS RELATING T REPRODUCTHGN AGEMENTS FOR STEREUPHGNIC GUND SYSTEMS William Spencer Percival, West Ealing, London, "England, assignor to Electric 81 Musical Industries Limited, Hayes, England, a company of Great Britain Filed Oct. 9, 1959, Ser. No. 845,469 Claims priority, application Great Britain Get. 11, 1955 3 Claims. (Cl. 179-1) This invention relates to reproduction arrangements for stereophonic sound systems.

One defect of stereophonic reproduction in which audio signals are reproduced by two spaced loudspeakers is that as the listener moves along the line parallel to the line joining the loudspeakers, the apparent position of the source of sound tends also to move. The result is that satisfactory stereophonic reproduction is available only for listeners on or near the centre line between the loudspeakers.

The object of the present invention is to provide an improved stereophonic sound reproducer which enables stereophonic reproduction to be obtained such that the apparent direction of reproduced sound is less dependent on the position of the listener than in arrangements such as indicated in the preceding paragraphs.

According to the present invention there is provided a stereophonic sound reproducer comprising means for providing an audio signal representing the waveshape of sound, means for providing a control signal which represents the direction or position of the source or sources of said sound, and is substantially independent of the loud ness of the sound, at least three loudspeaker units, and means for applying the audio signals to said loudspeaker units in relative proportions dependent upon the instantaneous value of said control signal, in such a way that the audio signal is applied to a single and different one of said loudspeaker units for different discrete values of said control signal and is proportioned between the respective two loudspeaker units for values of the control signal intermediate said discrete values.

The expression loudspeaker unit is used to denote either a single loudspeaker, or a group of loudspeakers, having for example different frequency responses, mounted in such relative positions that sound emanating from them would appear to the listener to come substantially from the same direction or position.

The invention may be applied to reproducers adapted to reproduce sounds from a radio broadcast comprising the audio signal and the control signal, in which case the means for providing the audio signal and the control signal may comprise amplifying and detecting circuits such as are conventional in broadcast radio receivers. The invention may nevertheless be applied to reproducers adapted to reproduce signals derived from twin track record tape or complex cut record discs. In such a case the input signal to the reproducer may comprise two audio signals; for example the sum and difference of two directional signals and the means for providing the control signal at the reproducer would need to be such as to derive the latter signals from the two audio signals.

In order that the present invention may be clearly understood and readily carried into elfect the invention will now be described with reference to the accompanying drawings, in which:

FIGURE 1 illustrates diagrammatically, and mainly in block form, one example of a stereophonic sound reproducer according to the present invention, and

FIGURE 2 illustrates a modification of FIGURE 1.

It will be assumed that the sound reproducer which is illustrated in FIGURE 1 is adapted to receive a broadcast carrier wave which is modulated in any suitable manner 3,055,980 Patented Sept. 25, 1962 ire The audio signal is denoted by the symbol V whilst the control signal is denoted by the symbol VI!L+VIIR The control signal, which for brevity may be denoted as V is substantially independent of the loudness of the sound, it has the value of +1 when it represents sound coming from the extreme left of the stereophonic base, it has the value 1 when it represents sound coming from the extreme right of the stereophonic base, and it has value zero when it represents sound coming from the centre of the stereophonic base. The control signal and the audio signal may for example be obtained at the transmitter by means of apparatus such as described in United States patent application Number 742,175, filed by W. S. Percival on June 16, 1958. This application describes the derivation of a control signal having the form and then changed to the other form, if necessary, at the receiver.

The receiver shown in FIGURE 1 comprises an antenna 1 which feeds the received carrier wave to conventional I represent the extreme right direction.

T right of centre yield zero output voltage.

amplifying and demodulating circuits 2 which are arranged to separate the audio signal from the control signal V The signals V and V as derived from the cir cuits Z are voltage signals and the polarity of the audio signal is such that it can be represented as V The audio signal -V is applied to one end of a resistor 3 which will be referred to subsequently and it is also ap plied to an amplifier 4- which is arranged to derive a current i proportional to the voltage V but is of positive polarity. The relationship between a voltage signal V and direction as represented by the diagram 5 which shows that V has a maximum positive value L to represent the extreme left direction, has zero value to represent the central direction, and has a maximum negative value R to The signal V is applied to a bridge network of four diodes 6, 7, 8 and 9, polarised as shown so that at the junction of the diodes 6 and '7 signals representing left of centre yield a positive voltage whereas central signals and those representing On the other hand, at the junction of the diodes 8 and 9, signals representing right of centre give a negative output voltage whereas central signals and signals representing left of centre give zero output voltage. The relationships between direction and voltage of the two output signals derived from the diode bridge circuit are represented by the diagrams 1t and 11. The voltage derived from the junction of 6 and 7 is applied to a non-phase inverting amplifier 12 and that derived from the junction of the diodes 8 and 9 is applied to a' phase inverting amplifier 13 and the two signals are then applied to the

field windings

14 and 15 of two Hall amplitude circuits, comprising Hall crystals represented by the

rectangles

16 and 17. It will be understood that the

field windings

14 and 15 are arranged to set up magnetic fields normal to the planes of the respective Hall crystals. The current i from the aforesaid amplifier 4 is fed through the'Hall crystals from one edge to another so that the current flows in a direction perpendicular to the respective magnetic fields. As a result, voltages are induced between the remaining edges of the Hall crystals, proportional to control current i and the respective magnetic fields. As shown the voltage induced across the Hall crystal 16 is applied to one end of the resistor 18 and through an audio amplifier 19 to a loudspeaker 20 which is placed at the left hand side of the auditorium. Similarly the voltage induced across the Hall crystal 17 is applied to one end of the resistor 21 and through an audio amplifier 22 to a loudspeaker 23 which is placed at the right hand side of the auditorium. The other ends of the

resistors

18 and 21, of the aforesaid resistor 3 are joined together, and any voltage appearing at this junction is applied via audio amplifier 25 to a loudspeaker 26 which is placed in the centre of the auditorium. The

resistors

3, 18 and 21 have equal resistances and the constant of proportionality between the voltage V applied to the resistor 3 and the current i is such that V is numerically equal to the maximum voltages obtained from the outputs of the

Hall crystals

16 and 17. The voltage V is however of opposite polarity to the outputs of the Hall crystals. The amplifier 25 is required to be phase inverting relative to the

amplifiers

19 and 22, and to have three times as much gain, in order to compensate for attenuation of the signal V at the junction of 3, 18 and 21.

The result of the arrangement shown is that the output of the Hall crystal 16 is a voltage proportional to both the audio signal V and the control voltage obtained from the amplifier 12, and conforming to the directional relationship 10. The output of the Hall crystal 17 is a voltage proportional to both the audio signal V and the control voltage obtained from the amplifier 13, and conforming to the inverted directional relationship 11. The voltage output from the Hall crystal 16 will equal to the voltage V when the direction is fully left, and in this condition because the

resistors

3, 18 and 21 have equal resistances, and because the output impedance of a Hall crystal can be assumed to be small, no voltage will appear at the input to the amplifier 25 or at the input to the amplifier 22. Therefore sound is reproduced only by the loudspeaker 20. Similarly when the direction is fully right, sound is reproduced only by the loudspeaker 23. For a central direction, the output voltage of both Hall crystals is zero and sound is reproduced only from the central loudspeaker 26. For directions between left and centre, sound is reproduced simultaneously from the

loudspeakers

20 and 26, the audio signal being in effect applied to these two loudspeakers in different ratios dependent upon the instantaneous value of the control signal. For a given sound of constant value moving from left to centre the output of the two

speakers

20 and 26 is also of constant volume. No sound is reproduced from the right end loudspeaker 23. Conversely for directions between centre and right, sound is reproduced from the

loudspeakers

23 and 26 in different ratios dependent upon the values of the control signal, no sound being reproduced by the left hand loudspeaker 20.

In the modification of the invention which is illustrated in FIGURE 2, one of the Hall crystal amplitude control circuits and one of the audio amplifiers before the loudspeakers are dispensed with. The control signal V A is applied directly to the

amplifiers

12 and 13, the amplifier 13 being as in FIGURE 1 a phase inverting amplifier. The outputs of the

amplifiers

12 and 13 are then applied to

diodes

30 and 31 which have a common load resistor 32, so that the voltage across the resistor 32 corresponds to the more positive of the voltages applied to the

diodes

30 and 31. The voltage across the resistor 32 therefore represents the modulus of the voltage V the relationship between voltage and direction being as indicated by the diagram 33. This voltage is applied to the field winding 34 of the single Hall crystal amplitude control circuit the crystal of which is denoted by the reference 35. The current L, is fed to this crystal in the manner described with reference to FIGURE 1 and the resultant voltage is applied to one end of the resistor 36 which corresponds either to the

resistance

18 or 21 of FIGURE 1. The other end of this resistor is joined as shown to the resistor 3. The output voltage from the Hall crystal 35 is also applied to an audio amplifier 37. The output of the amplifier 37 is applied to a switch 38 which though represented as a mechanical switch may in fact be an electronic switch. The switch is controlled by a polarized relay 39 in such a way that the output of the amplifier 37 can be applied either to the left hand loudspeaker 20 or to the right hand loudspeaker 23. The voltage at the junction of the

resistors

3 and 36 is applied via the phase inverting amplifier 25 to the central loudspeaker 26. The gain of the amplifier 25 is adjusted relative to that of the amplifier 37 to suit the attenuation of the

network

3, 36.

The relay 39 is polarized in such a way that whenever the control voltage V has a positive value, the output of the amplifier 37 is applied to the left hand loudspeaker 20 and whenever the control voltage V has a negative value, the output of the amplifier 37 is applied to the right hand loudspeaker 23. The arrangement takes advantage of the fact that at any instant one or other of the

loudspeakers

20 and 23 should reproduce no sound, and the operation of the relay 39 ensures that the switching over from one loudspeaker to the other occurs when the control voltage corresponds to a centre direction, in which circumstance neither of the

loudspeakers

20 and 23 should reproduce sound. Distortion of the sound output due to switching is therefore avoided. In other respects the operation of FIGURE 2 is similar to that described with reference to FIGURE 1.

In both FIGURE 1 and FIGURE 2, the proportioning of the audio signal among the loudspeakers is performed in effect by controlling the amplitude of the audio signal V in response to the modulus of the control signal V to produce an amplitude controlled signal, and by deriving a signal representing the difference of the uncontrolled audio signal V and the amplitude controlled signal, the differencing being effected in the

network

3, 18 and 21 of FIGURE 1 and the

network

3 and 36 of FIGURE 2. The difference signal is applied to the central loudspeaker and the amplitude controlled signal applied either to the loudspeaker 20 or the loudspeaker 23 depending on whether the control signal represents a direction or position to one side or other of the central direction or position. In the arrangement shown in FIGURE 1 two amplitude controlling circuits are used to form the amplitude controlled signal, the amplitude control circuit including the crystal 16 for controlling the audio signal with the control signal when the control signal has a positive value, and the amplitude control circuit including the crystal 17 for controlling the audio signal with the negative of the control signal when the control signal has a negative value.

The control signal V requires to have only a limited band width relative to the audio signal V although the control signal may extend in to the audio frequency band. The band width of the control signal may be for example from 0 to c./s. The

amplifiers

12 and 13 therefore require only a limited pass band, corresponding to that of the control signal. The

loudspeakers

20, 23 and 26 need not be single loudspeakers, but may comprise groups of loudspeakers positioned in such a way as to form single loudspeaker units. Furthermore although the arrangements described ensure that the sum of the signals applied to the three loudspeakers is always linearly proportional to the audio signal it may not always be necessary to arrange that this is so, as the acoustics, of a particular room may be such that better results are obtained with some other relationship. The amplitude controlled signals obtained from the amplitude control circuits may not be linearly related to the control signal, in order to obtain optimal conditions as judged by listening tests. The reproducer may include means for varying the relationship between the control signal V and direction, to provide for spread adjustments which can be controlled by the listener.

'In a further modification, it is arranged that the very low audio frequencies are diverted entirely to the amplifier 25 and the central loudspeaker 26. In this case only the loudspeaker 26 need have a full bass response. This modification takes advantage of the fact that stereo effects for very low frequencies are not so important as for higher frequencies and allows economy to be effected at the cost of only a small loss of stereo effects since good bass reproduction involves more expensive amplifiers and speakers.

In the arrangement shown in FIGURE 2, the switch 33 may be placed before the amplifier 37, and in this case a separate audio amplifier must be introduced for the speaker 23. In this modification the switch is situated at a point in which the signal level is relatively low and this facilititates the employment of an electronic switch. If an electronic switch is used, that is one employing thermionic or transistor valves, the impedances of the valves when conducting are of low value so that the power consumption is considerably less than that required for an additional amplifier.

The invention can also be applied to reproducers in which more than three loudspeaker units are provided, spaced one from another along the stereophonic base. In such an application of the invention, an expedient similar to that illustrated in FIGURE 2 is preferably adopted in order to reduce the number of audio amplifiers which are required. The switch equivalent to 38 is required to operate in such a way that the two audio amplifiers are always connected to the two speakers which are required to operate at a given instant. The switching is arranged to occur at instants when all the sound output is produced by one speaker and is such as to switch the amplifier not in service at that instant from the speaker on one side of the inoperative speaker to that on the other side.

What I claim is:

1. A stereophonic sound reproducer comprising means for providing an audio signal representing the waveshape of sound, means for providing a control signal which represents the direction or position of the source or sources of said sound, at least left hand, central and right hand loudspeakers and means responsive to the instantaneous value of said control signal comprising means for controlling the amplitude of said audio signal in response to the modulus of said control signal to produce an amplitude controlled signal, means for deriving a signal representing the difference between said audio signal and said amplitude controlled signal, means for applying said difference signal to said central loudspeaker, and means for applying said amplitude controlled signal to said left hand or said right hand loudspeaker, depending on Whether the control signal represents a direction or position to the left or right of a central direction or position.

2. A sound reproducer according to claim 1 comprising a first amplitude control circuit for producing said amplitude controlled signal when the control signal represents any direction or position to the left of said center direction or position, and a second amplitude control circuit for producing said amplitude controlled signal when the control signal represents any direction or position to the right of said central direction or position, said left hand loudspeaker being connected to receive the amplitude controlled signal from said first amplitude control circuit, and said right hand loudspeaker being connected to receive the amplitude controlled signal from said second amplitude control circuit.

3. A sound reproducer according to claim 1 comprising an amplitude control circuit common to both said left hand and right hand loudspeakers for producing said amplitude controlled signal, and a switch for applying the amplitude controlled signal from said amplitude control circuit either to the left hand or right hand loudspeaker depending on whether said control sign-a1 represents a direction or position to the left or the right of said central direction or position.

De Boer Apr. 10, 1946 Volkma-n June 14, 1960