Patent GB-1112233-A (GB)
Title: Electro-acoustic system for stereophonic reproduction
Inventor: KEIBS LOTHAR
IPC: H04S3/00
Filed: 1965-05-10
Abstract: 1,112,233. Stereophonic systems. DEUTSCHE POST, RUNDFUNK-UND FERNSEHTECHNISCHES ZENTRALAMT. 10 May, 1965, No. 19594/65. Heading H4R. In a system for stereophonic sound reproduction by at least three loudspeakers, in which system two primary signals X and Y or M and S are used and a control signal is obtained from the primary signals, the control signal produces control such that, when there is a single sound source, sound is only radiated by a single loudspeaker when the levels of the primary signals correspond to such sound source positioned in this loudspeaker and that sound is only radiated by two adjacent loudspeakers when the levels of the primary signals correspond to such sound source positioned between these two loudspeakers. The control signal is used to produce from the primary signals three signals A, B and C having the values:- The meaning of |S| is as follows: |S| =  The meaning X - Y > 0 and |S| =-S = S = X - Y when X - Y > 0 and |S| = - S when X - Y < 0. Similarly the meaning of X - Y | is as follows: Where the system has three loudspeakers these are arranged at left, centre and right and the signals A, B and C are fed to the left, centre and right loudspeakers respectively. If the system has more than three loudspeakers the three signals A, B and C are subdivided in a manner dependent upon the number of loudspeakers. Where three loudspeakers are used a control circuit, for producing | S may be obtained either by connecting the inputs of two amplifier stages in parallel and connecting their outputs in opposite phase and in series or, conversely, the inputs of two amplifier stages can be connected in opposite phase and in series and their outputs in parallel. The inputs are fed with signal S. A control signal is applied to the grids of the amplifier stages and ensures that, according to whether it is positive or negative, one amplifier stage is opened and the other blocked, so that either S or - S is produced at the output. The control signal is obtained by rectifying primary signals X and Y individually and using the difference between the rectified voltages as a control signal itself, or the difference can be used to control a switching tube which will produce the control signal. The signal S can then be combined, by addition or subtraction, by means of differential transformers or voltage divider circuits, with the information S and M. In place of controlled amplifier stages there may be used diode stages which are blocked or opened by means of a control signal, which is applied as a bias voltage, the control signal being obtained as above. If more than three loudspeakers are used more control circuits are required. If a number of acoustic sources are operating simultaneously the system does not function satisfactorily and is preferably combined with another system which then becomes less or more effective, in accordance with the prominence of a single source. Preferably the other system is such that the LEFT loudspeaker radiates the signal X and the RIGHT loudspeaker radiates the signal Y. The combination is represented by Fig. 2. Unit 1 either produces the signal | S which is combined with the signals S and M to produce the signals applied to the loudspeakers or it produces directly the signals applied to the loudspeakers. Alternatively, the unit 1 is fed at two separate inputs with signals X and Y and its output is either X or Y, whichever has the lower amplitude. Control is effected by a unit 7. The units 2 and 8 control the combination of the two systems. Unit 8 senses whether there is a single source, for example by separately amplifying the signals X and Y to equal fixed levels, finding the difference and rectifying such difference. For a single source the difference is zero. The control signal 10 ensures that the amplification factor of amplifier 2 is unity for a single source and zero for a completely spread source. Intermediate values of the amplification factor are also obtainable. The output voltage of amplifier 2 is subtracted from the voltage of signal Y and the difference is applied to loudspeaker 6. The output of unit 2 is applied to amplifier 3 and then to loudspeaker 5. Additionally the output voltage of unit 2 is subtracted from the voltage of signal X and the difference is applied to loudspeaker 4. The distribution of the primary signals may be effected separately for different frequency bands.