The present invention relates to a detection apparatus and method for detecting a phase difference between a first signal supplied to a first electro-acoustic transducer means, such as a first loudspeaker, and a second signal applied to a second electro-acoustic transducer means, such as a second loudspeaker.

It is desired to maintain a predetermined phase relationship between stereophonically related signals to obtain full benefit of stereophonic reproduction. Thus, if stereophonically related signals of incorrect phase are added together for transmission by a monophonic broadcasting station or if one side of a pair of stereo loudspeakers is connected out-of-phase with respect to the other by mistake, the center sounds reproduced at a receiving side will be attenuated and distorted. Such errors in phasing cause incorrect stereophonic reproduction and low frequencies are attenuated, leading to a loss of bass. In conventional systems, phase connections of loudspeakers therefore had to be manually checked.

Document US 3,148, 287 discloses a signal phase sensing and maintaining system for continuously maintaining a predetermined phase between related signals such as stereophonic sound signals. According to this prior art, left and right signals from corresponding left and right transmission channels are combined both additively and subtractively to produce a monophonic or centered component and a stereophonic component. The two components are continuously compared in magnitude and whenever the magnitude of the difference component is found to be greater than the sum component, the signal in one of the channels is immediately reversed to restore the proper phase relation. In this fashion, the proper phase relation between the two stereophonic signals is always maintained, regardless of erroneous system connections. In the practical implementation proposed in this document, movable switch contacts normally are maintained in engagement with fixed contacts so long as the sum of the signals in the two channels is greater in magnitude than the difference. However, if the sum becomes less than the difference as result of an inadvertent reversal in phase of one of the two signals, the switch is operated, reversing

the signal in its channel so that the proper phase relation between the two signals is maintained.

However, this prior art requires a quite complex and sophisticated mechanical construction and thus involves high production costs.

It is therefore an object of the present invention to provide a signal phase detection system for stereophonic reproduction systems, by means of which phase detection and correction can be achieved at moderate modifications.

This object is achieved by a detection apparatus as claimed in claim 1 and by a detection method as claimed in claim 10. Furthermore, the above object is achieved by an electro-acoustic transducer apparatus as claimed in claim 11 and a processing apparatus as claimed in claim 12.

Accordingly, two magnetic field or current detectors are provided for the respect first and second electro-acoustic transducer means to measure the respective directions of magnetic field or current in order to derive detection outputs which can be used to detect a phase difference and to provide a corresponding control output. Thereby, the system is able to fully detect and correct for out-of-phase connections at the electro-acoustic transducer means or for other phase differences caused during the supply of the first and second signals to the electro-acoustic transducer means.

The first and second detection means each may comprise respective non- invasive sensing means for sensing the induced magnetic field or current. The use of non- invasive sensing means provides the advantage that little modifications are required at the electro-acoustic transducer means, e. g. loudspeakers, such that the detection system can be easily implemented without major changes. In particular, the respective non-invasive sensing means may comprise Hall sensing means for magnetic field sensing. Such Hall sensing means provide the advantage of small size and can thus be easily mounted at the electro- acoustic transducer means. Furthermore, the respective non-invasive sensing means may be mounted onto respective housings of the first and second electro-acoustic transducer means at a location arranged on a central axis of respective coil means of the first and second electro- acoustic transducer means. This mounting location of the non-invasive sensing means assures maximum detection of the magnetic field induced by the current of the coil means.

The control means may be arranged to supply a predetermined test signal to the first and second electro-acoustic transducer means, and to generate the control output

based on the detection outputs corresponding to the predetermined test signal. As an example, the predetermined test signal may have a pulse shape. The current induced in the electro-acoustic transducer means will thus either generate a magnetic north or south pole depending on the phase of the supplied first and second signals, due to the fact that the pulse shaped test signal only comprises two discrete values. This enables easy detection of an out- of-phase relationship. Of course, other predetermined shapes of the test signal can be used as well.

Furthermore, the control output of the control means may be generated so as to invert the phase of one of the first and second signals, if the detection outputs indicate an out- of-phase relationship between the first and second signals. Thereby, automatic correction of out-of-phase connections and supplies can be assured. As an alternative, the control means may be arranged to generate an audible or visible output if the detection outputs indicate an out-of-phase relationship between the first and second signals. In this case, a user of the apparatus may then manually initiate the phase change or actually carry out a change of the connections at the electro-acoustic transducers when the audible or visible output has been noticed.

In the following, the present invention will be described in greater detail based on a preferred embodiment with reference to the accompanying drawings, in which: Fig. 1 shows a schematic block diagram of a phase detection apparatus according to the preferred embodiment; and Fig. 2 shows a schematic diagram of a cross section of a loudspeaker modification according to the preferred embodiment.

The preferred embodiment will now be described on the basis of a stereophonic sound reproduction system as shown in Fig. 1.

Fig. 1 shows a schematic diagram of a stereophonic sound reproduction system which comprises a processor 10, which may be a digital signal processor or microprocessor for sound processing having control inputs for inputting phase detection signals obtained from respective loudspeakers 32,34, and stereophonic signal outputs L and R for outputting stereophonic signals to be supplied via a power amplifier 20 to a left loudspeaker 32 and a right loudspeaker 34, respectively.

The power amplifier 20 supplies the respective stereophonic signals via respective left connection lines L+ and L-connected to the left loudspeaker 32, and right connection lines R+ and R-connected to the right loudspeaker 34. The left and right loudspeakers 32,34 have mounted thereon respective non-invasive magnetic field or current detectors 42,44 having respective detection outputs connected to corresponding control inputs of the processor 10. The detectors 42,44 may be any suitable detector which allows detection or measurement of a direction and/or amount of drive current or magnetic field strength generated in or supplied to the left and right loudspeakers 32,34.

According to an example, the non-invasive detectors may be Hall effect transducers or Hall sensors 42,44 mounted e. g. onto dust caps or any other suitable detection position of the respective left and right loudspeakers 32,34. For example, the mounting position may be on-axis with or on the central axis of a voice coil of the loudspeakers 32,34.

The function of the Hall sensors 42,44 is based on the physical principal of the Hall effect named after its discoverer E. H. Hall. According to this Hall effect, a voltage is generated transversely to the current flow direction in an electric conductor, if a magnetic field is applied perpendicularly to the conductor. As the Hall effect is most pronounced in semiconductors, the most suitable Hall element is a small platelet made of semiconductor material. Particularly, Hall sensors are provided having the Hall element with its entire evaluation circuitry integrated on a single silicon chip. This chip may be produced using' CMOS (Complementary Metal Oxide Semiconductor) technology. They may be provided in leaded or SMD (Surface Mounted Device) package. By these Hall sensors, the magnetic flux component perpendicular to the chip surface is measured, and a proportional electrical signal is generated which may be processed in the evaluation circuits integrated on a sensor chip.

The processor 10 may be arranged to output a predetermined test pulse, such that the pulse current to the voice coil of the left and right loudspeakers 32,34 will generate either a magnetic north or south pole on-axis to the coil, depending on a phase connection of the speaker connections L+, L-, R+, R-. This magnetic polarity is detected by the respective Hall sensors 42,44 and is fed back via the control input of the processor 10 for analysis.

If both loudspeakers 32,34 are connected in phase, the respective Hall sensors 42,44 will detect the same polarity, either both magnetic north poles or both magnetic south poles. Otherwise, if opposite magnetic polarity is detected by the two Hall sensors 42,44, this means that one of the channel is connected with the wrong polarity. Thus, the audio output for one of the channels L and R will be inverted by the processor 10 so that both outputs will be in-phase with each other.

As an alternative example, the processor 10 may generate an audible output, e. g. via at least one of the loudspeakers 32,34, or a visible output, e. g. via a light emitting diode (LED) or the like, to thereby notify a user to manually change the connection of one of the loudspeakers 32,34.

Additionally, absolute direction of coil movement can be detected given the specifications of the loudspeakers 32,34, to thereby provide ability for absolute phase correction.

Accordingly, if the connection lines R+ and L+ are connected to the respective positive terminals of the respective loudspeakers 32,34, the processor 10 will judge correct connection of the speakers 32,34. Otherwise, if one of the connection lines R+ and L-is connected to the negative terminal, the processor 10 will determine an out-of-phase relationship and will correspondingly correct the phase of one of the signals L and R. If both connection lines R+ and L+ are connected to the respective negative terminals, the loudspeakers 32,34 are deemed in phase, but the absolute phase is wrong. This can be corrected if an ability for absolute phase correction is implemented in the processor 10.

It is noted that the above correction also enables phase correction of the signals if the loudspeakers 32,34 are connected in the right manner, but one of the signals L and R is supplied to the processor 10 at a wrong phase relationship.

Fig. 2 shows a schematic diagram of a cross section, which might correspond to each one of the loudspeakers 32,34, to indicate a possible location of the Hall sensors 42, 44. The bold arrows indicate magnetic flux directions generated by a voice coil 302 provided on a magnetic south pole region 304 of a permanent magnet of the loudspeaker. The voice coil 302 is movably arranged in a gap portion, which may be a circular gap portion, between the magnetic south pole region 304 and opposing north pole regions 308 of the permanent magnet. Furthermore, the voice coil 302 is fixed to a movable conical portion 306 of the loudspeaker. Based on the direction of current flowing through the voice coil 302, the movable conical portion 306 is either pushed in the upper direction or in the lower direction of Fig. 2. The current in the voice coil 302 is generated based on the respective signal supplied via the connection lines L+, L-or R+, R-, respectively. The dotted bold arrow in Fig. 2 indicates a first voice coil flux direction and the continuous bold arrow indicates a second voice coil flux direction, e. g. N (north) or S (south) direction, respectively, which depend on the polarity connection of the loudspeakers 32,34. This magnetic flux direction is detected by the Hall sensors 42,44 which are mounted e. g. on the dust cap on-axis to the voice coil 302. Of course, any other suitable position can be selected for mounting the Hall

sensors 42,44 or any other non-invasive current or magnetic field sensor adapted to detect the magnetic field generated by the voice coil 302 or the current flowing through the voice coil 302.

The invention is applicable for use in all systems with loudspeakers or other electro-acoustic transducers driven by conventional drivers, including cone speakers, flat panel speakers and the like. In particular, the present invention facilitates the implementation of automatic power-up diagnostics for speaker phase connection in audio systems.

In summary, the present invention relates to a detection system and method for detecting a phase difference between a first signal supplied to a first electro-acoustic transducer means and a second signal supplied to a second electro-acoustic transducer means, wherein first and second detection means are provided for detecting magnetic fields or currents induced by said first and second signals into the first and second electro-acoustic transducer means. A control output is generated based on a comparison of detection outputs of the first and second detection means. This control output can then be used for correcting the phase relationship between the first and second signals or for providing a corresponding audible or visible indication to a user. Thereby, detection and correction of out-of-phase connections of loudspeakers or other electro-acoustic transducers can be provided at moderate system configurations. This automatic detection and correction configuration leads to a user-friendly phase detection system.

It is noted that the present invention is not restricted to the above-preferred embodiment, and various modifications with respect to the location and kind of current or magnetic field sensors are intended to be covered by the invention. The preferred embodiments may thus vary within the scope of the attached claims.