DE4021787A1 - Electro-acoustic system e.g. for telephone appts. - controls coupling between loudspeaker and microphone via space to suppress acoustic feedback using filter and/or modulator - Google Patents

Abstract

The electroacoustic system processes acoustic signals by means of electroacoustic components ready for transmission. The system includes at least one closed feedback loop (1) which is formed on the basis of acoustic and electrical coupling effects. At least two frequency-controlled components (10) e.g. comb filter, modulator are included in each closed feedback loop (1) in order to suppress acoustic feedback noise by ensuring that the loop gain in all feedback loops is less than one. The feedback loop (1) forms its acoustic coupling between a loudspeaker (6) and microphone (7), through space (4), and its electrical coupling between two amplifiers (8), through a bifurcation circuit (5). USE - For telephone systems, including loudspeaker telephones etc.

Description

In electro-acoustic systems - for example telephone systems or radio systems - there are usually several closed feedback loops due to acoustic and electrical couplings. With unfavorable amplification conditions of the electro-acoustic components, the loop reinforcements of the feedback loops can be greater than 1; As a result, vibrations can form that lead to unwanted and disruptive echoes - the whistling tone on the radio with simultaneous phone calls is an example. In particular, especially with the hands-free phone - as can be seen from the schematic drawing in FIG. 3, the closed feedback loop 1 is formed as a result of the acoustic coupling through the space 4 and the electrical coupling through the hybrid circuit 5 - an annoying whistle occurs due to the spatial arrangement of the system components - loudspeaker 6 and microphone 7 are arranged directly next to one another in a housing - and due to the necessary amplification conditions of the two amplifiers 8 for operating the telephone device.

To disturbing feedback effects in electro-acoustic To avoid systems, it is known to carry over supply properties of the system by means of a so-called to reproduce the compensation filter and that through  this filter transmitted interference signal from the actual one Subtract transmission signal - the useful signal; this is realized in that the feedback loop is separated at one point and the at separated the signals using correlative methods will. However, this method requires a high one Expenditure; because the transmission properties of the electro acoustic systems are often time-varying Formation of the compensation filter very adaptive filter high order needed. Because of the large space Such compensation circuits may do so not - which is often desired and required - in egg integrated IC. There is also a separation of useful and interference signals using correlative methods only incompletely realizable because the theory stocha stic signals only very limited to speech signals is reversible; only short-term correlations are technical possible. With reasonable effort with such methods, the disturbing feedback not suppress or insufficient echo achieve damping.

It is also known to use time-dependent damping that integrate into the feedback loop to get there by reducing the loop gain and thus to prevent the feedback mechanism. Example wise in hands-free phones by so-called "Voice switch" circuits microphone and speaker branch depending on the operating mode (transmit or Receiving mode) mutually damped. However then full duplex operation is not possible, i. H. at the same time Hearing and speaking is when using such Circuits not possible. Unfavorable background noise sche - for example the sounds of typewriters or pneumatic hammers - lead to malfunctions in the  Signal detection and thus to the failure of the communications tion. If both transmit and receive signals before there are problems with the assignment of the Be drive state; for example, always the lukewarm other speakers at the expense of the other participant switched. When the operating status changes, the Choosing the right time for switching is problematic - for example, too slow or too fast on speak on operating status changes for ver swallow starting or ending syllables.

The invention has for its object a verbes specified electro-acoustic system, in which the disadvantages mentioned above can be avoided. This object is achieved by the features solved in the characterizing part of claim 1. Advantageous further developments of the invention result from the subclaims.

The arrangement for interference suppression according to the invention in electro-acoustic systems combines several pre parts in:

• - It is time invariant, so there are no adaptations necessary, the settling times are very short.
• - It can be implemented with little effort, the required components can be in one circuit circle (IC) can be integrated.
• - A full duplex operation, i.e. H. the simultaneous Sending and receiving signals - a comm communication in both directions - is always possible Lich.  
• - It needs - regardless of the environmental conditions conditions - to be set only once and is in contrast to known circuits extremely disturbing insensitive.

According to the invention, the interference suppression is as follows ß achieved: by the first frequency influencing Component - preferably a frequency selective construction element - becomes the transmission behavior of the electro acoustic system in that a alternating sequence of pass and blocking areas in the Frequency band of the transmission signal is formed. The Exclusion areas and passband areas are so ge chooses that on the one hand the system no longer vibrates can train - d. H. the damping of the system is in entire useful frequency range larger than the loops amplification - but still the useful signal is sufficiently let through. By the second fre Quence-influencing component - either another frequency selective component or a frequency ver pushing component - are created feedback lung echoes so changed that they are also in the back coupling loop are damped.

The number or type of frequency influencing Components - preferably filters as frequency selective Components and modulators as frequency shifting Components - can depend on the complexi system or its transmission behavior be given arbitrarily.

For example, as frequency-selective components Comb filters - these are pass-through and blockers range in the frequency domain in a periodic sequence arranged - or filter with a non-periodic Arrangement of the passband and restricted areas who selected  the. For example, the respective width of the Passband depending on the frequency value The beginning of the pass band can be selected, for example always as a percentage of this frequency value have the same relative amount.

As frequency-shifting components, for example wise modulators can be selected, with either On sideband modulators Frequency shift only in one direction - or Double sideband modulators - the Fre quenzband relative to the center frequency of the Durchlaßbe Reich shifted in both directions - questionable com men. Here too, the frequency shifting Elements either in the entire frequency range a con constant displacement can be specified or the ver shift is dependent on the respective frequency value made - for example always around the same relative proportion. The characteristic of Fre the frequency shift depends on the desired th sound quality of the transmitted acoustic In formation.

The invention will be explained in more detail below with reference to FIGS. 1 to 3. It is in the

Fig. 1, the operation of two comb filters in the frequency range, in the

Fig. 2 shows the operation of a Kammfil age and a modulator in the frequency domain and in the

Fig. 3 shows a hands-free phone arrangement as an embodiment of an electro-acoustic system's.

In the simplest version, the two frequency-influencing components 10 in the feedback loop 1 of an electro-acoustic system are implemented by two comb filters 2 , which according to FIG. 1 in the frequency range a - mutually complementary - periodic sequence of pass bands 2 a and blocking areas 2 b exhibit. Each comb filter 2 thereby suppresses one half of the frequency spectrum, and consequently prevents the occurrence of feedback vibrations. With this arrangement, a duty cycle of the filter - this indicates the ratio of passband to stopband bandwidths - of 1: 1. Although complete speech intelligibility can be achieved, the listening comfort - in the sense of optimal sound quality - can still be improved.

The duty cycle can be reduced by introducing modulators the filter significantly increase and thereby the sound quality improve the electro-acoustic system.

In the embodiment of FIG. 2, the frequency-influencing elements consist of a comb filter 2 and a modulator 3 ; With this arrangement, for example, a duty cycle of 3: 1 can be achieved, but disturbing echoes are still completely suppressed.

In Fig. 3 a hands-free phone is provided as an embodiment of an electro-acoustic system, which a closed feedback loop 1 due to the acoustic coupling between speaker 6 and microphone 7 through the room 4 and the electrical coupling between the two amplifiers 8 through the hybrid 5 has.

Both in the "microphone" branch and in the "loudspeaker" branch, two modulators 3 and a filter 2 are arranged as frequency-influencing components 10 . This arrangement is suitable to prevent feedback effects in the feedback loop 1 in such a system, so that full speech suppression can be achieved with full speech intelligibility.

The arrangement for interference suppression according to the invention can be used with a wide variety of electro-acoustic systems men - which are usually more than just a closed one Show feedback loop - are used. For each closed feedback loop min at least two frequency-influencing components, of which at least one frequency-selective component, vorese will be.

For example, with hands-free telephones, Mobile phones make calls with good ver consistency can be significantly improved with telephone Switching systems, radio systems or radio devices the disturbing feedbacks are completely under presses.

Claims (13)

1. Electro-acoustic system which processes and transmits acoustic signals by means of electro-acoustic components, and which has at least one closed feedback loop, which is formed due to acoustic and electrical coupling effects, characterized in that to suppress disturbing acoustic cal feedback effects in each closed Feedback loop ( 1 ) of the electro-acoustic system at least two frequency-influencing components ( 10 ) are provided, which are chosen such that the loop gain in all closed feedback loops ( 1 ) is less than 1. 2. Electro-acoustic system according to claim 1, characterized in that as frequency-influencing components ( 10 ) frequency-selective components ( 2 ) and frequency-shifting components ( 3 ) are provided, and that each closed feedback loop ( 1 ) at least one frequency-selective component ( 2 ) contains. 3. Electro-acoustic system according to claim 1 or 2, characterized in that the frequency-selective construction elements ( 2 ) and frequency-shifting components ( 3 ) are time invariant. 4. Electro-acoustic system according to one of claims 1 to 3, characterized in that the frequenzselek tive component ( 2 ) is a filter rich in the frequency range a plurality of pass bands ( 2 a) with low Si signal attenuation by blocking areas ( 2nd b) are separated from one another with high signal attenuation. 5. Electro-acoustic system according to claim 4, characterized in that a comb filter is seen as a filter ( 2 ), which has a periodic arrangement of the pass regions ( 2 a) and blocking regions ( 2 b) ( Fig. 1). 6. Electro-acoustic system according to claim 4, characterized in that the sequence of the pass regions ( 2 a) and blocking regions ( 2 b) of the filter is non-periodic. 7. Electro-acoustic system according to claim 6, characterized in that the widths (Δf) of the pass areas ( 2 a) depend on those frequency values (f _A ) at which the pass ranges ( 2 a) begin. 8. Electro-acoustic system according to one of claims 1 to 7, characterized in that the frequenzver shifting elements ( 3 ) move the sequence of the passband ( 2 a) and blocking areas ( 2 b) by a constant frequency frequency amount ( Fig. 2) . 9. Electro-acoustic system according to one of claims 1 to 7, characterized in that the frequenzver shifting elements ( 3 ) shift the sequence of the pass bands ( 2 a) and blocking areas ( 2 b) such that the amount of the frequency shift of the Output frequency depends. 10. Electro-acoustic system according to one of claims 1 to 9, characterized in that the frequenzver shifting elements ( 3 ) are modulators. 11. Electro-acoustic system according to claim 10, characterized in that the modulators ( 3 ) are single band modulators. 12. Electro-acoustic system according to claim 10, characterized in that the modulators ( 3 ) Zweisi tenband modulators. 13. Use of an electro-acoustic system after one of claims 1 to 12 to avoid disruptive acoustic feedback effects or echoes in tele systems, radio systems or radios.