Abstract:
A method and system for inhibiting noise produced by one or more sources of undesired sound from pickup by a speech recognition unit, where a respective transducer is located proximate each source of undesired sound for converting each source of undesired sound to a corresponding electrical signal, and a noise reduction system is coupled to each of the transducers for converting each electrical signal to an equivalent anti-phase electrical signal of equal amplitude. An output of the noise reduction or a signal corresponding thereto is fed to the speech recognition unit so each of the anti-phase electrical signals cancels or reduces a corresponding electrical signal produced by the speech recognition unit upon picking up the undesired sound from the respective source.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to noise reduction systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is known to use noise reduction systems in vehicles in order to reduce acoustic interference when using mobile telephones. JP 8248963 to Mazda Motor Corporation of Japan published Sep. 27, 1996 and entitled “Noise Reduction Device For Vehicle” describes a commonly used approach where the noise reduction device is provided with a microphone and a speaker, and is constituted so as to reduce a noise by generating a sound of a phase opposite to the noise in the vehicle collected by the microphone from the speaker. Likewise, JP 10254457 to Isuzu Motors Ltd. of Japan published Sep. 25, 1998 and entitled “Reducing Method of Noise Within Vehicle Compartment and Device Therefor” is directed to noise reduction over a wide frequency band by forming a canceling sound producing signal based on measured actual noise, producing a canceling sound from a speaker by using the canceling sound producing signal and interfering it with a noise.  
           [0003]    To this end, a microphone for detecting noise and a speaker for emitting a canceling sound producing means are arranged within a vehicle compartment. The microphone measures sound generated within the vehicle compartment, the phase and acoustic pressure are detected in two and more resonant frequency bands of cavity resonance and a controller produces a canceling sound producing signal. The speaker emits a canceling sound for interfering with the noise thereby reducing the noise.  
           [0004]    U.S. Pat. No. 5,485,523 (Tamamura et al.) published Jan. 16, 1996 and entitled “Active noise reduction system for automobile compartment” discloses an automobile compartment noise reduction system, where an ignition pulse signal is processed to obtain a vibration noise source signal with a frequency spectrum composed of 0.5 n (integers) order components of the engine r.p.m. as the primary source signal. The signal is applied to an adaptive filter and an LMS calculating circuit via a speaker-microphone transmission characteristic correcting circuit. The primary source signal is synthesized by the filter into a cancel signal and then outputted through a speaker as canceling sound. The canceling sound is received by at least one error microphone at a noise receiving point as an error signal. The error signal is applied to the LMS calculating circuit. The LMS circuit updates the filter coefficients of the adaptive filter on the basis of the primary source signal and the error signal so that the error signal can be minimized.  
           [0005]    Owing to the high background noise inside the vehicle compartment, it is very difficult for speech recognition systems to be effective. Noise sources include wind, road, air conditioning system, sound system.  
           [0006]    Currently available systems for reducing the background noise in speech recognition systems or in communication devices of the type described above use a directional microphone or several microphone elements, in order to increase the microphone sensitivity in the direction of the speaker. The limitation of such systems is that they cannot hear the passenger near the driver, and cannot reduce the noise coming from the car audio system.  
           [0007]    Other systems increase the speech recognition, using algorithms that are based on the speech statistics parameters. The limitation of this method is that its effectiveness is dramatically reduced in the presence of background noise.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object of the invention to provide an alternative method and system for inhibiting the effect of unwanted noise from being picked up by a speech operated system.  
           [0009]    The invention finds particular application when the speech operated system is an automatic system that is used to understand and execute voice instructions, for example, a vocally operated mobile telephone that is adapted to obey vocal instructions. Within the context of the following description and claims, such a system will be referred to as a speech recognition unit. However, the system also improves telephone communications in noisy environments such as vehicles, even though humans do understand even in a noisy environment. Machines do not have the intelligence to discriminate genuine acoustic signals from background noise, so while the present invention is beneficial for telephone pick up, its principal utility is for speech recognition systems.  
           [0010]    To this end there is provided in accordance with a first aspect of the invention a method for inhibiting noise produced by one or more sources of undesired sound from pickup by a speech recognition unit, the method comprising:  
           [0011]    obtaining separate electrical source signals each relating to a respective source of undesired sound,  
           [0012]    producing a plurality of anti-phase electrical signals each corresponding to a respective one of the electrical source signals and of equal amplitude thereto, and  
           [0013]    injecting into the speech recognition unit a signal corresponding to each of the anti-phase electrical signals so that it cancels or reduces a corresponding electrical signal produced by the speech recognition unit upon picking up the undesired sound from the respective source.  
           [0014]    A system in accordance with invention for inhibiting noise produced by one or more sources of undesired sound from pickup by a speech recognition unit, comprises:  
           [0015]    an electrical circuit responsive to corresponding electrical source signals each relating to a respective source of undesired sound for producing in respect of each of said electrical source signals an equivalent anti-phase electrical signal of equal amplitude fed to an output of the circuit;  
           [0016]    the output of the circuit or a signal corresponding thereto being fed to the telephone mouthpiece so that each of the anti-phase electrical signals cancels or reduces a corresponding electrical signal produced by the speech recognition unit upon picking up the undesired sound from the respective source.  
           [0017]    Preferably, the system is disposed within a vehicle for reducing extraneous background noise from being picked up by a mobile telephone pickup. Such a system is a source-orientated system that electrically or acoustically reduces the noise originating from dedicated noise sources within the vehicle, at the microphone point of speech recognition systems or communication devices such as cellular phones. Thus, according to one embodiment, an electrical output of the circuit is directly summed with the microphone electrical signal within the telephone pickup so to cancel or reduce a corresponding electrical signal produced thereby upon picking up the undesired sound from the respective source. Alternatively, the anti-phase electrical output may be converted back to an acoustic signal that is fed to the telephone pickup via a loudspeaker placed proximate thereto.  
           [0018]    In either case, a principal benefit of such a system is that the signal to noise ratio is increased at the pickup. This improves the performance of speech recognition systems and other devices that utilize microphones within a vehicle, while allowing the use of an omni-directional microphone within the telephone pickup. This is preferable to use of a uni-directional microphone, which is not only more expensive but less suitable than an omni-directional microphone in a vehicle environment where a motorist must be free to move his head relative to the telephone pickup.  
           [0019]    The system may operate by its own or as additional improvements to other technologies for signal to noise improvement.  
           [0020]    The system handles dedicated noise sources within a vehicle: air conditioning system, audio system, motor noise, vibrations at the microphone point etc. The advantage of the system is that there is a large reduction of the disturbing sources, while the microphone remains omni-directional.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:  
         [0022]    [0022]FIG. 1 is a schematic representation of a system according to the invention;  
         [0023]    [0023]FIG. 2 is a schematic representation of a first implementation of a system according to the invention for reducing multiple noise sources;  
         [0024]    [0024]FIG. 3 is a schematic representation of an alternative implementation of a system according to the invention for reducing multiple noise sources;  
         [0025]    [0025]FIG. 4 is a schematic representation of another implementation of a system according to the invention for reducing multiple noise sources. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    [0026]FIG. 1 is a schematic representation of a system  10  according to the invention comprising a speech recognition unit  11  having a microphone  12 . A noise source  13  is located within sufficient proximity to the speech recognition unit  11  that sound produced by the noise source  13  is capable of being picked up by the microphone  12  and interfering with its operation. A sensing microphone  14  is set up proximate the noise source  13  and is connected to a noise reduction system shown generally as  15  that includes an electronic circuit that is responsive to a noise signal fed thereto for estimating in real time a transfer function, which creates an active electrical signal representing an inverse of the noise signal. The output of the noise reduction system is multiplied by −1 (i.e. inverted) and converted to an equivalent analog signal by a D/A converter  16 , whose output is added directly to the microphone output  12  by an adder  17 .  
         [0027]    The noise reduction system  15  includes a digital filter such as finite impulse response (FIR) filter  18  having an input that is coupled to the output of the sensing microphone  14  and which is used to estimate the transfer function. The FIR filter  18  is controlled by a real time convergence system  19  whose input is likewise coupled to the output of the sensing microphone  14 . The output of the FIR filter  18  is inverted by an inverter  20 . The real time convergence system  19  receives a feedback signal from the analog input signal fed to the speech recognition unit and which is digitized by an A/D converter  21 . The real time convergence system  19  converges to the transfer function and may be based on any of several well-known convergence algorithms such as LMS (least mean square), RLS and so on.  
         [0028]    [0028]FIG. 2 is a schematic representation of a first implementation of a system  30  according to the invention for reducing multiple noise sources. Identical reference numerals are used to refer to components that are common to the system  30  and the system  10  described above with reference to FIG. 1 of the drawings. Thus, the system  30  includes a speech recognition unit  11  having a microphone  12 , which is to be isolated from acoustic noise generated by multiple noise sources designated as  13   a  . . .  13   n . A respective sensing microphone  14   a  . . .  14   n  is placed near each of the n noise sources, each microphone being connected to a respective noise reduction system  15   a  . . .  15   n  whose respective analog outputs are summed together with the analog signal fed to the equipment microphone  12  by an adder  17 . An analog output of the adder  17  is converted to an equivalent digital signal by an A/D converter  21 , and fed back to each of the noise reduction systems  15   a  . . .  15   n.    
         [0029]    The noise reduction systems  15   a  . . .  15   n  may be identical to that described above with reference to FIG. 1 or it may be a multi-channel unit having n-inputs for receiving the n noise signals, an input for receiving the feedback signal and an output for producing a composite output signal. In the case that discrete noise reduction systems are used, as shown in FIG. 2, it is also possible to use a separate A/D converter for each noise reduction system.  
         [0030]    [0030]FIG. 3 is a schematic representation of a second implementation of a system  40  according to the invention for reducing multiple noise sources, wherein noise reaching the microphone  12  of the speech recognition unit  11  is reduced without actually connecting to the microphone  12 . Again, identical reference numerals are used to refer to components that are common to the systems  10  and  30  described above with reference to FIGS. 1 and 2 of the drawings.  
         [0031]    All of the digital outputs of the noise reduction systems  15   a  . . .  15   n  are summed together by a summation unit  41  producing a composite summed output signal that is converted to an equivalent analog signal by a D/A converter  16 . It is also possible to use separate D/A converters for each of the noise reduction systems  15   a  . . .  15   n  as explained above with reference to FIG. 2. The output of the D/A converter  16  is amplified by an amplifier  42  and vocalized by a loudspeaker  43  located proximate the microphone  12  for generating counter noise that cancels the acoustic noise picked up by the microphone  12 . A feedback microphone  44  is likewise located near the loudspeaker  43  for generating the analog feedback signal, which is digitized by the A/D converter  21  and fed to the noise reduction systems  15   a  . . .  15   n . The sensing microphones  14   a  . . .  14   n  are preferably uni-directional microphones that are located proximate the microphone  12  of the speech recognition unit  11 . By such means, each sensing microphone  14   a  . . .  14   n  senses the actual noise that reaches the speech recognition unit  11  so that the counter-noise output by the loudspeaker  43  exactly nullifies the effect of the respective noise component reaching the speech recognition unit  11 .  
         [0032]    [0032]FIG. 4 is a schematic representation of a third implementation of a system  50  according to the invention for reducing multiple noise sources, wherein noise reaching the microphone  12  of the speech recognition unit  11  is reduced without actually connecting to the microphone  12 . However, the system  50  is adapted for use with a digital speech recognition unit where the input signal is digital rather than analog. Again, identical reference numerals are used to refer to components that are common to the systems  10 ,  30  and  40  described above with reference to FIGS. 1, 2 and  3  of the drawings.  
         [0033]    All of the digital outputs of the noise reduction systems  15   a  . . .  15   n  are fed to a two-way digital communication port  51 . In a single channel system, the digital communication port  51  conveys the digital inverse signal directly to the speech recognition unit  11 . In a multi-channel system, the digital communication port  51  is coupled to the output of an adder  52  that serves as a summing unit for summing the digital inverse signals digitally. In either case, since the speech recognition unit  11  is digital, there is no need for A/D and D/A conversion. The digital output of the digital communication port  51  is fed directly as an electrical digital signal to the speech recognition unit  11 . The digital communication port  51  outputs a composite error digital signal that is fed back to all of the noise reduction systems  15   a  . . .  15   n  in a similar manner to what is done in the analog system  40  shown in FIG. 3. Clearly, in a single channel system only a single feedback signal is produced and fed to the single noise reduction system.  
         [0034]    In any of the systems  10 ,  30 ,  40  or  50  one or more sensing microphone  14  may be replaced by a transducer for producing an analog signal representative of a physical property that manifests itself as acoustic noise. For example, one or more sensing microphones  14  might be replaced by an accelerometer for sensing acoustic noise derived from vibrations that must be reduced at the microphone  12  of the speech recognition unit  11 . Alternatively, one or more of the sensing microphones  14  might be replaced by direct connection to a loudspeaker in a sound system. It will be appreciated that the noise reduction system  15  may also include different kinds of input transducers at the same time. It is also possible to dispense with any of the microphones or other transducers in any of the systems if direct access is possible to an electrical source signal relating to a respective source of undesired sound.