Patent Publication Number: US-9888335-B2

Title: Method and apparatus for processing audio signals

Description:
RELATED APPLICATION 
     This application was originally filed as PCT Application No. PCT/FI2009/050559 filed Jun. 23, 2009. 
     The present invention relates to processing of audio signals. 
     BACKGROUND 
     An audio processing system may comprise several microphones arranged to capture several audio signals. The audio signals may be processed for transmission via a transmission path at a high bit rate. 
     However, there may be a need to reduce the bandwidth needed for transmitting the audio signals. The audio processing system may be used e.g. as a part of a teleconference system. 
     It is known that parametric coding techniques, e.g. binaural cue coding (BCC), may be used to reduce the bit rate in multi-channel audio transmission. 
     SUMMARY 
     An object of the present invention is to provide apparatus for processing audio signals. A further object of the invention is to provide a method for processing audio signals. 
     According to a first aspect of the invention, there is provided an apparatus disclosed herein. 
     According to a second aspect of the invention, there is provided a method disclosed herein. 
     According to a third aspect of the invention, there is provided a computer program disclosed herein. 
     According to a fourth aspect of the invention, there is provided a computer readable medium disclosed herein. 
     An apparatus according to the present invention ( 300 ) may comprise:
         one or more inputs (IN 1 , IN 2 ) to receive two or more different audio signals (S 1 , S 2 ),   an input (IN VDI1 ) to receive a direction signal (S VDI1 ), and   a signal processing unit ( 100 ) arranged to generate a processed audio signal (S AUDIO1 ) from said two or more different audio signals(S 1 , S 2 ), said processed audio signal (S AUDIO1 ) comprising an enhanced audio signal (S ENC1 ) corresponding to a sound (SND 2 ) originating from a location (x 2 ,y 2 ) indicated by said direction signal (S VDI1 ).       

     Thanks to enhancing an audio signal based on a direction signal provided by a direction indicator, the total bit rate needed for the audio transmission may be reduced. The information of the desired direction of arrival may be utilized to improve the quality of the spatial audio coding and representation. 
     Conventional spatial audio coding schemes treat the whole audio scene equally with the intention to represent the whole audio image at the best possible perceptual quality at a given bit rate. However, e.g. 
     for conversational services there may be a need to represent only the essential content of interest, and to consider the remaining audio scene as ambience in order to optimize the audio quality in the direction of interest. 
     The direction of interest within an auditory image may be determined by using a direction detecting unit. For example, the direction detecting unit may be a gaze direction detecting device. 
     The auditory image may be captured by concentrating on the determined direction of interest, e.g. by using a directional microphone array. 
     Audio signal components of the auditory image in the determined direction of interest may be encoded using a higher bit rate, wherein the remaining audio components may be encoded at a lower bit rate. In other words, audio signals originating from a selected direction of arrival may be coded more accurately than the rest of the audio image. 
     In an embodiment, the direction signal may be provided by a gaze direction tracking device. Consequently, an audio signal in the direction of interest may be enhanced. As an example, a participant of a teleconference may enhance the voice of a most relevant speaker simply by looking at said speaker or by looking at a displayed image of said speaker. 
     In an embodiment, a direction signal provided by the direction indicator may be used to steer the direction of maximum sensitivity of a directional microphone array. 
     The system and the method according to the invention may provide efficient and flexible coding of spatial audio content concentrated towards the direction of interest in a surrounding audio scene. Spatial audio parameterization may be extracted from the direction of interest indicated by a gaze direction detecting device or other means for pointing out the direction of interest. In an embodiment, the rest of the audio scene may be handled with coarser parameterization and coding. In an embodiment, the rest of the audio scene may be handled as ambience noise with minimum number of parameters and with a low bit rate. 
     The embodiments of the invention and their benefits will become more apparent to a person skilled in the art through the description and examples given herein below, and also through the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following examples, the embodiments of the invention will be described in more detail with reference to the appended drawings, in which 
         FIG. 1 a    shows an audio processing system, wherein a direction selecting unit is located on the transmitting side of the system, 
         FIG. 1 b    shows an audio processing system, wherein a direction selecting unit is located on the receiving side of the system, 
         FIG. 2  shows the gaze direction of an eye, 
         FIG. 3  shows a gaze direction detecting device, 
         FIG. 4  shows an image of an eye, as captured by an image sensor of the gaze direction detecting device, 
         FIG. 5 a    shows an eye looking at real objects, wherein the gaze direction of the eye is monitored by a gaze direction detecting device, 
         FIG. 5 b    shows an eye looking at images of objects, wherein the gaze direction of the eye is monitored by a gaze direction detecting device, 
         FIG. 5 c    shows an eye looking at virtual images of objects, wherein the gaze direction of the eye is monitored by a gaze direction detecting device, 
         FIG. 6  shows an audio processing system, 
         FIG. 7 a    shows an audio processing system comprising a directional microphone array, 
         FIG. 7 b    shows an audio processing system comprising a directional microphone array 
         FIG. 7 c    shows an audio processing system comprising a directional microphone array and a filtering unit, 
         FIG. 8 a    shows a parametric audio encoder, 
         FIG. 8 b    shows a parametric audio encoder arranged to provide spatial audio parameters based on audio signals captured by additional microphones, 
         FIG. 8 c    shows a a directional microphone array arranged to provide a downmixed signal for parametric audio encoding, 
         FIG. 8 d    shows adjusting spatial audio parameters based on a direction signal, and 
         FIG. 9  shows creating a virtual sound field to a listener based on the position and orientation of said listener. 
     
    
    
     All drawings are schematic. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 a   , an audio processing system  300  may comprise two or more microphones M 1 , M 2 , M 3 , a direction selecting unit VDI 1 , a signal processing unit  100 , and one or more electro-acoustic transducers SPK 1 , SPK 2 . 
     Sounds may originate from sound sources A 1 , A 2 , A 3 , which are in different spatial locations (x 1 ,y 1 ), (x 2 ,y 2 ), (x 3 ,y 3 ). The sound sources A 1 , A 2 , A 3  may be any audio sources in an auditory scene, e.g. participants attending a meeting. 
     Sounds SND 1 , SND 2 , SND 3  emitted from the sources A 1 , A 2 , A 3  may be captured by the microphones M 1 , M 2 , M 3  in order to provide audio signals S 1 , S 2 , S 3 , respectively. 
     The microphones M 1 , M 2 , M 3  may have different sensitivities for sounds emitted from the sources A 1 , A 2 , A 3  e.g. due to different distances between the sound sources and the microphones, due to directional emission patterns of sound sources, and/or due to directional sensitivity of a microphone. 
     For example, the first microphone M 1  may be sensitive primarily to sounds emitted from the first source A 1 , and the first microphone M 1  may be less sensitive to sounds emitted from the second source A 2  and the third source A 3 . The second microphone M 2  may be sensitive to sounds emitted from the second source A 2 , the third microphone may be sensitive to sounds emitted from the third source A 3 , etc. 
     The audio signals S 1 , S 2 , S 3  may be converted into digital form, and processed for transmission and/or storage e.g. in a memory device. If desired, a processed audio signal S AUDIO1  corresponding to said audio signals S 1 , S 2 , S 3  may be sent via a transmission path  400 . In order to reduce the total bit rate required for the transmission, the signal processing unit  100  may be arranged to allocate a higher bit rate for audio signals originating from a selected spatial location indicated by the direction selecting unit VDI 1 , and the signal processing unit  100  may be arranged to allocate a lower bit rate for audio signals originating from the other locations. 
     In particular, the signal processing unit  100  may be arranged to enhance an audio signal originating from a spatial location indicated by the direction selecting unit VDI 1 . In other words, the signal processing unit  100  may be arranged to suppress audio signals originating from the other locations. 
     The direction selecting unit VDI 1  provides a signal S VDI1 , which may comprise information about the direction of the selected spatial location with respect to a reference direction. The direction may be expressed e.g. by a horizontal angle α between the direction SY and a line SELD drawn from the direction selecting unit VDI 1  to the selected location. 
     The signal S VDI1  may also comprise information about a vertical angle between the direction SY and the line SELD. 
     The direction selecting unit VDI 1  may be operated by a user A 5 . The direction selecting unit VDI 1  may be e.g. a gaze direction detecting device, a satellite navigation device, an electronic compass, a gyroscopic compass, or an integrating accelerometer. The electronic compass may comprise e.g. a magnetic field sensor based on the Hall effect. 
     The direction selecting unit VDI 1  may be arranged to detect the gaze direction of the user A 5 , or the user may manually direct a compass or accelerometer towards the selected location. A satellite navigating device, e.g. a GPS device (Global Positioning System) may be moved in a desired direction in order to provide a direction signal S VDI1 . 
     The signal processing unit  100  may comprise a filtering unit  20 . The filtering unit  20  may comprise a set of filters F 1 , F 2 , F 3 . An audio signal S 1  captured by the first microphone M 1  may be filtered by a first filter F 1 , an audio signal S 2  captured by the second microphone M 2  may be filtered by a second filter F 2 , and an audio signal S 3  captured by the third microphone M 3  may be filtered by a third filter F 3 . 
     The filters F 1 , F 2 , F 3  of the filtering unit  20  may be arranged to change the level of at least one of the audio signals (e.g. S 2 ) with respect to the other audio signals (e.g. S 1 , S 3 ) such that an audio signal originating from a spatial location indicated by the direction selecting unit VDI 1  may be enhanced. The filters may change the level of at least one of the audio signals S 1 , S 2 , S 3  according to to the direction signal SVDI 1  provided by the direction selecting unit VDI 1 . 
     Each filter F 1 , F 2 , F 3  may comprise a filter coefficient or coefficients k 1 , k 2 , k 3 . The symbol k 1  may denote a single scalar multiplier. The coefficients k 1 , k 2 , k 3  may be scalar multipliers. For example, the audio signal S 1  may be multiplied by a first filter coefficient k 1 , the audio signal S 2  may be multiplied by a second filter coefficient k 2 , and the audio signal S 3  may be multiplied by a third filter coefficient k 3  so as to enhance the selected audio signal. 
     Each symbol k 1 , k 2 , k 3  may also denote a set of filter coefficients. In particular, the symbol k 1  may denote an array representing coefficients of a digital filter F 1 . For example, when a direction corresponding to the location (x 2 ,y 2 ) of the second source A 2  has been selected, then the signal processing unit  100  may be arranged to set the values of second filter coefficients k 2  greater than the values of first filter coefficients k 1  and third filter coefficients k 3 . Consequently, the level of the audio signal S 2  of the second microphone M 2  may be selectively enhanced in order to provide an enhanced audio signal S ENC . 
     The levels of the audio signals may be adjusted when they are in analog or digital form. 
     The signal processing unit  100  may comprise an encoder  30 . The encoder  30  may be a parametric encoder (see  FIG. 8 a   ). In particular, the encoder may be arranged to provide a binaural cue coded signal (BCC). The encoder  30  may be arranged to convert time domain signals into frequency domain. The levels of the audio signals may also be changed in the frequency domain instead of adjusting the levels in the time domain. For example, fourier transformed signals may be multiplied in the frequency domain by coefficients k 1 , k 2 , k 3 , . . . instead of multiplying the audio signals S 1 , S 2 , S 3  in the time domain. 
     The signal processing unit  100  may be arranged to provide a processed audio signal S AUDIO1 , which comprises an enhanced audio signal S ENC  corresponding to sounds originating from a location indicated by the direction signal. For example, the enhanced audio signal S ENC  may correspond to sounds SND 2  originating from a location (x 2 ,y 2 ). 
     The processed audio signal S AUDIO1  may be e.g. a monophonic audio signal. A monophonic audio signal S C1  may be reproduced via a single transducer SPK 1 . However in that case the auditory image is not reproduced at the receiving end of the system  300 . 
     When the receiver decodes and renders only the downmixed signal, the listener may concentrate only to the audio source, which has been selected by the direction selecting unit VDI 1 . 
     In order to reproduce spatial effects, the system  300  may comprise a decoder  200  arranged to provide two or more audio signals based on a coded audio signal S AUDIO1  The separated audio signals may be reproduced via two or more electro-acoustic transducers SPK 1 , SPK 2  so that a listener A 4  at the receiving end of the system  300  may hear the reproduced audio image. The transducers SPK 1 , SPK 2  may be e.g. loudspeakers or headphones. 
     The coded audio signal S AUDIO1  may be binaural cue coded (BCC), and the decoder  200  may be arranged to convert the coded audio signal S AUDIO1  into two different audio channels for stereo reproduction via the transducers SPK 1 , SPK 2 . 
     The processed audio signal S AUDIO1  may be binaural cue coded (BCC), and the decoder  200  may be arranged to convert the audio signal S AUDIO1  into three or more audio channels for reproduction via loudspeakers. For example, the decoder  200  may be arranged to convert the audio signal S AUDIO1  into 5.1 surround sound or 7.1 surround sound. A 5.1 surround sound system has five loudspeakers positioned at different directions with respect to a listener, and a low frequency effects channel (LFE). A 7.1 surround sound system has seven loudspeakers positioned at different directions with respect to a listener, and a low frequency effects channel (LFE). 
     In general, the decoder  200  may be arranged to provide 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more different audio channels based on a processed audio signal S AUDIO1    
     If desired, the signal S VDI1  provided by the direction selecting unit VDI 1  may also be transmitted via the transmission path  400 . The direction signal S VDI1  may be coupled to the decoder  200 . Consequently, a reproduced audio signal can be rendered according to the direction indicated by the direction signal S VDI1 , if desired. 
     The audio signals S 1 , S 2 , S 3  captured by the microphones M 1 , M 2 , M 3  may be coupled to respective inputs IN 1 , IN 2 , IN 3  of the signal processing unit  100 . The signal processing unit  100  may comprise an interface IF 1  for providing the processed audio signal S AUDIO1  for transmission by a transmitter (not shown) and/or for receiving the direction signal S VDI1  from a receiver (not shown). However, the signal processing unit  100  may comprise a transmitter and/or the decoder  200  may comprise a receiver. 
     The signal processing device  100  may comprise a memory MEM 1  for e.g. storing computer program code for operating said signal processing device  100 . 
     If the distance W VM  between the direction indicator VDI 1  and the microphones M 1 , M 2 , M 3  is large when compared with the minimum distance L 1  between the sources A 1 , A 2 , A 3  and the microphones M 1 , M 2 , M 3 , then the data processing unit  100  may be arranged to take into account the position of the direction indicator VDI 1  with respect to the microphones M 1 , M 2 , M 3  and/or the distances between the sound sources A 1 , A 2 , A 3  and the microphones. 
     The distance WVM between the direction indicator VDI 1  and the microphones M 1 , M 2 , M 3  is preferably smaller than 25% of the minimum distance L 1  between the sources A 1 , A 2 , A 3  and the microphones M 1 , M 2 , M 3 . 
     If desired, the audio signals S 1 , S 2 , S 3  may also be multiplexed before coupling to the signal processing device  100  via a single input IN 1 . The signal processing device  100  may comprise a demultiplexer arranged to distribute the audio signals S 1 , S 2 , S 3  to different filters F 1 , F 2 , F 3 , respectively. 
     SX, SY, and SZ denote orthogonal directions. The direction SY may be interpreted e.g. as a reference direction. The direction SX may be interpreted e.g. as a horizontal direction, and the direction SZ may be interpreted e.g. as a vertical direction. 
     Referring to  FIG. 1 b   , the direction signal SVDI 1  may also be sent via a transmission path  400 . In particular, the direction selecting unit VDI 1  may be operated by the listener A 4  at the receiving end of the system  300 . 
     The direction indicator VDI 1  may be located on the receiving side of the system  300 , and the direction signal SVDI 1  may be sent via the transmission path  400  to the transmitting side in order to control the signal processing unit  100 . 
     The system  300  may further comprise a camera CAM 1  for capturing visual images corresponding to the audio sources A 1 , A 2 , A 3 . The camera CAM 1  may be arranged to send an image signal S IMG1  via the transmission path  400  to the receiving side. A respective image corresponding to the audio sources may be displayed e.g. on a screen (see  FIG. 6 ). The signals S AUDIO1 , S IMG1 , S VDI1  may also be sent via different transmission paths  400 . For example, the visual image signal S IMG1  and the processed audio signal S AUDIO1  may be sent via a television broadcasting network, and the direction signal S VDI1  may be sent from a remote listener A 4  to a TV studio via a mobile telephone network. 
     Referring to  FIG. 2 , the direction signal SVDI 1  may be provided e.g. by using a gaze direction detecting device.  FIG. 2  shows the gaze direction GZD of any eye E 1 . By monitoring the gaze direction GZD, it may be determined whether the eye E 1  is looking at an object A 1 , A 2 , or A 3 . In particular, the objects may be audio sources. The gaze direction GZD may be defined e.g. by a horizontal angle α between the gaze direction GZD and a reference direction REF 1 , and by a vertical angle β between the gaze direction GZD and the reference direction REF 1 . The reference direction REF 1  may be e.g. aligned with the direction SY. 
       FIG. 3  shows a gaze direction detecting device  700 . The gaze direction detecting device  700  may comprise one or more light sources  710 ,  720  and an imaging unit  730 . The light sources  710 ,  720  may be arranged to emit light beams LB 1 , LB 2 . 
     The gaze direction detecting device  700  may further comprise an image analysis unit  750  to determine the gaze direction GZD on the basis of an image IMG E1  captured by the imaging unit  730 . The gaze direction detecting device  700  may further comprise a damping unit  770  to reduce fluctuations in the direction signal S VDI1 . 
     The light sources  710 ,  720 , the image analysis unit  750  and/or the damping unit  770  may also be external components. For example the sun or another external substantially point-like light source may also be utilized as a light source  710 . 
     In particular, the light beams LB 1 , LB 2  provided by the light sources  710 ,  720  may be substantially collimated at the position of the eye E 1  in order facilitate accurate determination of the gaze direction GZD. 
       FIG. 4  shows an image IMG E1  of the eye E 1  captured by the imaging unit  730  of the gaze direction detecting device  700 . Light emitted from a light source  710 ,  720  is reflected from the surface of the eye E 1 . Consequently, the image IMG E1  may comprise one or more reflection spots G 1 , G 2  known as the Purkinje images. The horizontal gaze angle α and/or the vertical gaze angle β may be determined based on the position of the pupil P with respect to the Purkinje images G 1 , G 2 . 
     The use of two or more Purkinje images G 1 , G 2  improves accuracy and may make the determination of the gaze angles α and β substantially independent of the distance between the eye E 1  and the tracker device  700 . However, in some embodiments of the invention, a lower accuracy may be tolerated, and it may be sufficient if the gaze angles α and β are estimated by using only one Purkinje image G 1  or G 2 . 
     Some mobile telephones comprise a camera unit. Even the camera unit of a mobile telephone may be used as an imaging unit of a gaze direction detecting device  700  if a signal processing device is arranged to determine the gaze direction based on an image IMG E1  captured by said camera unit. Signal processing capabilities of said mobile telephone or an external portable computer may be used for analyzing the gaze direction. 
     Referring to  FIG. 5 a   , a gaze direction detecting device  700  may be arranged to detect whether the eye E 1  is looking at the location of the sound source A 1 , A 2 , or A 3 . The direction selecting unit VDI 1  may be located at the transmitting side of the audio transmission system  300 . The direction selecting unit VDI 1  may be a gaze direction detecting device  700  arranged to monitor the gaze direction of the user A 5  ( FIG. 1 ). 
     The direction selecting unit VDI 1  or the signal processing unit  100  may comprise a damping unit  770  to eliminate rapid fluctuations of the direction signal S VDI1 , because rapid fluctuations in the audio image experienced by the listener A 4  may be rather annoying. For example, the direction selecting unit VDI 1  may be arranged such that the eye E 1  has to look at new location at least during a predetermined time period before the value of the direction signal S VDI1  is changed. The predetermined time period may be e.g. 10 seconds. The signal processing unit  100  may be arranged such that the eye E 1  has to look at new location at least during a predetermined time period before the values of the filter coefficients k 1 , k 2 , k 3  of the filtering unit  20  are altered. 
     Alternatively, the system  300  may comprise several direction selecting units VDI 1  operated by several users, and the direction signal S VDI1  may be determined by voting. In other words, the total range of possible directions may be divided into a set of adjacent sectors, and the number of direction selecting units indicating directions within each range may be counted. A direction corresponding to the sector with the highest count may be used to indicate the selected location. For example, the gaze direction of e.g. ten participants may be monitored, and if e.g. six of them are looking at a certain location, then the signal processing unit  100  may be arranged to enhance audio signals originating from said location. For monitoring the gaze directions, images of the eyes of several participants may be captured simultaneously by a single camera, if sufficient image resolution can be provided. 
     Referring to  FIG. 5 b   , visual images IMG 1 , IMG 2 , IMG 3  corresponding to the audio sources A 1 , A 2 , A 3  may be also be displayed on a screen SCR 1 , and the gaze direction may be determined by a gaze direction detecting device  700 . 
     Referring to  FIG. 5 c   , virtual images IMG 1 , IMG 2 , IMG 3  corresponding to the audio sources A 1 , A 2 , A 3  may also be displayed by a virtual display  800 . The listener A 4  may place the virtual display near his eye E 1  such that when light provided by the virtual display impinges on his eye, he perceives an impression of a large virtual image displayed at an infinite distance. 
     For example a person may wear goggles  900 , which comprise a gaze direction detecting device  700  and a virtual display  800 . The goggles may further comprise transducers SPK 1 , SPK 2 . 
     The patent publication WO2007/085682 and the patent application PCT/FI2008/050065 disclose gaze direction detecting devices suitable for the purpose. PCT/FI2008/050065 also discloses goggles, which comprise a gaze direction detecting device and a virtual display. 
       FIG. 6  shows an audio processing system  300  comprising a first side P 1 , a second side P 2 , and the data transmission path  400 . Sounds emitted from the audio sources A 1 , A 2 , A 3  may be captured by two or more microphones M 1 , M 2 , M 3 . The audio sources A 1 , A 2 , A 3  may be e.g. participants of a meeting. The system  300  may be e.g. a teleconference system. 
     The signal processing unit  100  may be arranged to enhance an audio signal originating from a spatial location indicated by the direction selecting unit VDI 1 . The audio signal S AUDIO1  may be sent via the transmission path  400  to the second side P 2 , where it may be reproduced via one or more transducers K 1 , K 2 . 
     The direction indicator VDI 1  may be located on the second side P 2  of the system  300 . The direction indicator VDI 1  may be e.g. a gaze direction detecting device  700 , which is arranged to provide a direction signal S VDI1  based on the gaze direction of a listener A 4 . The direction signal S VDI1  may be sent from the second side P 2  to the first side P 1  where it may be coupled to the signal processing unit  100 . 
     Video or still images may be captured by a camera CAM 1  on the first side P 1 , and a corresponding image signal S IMG1  may be sent via the transmission path  400  to the second side P 2 . Video or still images IMG 1 , IMG 2 , IMG 3  corresponding the audio sources A 1 , A 2 , A 3  may be displayed on a screen or on a virtual display SCR 1  on the second side P 2 . 
     Thus, the gaze direction detecting device may be arranged to determine whether the listener A 4  is looking at the image IMG 1 , IMG 2 , IMG 3 , and the direction angle α for audio enhancement may be set, respectively. 
     The system may comprise an encoder to provide e.g. a parameter-coded audio signal S AUDIO1 . The audio signal S AUDIO1  may be decoded by a decoder  200  on the second side P 2  and reproduced via transducers SPK 1 , SPK 2 . 
     The system  300  may further comprise a second camera CAM 4  for capturing images of the listener A 4 . A corresponding image signal S IMG4  may be sent via the transmission path  400  from the second side P 2  to the first side P 1 . The image IMG 4  of the listener A 4  may be displayed on a screen SCR 1  on the first side P 1 . 
     The second camera CAM 4  may also be used as an imaging unit of the gaze direction detecting device. 
     Also an audio signal S AUDIO4  may be sent from the second side P 2  to the first side P 1 . The audio signal S AUDIO4  may be captured by a microphone M 4  and reproduced by a transducer SPK 4 . 
     Referring to  FIG. 7 a   , a plurality of microphones M 1 , M 2 , M 3  may be arranged to operate as a directional microphone array ARR 1 . The direction of maximum sensitivity of the directional microphone array ARR 1  may be controlled by the direction selecting unit VDI 1  so as to enhance audio signals originating from a selected location. In particular, direction of maximum sensitivity of the directional microphone array ARR 1  may be controlled by a gaze direction detecting device  700 . 
     The microphones of a directional microphone array ARR 1  may also be binaural microphones. 
     The signal processing unit  100  may comprise a delay bank  52  and a summing unit  53 . An audio signal S 1  captured by a first microphone M 1  may be delayed by a first time period τ 1 , An audio signal S 2  captured by a second microphone M 2  may be delayed by a second time period τ 2 , An audio signal S 3  captured by a third microphone M 3  may be delayed by a third time period τ 3 , The delays τ 1 , τ 2 , τ 3  may be adjusted such that audio signals originating from the selected location and captured by the microphones M 1 , M 2 , M 3  are in the same phase when they are combined in the summing unit  53 . The delayed audio signals may be combined e.g. by summing or averaging. The selected location is indicated by the direction signal S VDI1 . 
     The directional microphone array ARR 1  may comprise e.g. two or more microphones M 1 , M 2 , M 3 . The minimum distance L 1  between the audio sources A 1 , A 2 , A 3  and a microphone M 1 , M 2 , M 3  may be greater than the maximum distance W 13  between the microphones M 1 , M 2 , M 3 . The use of e.g. three or more microphones may provide improved directional selectivity. 
     The output of the summing unit  53  may be enhanced audio signal S ENC . If monophonic sound is acceptable, the output of the summing unit  53  may be used as the signal S AUDIO1 , which is sent via the transmission path to the receiving side of the system  300 . 
     The direction of maximum sensitivity of the directional microphone array ARR 1  may be changed without moving the microphones M 1 , M 2 , M 3  with respect to the audio sources A 1 , A 2 , A 3 . 
     The direction MAXD of maximum sensitivity may be defined e.g. by an angle γ between said direction MAXD and a reference direction SY. Thus, apparatus  300  may be arranged such that the angle γ of maximum sensitivity depends on the gaze angle α. 
     Referring to  FIG. 7 b   , the audio signals S 1 , S 2 , S 3  provided by individual microphones M 1 , M 2 , M 3  of a directional array ARR 1  may be processed by using beamforming filters H 1 , H 2 , H 3 . 
     The output of the array ARR 1  of  FIG. 7 b    is given by the equation 
                       S   ENC     ⁡     (   n   )       =       ∑     m   =   1     M     ⁢       ∑     k   =   0       L   -   1       ⁢         H   m     ⁡     (   n   )       ⁢       S   m     ⁡     (     n   -   k     )                     (   1   )               
where n denotes discrete time index, M denotes the number of audio signals S 1 , S 2 , S 3 , and L denotes the length of the beam forming filters H 1 , H 2 , H 3 .
 
     The most trivial selections for the filters H 1 , H 2 , H 3 , . . . are delay lines, as shown in  FIG. 7 a   . In that case output of the array ARR 1  is given by 
                       S   ENC     ⁡     (   n   )       =       ∑     m   =   1     M     ⁢       S   m     ⁡     (     n   -     τ   m       )                 (   2   )               
where τ 1 , τ 2 , τ 3 , . . . denote the time delays of each signal S 1 , S 2 , S 3 , . . .
 
     The directionality may also be implemented in the frequency sub-band domain or e.g. in the DFT (discrete fourier transform) transform domain. In that case the delay for each audio signal A 1 , A 2 , A 3  may be frequency-dependent. 
     Referring to  FIG. 7 c   , the output of a directional microphone array ARR 1  may be weighed together with outputs S 1 , S 2 , S 3  of individual microphones M 1 , M 2 , M 3 . In particular, one or more of said individual microphones M 1 , M 2 , M 3  may be part of said directional microphone array ARR 1 . 
     The output of the directional microphone array ARR 1  may be enhanced with respect to the outputs of the individual microphones in order to provide an enhanced audio signal S ENC . The output of the directional microphone array ARR 1  and the audio signals S 1 , S 2 , S 3  of the individual microphones M 1 , M 2 , M 3  may be filtered by using respective filters F 0 , F 1 , F 2 , F 3 . In particular, the output of the directional microphone array ARR 1  and the audio signals S 1 , S 2 , S 3  of the individual microphones M 1 , M 2 , M 3  may be multiplied with respective filter coefficients k 0 , k 1 , k 2 , k 3 . 
     The enhanced audio signal S ENC  captured by the directional microphone array ARR 1  may be sent at a high bit rate, and audio signals S 1 , S 2 , S 3  captured by one or more of the individual microphones M 1 , M 2 , M 3  may be sent at a lower bit rate. 
     The audio signal captured by the directional microphone array may convey primary audio information, e.g. spoken words or sounds directly emitted from a musical instrument. The audio signals captured by the individual microphones may convey secondary information which may be utilized when reproducing the audio image, e.g. background noise, echos from the walls, or applause. 
       FIG. 8 a    shows a parametric encoder  30 . Parametric audio coding methods enable multi-channel and spatial audio coding and representation. The original audio signals may be represented as a downmixed signal S SUM  together with a bit stream of parameters describing the spatial audio image. The downmixed signal comprises a reduced number of audio channels. For example, the downmixed signal may be a monophonic sum signal or a two channel (stereo) sum signal. 
     The parameters may comprise parameters describing e.g. inter-channel level difference (ILD), inter-channel time difference (ITD), and inter-channel coherence (ICC) 
     This kind of coding scheme may allow extremely efficient compression of multi-channel signals. Furthermore, given that the extracted spatial information is adequate, it may allow decoding into any other spatial mixing format, i.e. for any other loudspeaker set-up. For example, music or conversation captured with binaural microphones may be reproduced through e.g. a 5.1 loudspeaker system. 
     The encoder  30  may comprise a downmix unit  31 , a mono audio encoder  32 , a spatial analysis unit  33 , a parameter encoder  34 , and a bit stream formatting unit  35 . In particular, the encoder  30  may be arranged to provide a binaural cue coded (BCC) signal S AUDIO1 . 
     For a detailed description of the BCC approach, a reference is made to: F. Baumgarte and C. Faller: “Binaural Cue Coding—Part I: Psychoacoustic Fundamentals and Design Principles”; IEEE 
     Transactions on Speech and Audio Processing, Vol. 11, No. 6, November 2003, and to: C. Faller and F. Baumgarte: “Binaural Cue Coding—Part II: Schemes and Applications”, IEEE Transactions on Speech and Audio Processing, Vol. 11, No. 6, November 2003. 
     Referring to  FIG. 8 b   , the spatial audio parameters ILD, ITD, and/or ICC may also be determined from further audio signals SL, SR provided by additional microphones M LEFT , M RIGHT . In other words, the spatial audio parameters may also be determined from signals which are not used for downmixing. 
     In particular, the additional microphones M LEFT , M RIGHT  may constitute a set of binaural microphones. The additional microphones M LEFT , M RIGHT  may be attached e.g. to different sides of a mobile phone or to headphones. The headphones may be worn by the user A 5 . 
     Referring to  FIG. 8 c   , an enhanced monophonic output S ENC  of a directional microphone array ARR 1  may also be used as the downmixed signal S SUM  as such, i.e. in that case it is not necessary to utilize the downmixing unit  31  shown in  FIG. 8   b.    
     The spatial audio parameters ILD, ITD, and/or ICC may be determined from audio signals SL, SR provided by additional microphones M LEFT , M RIGHT . 
     Alternatively, The spatial audio parameters ILD, ITD, and/or ICC may be determined from two or more audio signals S 1 , S 2 , S 3  provided by individual microphones M 1 , M 2 , M 3  of the directional microphone array ARR 1  ( FIG. 7 c   ). 
     The audio image experienced by the listener A 4  may be modified according to the direction signal S VDI1 . 
     As was noticed in the context of  FIG. 1 a   , the direction signal S VDI1  may also be sent to the decoder  200  to be utilized in the rendering. 
     If the direction signal S VDI1  is provided at the receiving end, a monophonic enhanced signal S ENC  provided by a directional microphone array ARR 1  may also be rendered in the selected direction by using panning laws. In that case the BCC rendering may even be completely omitted and the user may only concentrate to the audio source the capturing user was concentrating to. 
     Referring to  FIG. 8 d   , the spatial parameters provided by the encoder  300  may be modified according to the direction signal S VDI1  in the transmitting end of the system  300 . Consequently, the apparent direction of the reproduced sounds may be adjusted even without sending the direction signal S VDI1  via the transmission path  400 . 
     The modification of the spatial audio parameters enables at least two different possibilities for the rendering, i.e. the audio image may be rendered to the desired direction of arrival or the audio image may be rendered to the center of the audio image. 
     In case of a BCC coded signal S AUDIO1 , the apparent direction of the reproduced sounds may be adjusted e.g. by modifying the interchannel time difference (ITD) parameters, and by modifying the interchannel level difference (ILD) parameters. 
     The encoder  30  may further comprise a parameter modifying unit  37  arranged to modify the values of the parameters ILD, ITD, and/or ICC based on the direction signal SVDI 1 . Thus, the parameter modifying unit  37  may arranged to calculate a modified inter-channel level difference parameters ILD N  from inter-channel level difference parameters ILD provided by the spatial analysis unit  33 . The parameter modifying unit  37  may arranged to calculate a modified inter-channel time difference ITD N  parameters from inter-channel time difference ITD parameters provided by the spatial analysis unit  33 . The parameter modifying unit  37  may arranged to calculate modified inter-channel coherence parameters ICC N  from inter-channel coherence parameters ICC provided by the spatial analysis unit  33 . 
     The time delay associated with ITD parameter may be adjusted according to the following equation:
 
τ q,NEW =τ q −τ m   (3)
 
where τ q  denotes time domain transformation of the interchannel time difference parameter associated with the qth frequency sub-band, τ q,NEW  denotes time domain transformation of the new modified interchannel time difference parameter associated with the qth frequency sub-band, and τ m  denotes a time delay corresponding to the direction indicated by the direction signal S VDI1 .
 
     The interchannel level difference (ILD) parameters may be modified by calculating gain coefficients g LEFT  and g RIGHT  as follows: 
                     g   LEFT     =           θ   RIGHT     -   ϕ         θ   RIGHT     -     θ   LEFT                   (     4   ⁢   a     )                 g   RIGHT     =           θ   LEFT     -   ϕ         θ   LEFT     -     θ   RIGHT                   (     4   ⁢   b     )               
where φ denotes the direction angle corresponding to the direction signal S VDI1 , θ LEFT  denotes an angle to a left transducer SPK 1 , θ RIGHT  denotes an angle to a right transducer SPK 2 . If the positions of the transducers are selected to correspond to the left channel and to the right channel of head-mounted microphones, then θ LEFT =−π/2 and θ RIGHT =π/2. The interchannel level difference (ILD) parameters may now be modified as follows:
 
                     Δ   ⁢           ⁢     L     q   ,   NEW         =     Δ   ⁢           ⁢     L   q     ⁢       log   10     ⁡     (       g   LEFT       g   RIGHT       )                 (   5   )               
where ΔL q  denotes an interchannel level difference parameter associated with a q:th frequency sub-band, and ΔL q  denotes a new modified interchannel level difference parameter associated with the q:th frequency sub-band.
 
     Also the inter-channel coherence parameters ICC may be modified. However, that is not necessary. In other words, the parameters ICC N  may be equal to ICC. 
     The modified parameters ILD N  and ITD N  may now be quantized and provided for transmission to the decoder  200  via the transmission path  400 , or they may be stored e.g. in a memory for subsequent use or transmission. 
     In case of BCC coding, the encoder  30  may be arranged to operate such that the inter-channel level difference parameters and the inter-channel time difference parameters corresponding to the most important audio source indicated by the direction signal S VDI1  are substantially equal to zero. The inter-channel level difference parameters and the inter-channel time difference parameters corresponding to secondary audio sources may substantially deviate from zero, respectively. Thus, the inter-channel level difference parameters and/or the inter-channel time difference parameters may be quantized by using relatively coarse quantization in the encoding unit  34 , without significantly degrading the quality of the reproduced audio signal corresponding to the most relevant audio source. The quality of reproduced audio signals corresponding to the secondary audio sources may be degraded, because they are of secondary importance. 
     The processed signal S AUDIO1  may also comprise parameters, which describe the estimated direction of arrival of each sound SND 1 , SND 2 , SND 3  emitted from the sound sources A 1 , A 2 , A 3 . Thus, BCC parameterization may be replaced or augmented with directional information. For example, each sub-band and time frame of a downmixed signal may be associated with a direction parameter DOF q , and a processed audio signal S AUDIO1  may comprise a downmixed signal together with determined direction parameters DOF q . The downmixed signal may be e.g. a sum of audio signals S 1 , S 2 , S 3 . 
     In case of directional parameters, a parameter modifying unit may be arranged to determine a modified direction parameter DOF q,NEW  e.g. by the equation:
 
 DOF   q,NEW   =DOF   q −φ  (6)
 
where DOF q,NEW  denotes a modified direction parameter associated with a q th  frequency sub-band, DOF q  denotes a direction parameter associated with a q th  frequency sub-band provided by a spatial analysis unit, and φ denotes a direction angle corresponding to a direction indicated by the direction signal S VDI1 .
 
     The processed audio signal S AUDIO1  may comprise the spatial audio parameters ILD, ITD, ICC, and/or DOF q . However, the spatial audio parameters may also be stored or sent via the transmission path  400  separately. 
     The parameters ILD, ITD, and/or DOF q  determine the locations of audio sources in a reproduced auditory image, i.e. the parameters determine the locations of the audio sources in the subsequent decoding step of the processed audio signal S AUDIO1 . 
     Modification of the spatial audio parameters ILD, ITD, and/or DOF q  enables controlling of the audio source locations in the subsequent decoding step. Thanks to modifying the spatial audio parameters, the location of the sound sources in the reproduced auditory image may be adjusted even when the microphones M 1 , M 2 , M 3  remain stationary with respect to the audio sources A 1 , A 2 , A 3 . For example, sounds originating from the selected directions may be kept at a predetermined location of the reproduced auditory image even when the selected direction is changed. In other words, the parameters ILD, ITD may be adjusted such that a first sound SND 1  originating from a first audio source A 1  appears to come from a predetermined location of the reproduced auditory image when the direction of said first audio source is indicated by the direction signal S VDI1 , and a second sound SND 2  originating from a second audio source A 2  appears to come from the same predetermined location of the reproduced auditory image when the direction of said second audio source is indicated by the direction signal S VDI1 . The sounds originating from the selected directions may be kept e.g. at the center of the reproduced auditory image. The reproduced auditory image may also be e.g. rotated according to the direction signal S VDI1 . 
     Instead of enhancing, a direction indicated by the direction indicator VDI 1  may also be used to suppress audio signals originating from a location corresponding to said indicated direction. Thus, disturbing sounds originating from a specific location may be suppressed or even completely eliminated from an audio image sent via the transmission path  400 . 
     The signal processing unit  100  may be arranged to enhance sounds originating from a first selected location and to substantially eliminate sounds originating from a second location. The locations may be indicated by a gaze direction detecting device  700 , and the data processing unit  100  may be arranged to take the first location and the second location simultaneously into account by a command inputted via a user interface. For example, the gaze direction detecting device  700  may comprise an “enhance” button and an “eliminate” button. If the user A 4  wishes to enhance sounds originating from a first direction, he may look at said first direction and push the “enhance” button. If the user wishes to suppress sounds originating from a second location, he may look at said second direction and push the “eliminate” button. 
     The transmission path  400  may be e.g. internet, radio link, mobile telephone network, or a satellite communication system. 
     The audio signals may be stored in a memory before or simultaneously with reproduction. 
     The signal processing unit  100  may be implemented in a programmable data processing unit, e.g. in a computer. The signal processing unit  100  may comprise a computer readable medium (MEM 1 ) comprising program code, which when executed by a data processor is for enhancing and/or suppressing sounds according examples presented above. 
     The audio signals provided by the microphones M 1 , M 2 , M 3  and the direction signal S VDI1  provided by the direction selecting unit VDI 1  may be coupled to the data processing unit via one or more inputs IN 1 , IN 2 , IN 3 , and the data processing unit  100  may be arranged to send a processed audio signal S AUDIO1  e.g. via internet and/or via a mobile telephone network. 
     The relationship between a direction indicated by the direction determining unit VDI 1  and the direction of maximum sensitivity is trivial when the distance L 1  between the audio sources and the microphones is large when compared with the distance W 13  between the microphones, and when the distance W VM  between the direction determining unit VDI 1  and the microphones is small when compared with the distance L 1  between the audio sources and the microphones. These conditions are typically fulfilled when a gaze direction detecting device is used in the vicinity of a directional microphone. 
     If the distance between the direction detecting unit VDI 1  and the microphones is large, the data processing unit  100  may be initialized before use. 
     The initialization comprises finding a function, which describes how the values of the signal S VDI1  provided by the direction selecting unit VDI 1  can be mapped to the actual direction or location of maximum sensitivity of the audio capturing set-up. 
     The data processing unit  100  may be initialized e.g. by calibration. For example, a test sound source may be moved in a room or in a television studio, while the position of said sound source is all the time followed by the direction selecting unit VDI 1 . The data processing unit may be arranged to determine and store the values of the coefficients of the filtering unit  20  and/or the delays of the delay bank  52  based on the calibration such that the direction of maximum sensitivity can in each case be associated with the direction indicated by the direction selecting unit VDI 1 . 
     Alternatively, the signal processing unit  100  may be initialized by a method comprising:
         emitting sound from a sound source,   varying the location of maximum sensitivity of the microphone array,   looking at said sound source or an image corresponding to said sound source, and   sending a command to the signal processing unit  100  via a user interface when the intensity of a reproduced sound of said sound source reaches a maximum.       

     For example, a participant A 2  may be asked to speak, while the sensitivity direction of a directional microphone array ARR 1  is scanned. The listener A 4  may look at the participant A 2  or a displayed image of said participant, while a gaze direction detecting device  700  is arranged to monitor the gaze direction of the listener A 4 . The listener may push a calibration button when the sound of the participant A 2  appears to reach the loudest volume. For complete calibration, the same procedure may be repeated also for the participant A 1  and the participant A 3 . 
     Alternatively, the positions of the microphones M 1 , M 2 , M 3 , the estimated positions of the sound sources A 1 , A 2 , A 3 , the position of the direction selecting unit VDI 1 , and a reference direction of the direction selecting unit VDI 1  may be inputted to a data processing unit via a keyboard or a graphical user interface. The data processing unit may be arranged to calculate the coefficients of the filtering unit  20  and/or the delays of the delay bank  52  for each direction indicated by the direction selecting unit VDI 1  based on said positions. 
     In an embodiment, it is not even necessary to display visual images to the listener A 4  on the receiving end of the system  300 . The listener A 4  may e.g. detect on the basis of a reproduced auditory image whether the most interesting audio source is located in the left part, in the center part, or in the right part of an auditory image. Consequently, the listener A 4  can operate a direction indicator VDI 1  such that the location of the most interesting audio source is selected. 
     If desired, the transmitted signals may be multiplexed at the transmitting end of the system  300  and demultiplexed at the receiving end of the system  300 . The system  300  may comprise two or more transducers SPK 1 , SPK 2  to reproduce an audio image. 
     If desired, decoded audio signals may also be filtered at the receiving end of the system  300 , in order to restore the level of the enhanced audio signal with respect to the other audio signals. The decoder  200  may comprise a filter bank (not shown). For example, if the level of the audio signal S 2  has been increased at the transmitting end by using a filter F 2 , the corresponding decoded audio signal may be suppressed at the receiving end. Consequently, a higher bit rate may be allocated for audio signals originating from the most relevant direction, while the distribution of the levels of the sounds reproduced by the transducers SPK 1 , SPK 2  may substantially correspond to the original distribution of the levels of the original sounds SND 1 , SND 2 , SND 3 . For example, new sets of filter coefficients may be determined at the receiving end based on the direction signal S VDI1 . Alternatively, the values of the filter coefficients k 1 , k 2 , k 3  may be sent via the transmission path  400  from the signal processing unit  100  to the decoder  200 , where decoded audio signals may be multiplied e.g. by inverse values 1/k 1 , 1/k 2 , 1/k 3  in order to restore the original sound level distribution between the different audio channels. 
     The system  300  may comprise a position detecting unit for determining the absolute position of the direction determining unit VDI 1 . For example, the direction determining unit VDI 1  may comprise a GPS navigator (Global Positioning System). Also the position of the direction determining unit VDI 1  may be stored in a memory or sent via the transmission path  400 . 
     Further embodiments may facilitate linking sounding objects to their physical location even when they are recorded from a distance. 
       FIG. 9  shows creating a virtual sound field to a listener A 4  based on the position and orientation of said listener A 4 . A sound source A 2  may be located at coordinates (x 2 ,y 2 ). A user A 5  may operate a direction detecting unit VDI 1  at coordinates (x 5 ,y 5 ). The listener A 4  may be located at coordinates (x 4 ,y 4 ). The coordinates (x 4 ,y 4 ) and (x 5 ,y 5 ) may be determined e.g. by satellite navigation devices carried by the user A 5  and the listener. The orientation angle δ 1  of the listener A 4  may be determined e.g. by a magnetic compass. 
     The user A 5  may operate a direction detecting unit VDI 1  such that the direction angle α of the sound source A 2  with respect to a reference direction (e.g. the direction SY) is known. 
     In some embodiments, the direction angle α of the sound source A 2  with respect to a reference direction may also be determined by summing a first angle and a second angle. Said first angle may be determined e.g. by a compass, and said second angle may be determined e.g. by the direction detecting unit VDI 1 . 
     Consequently, the coordinates (x 2 ,y 2 ) of the sound source A 2  may be estimated based on the coordinates (x 5 ,y 5 ), based on the direction angle α, and based on the distance between the user A 5  and the sound source A 2 . The distance may be estimated and inputted to a signal processing device. 
     The direction detecting unit VDI 1  may also comprise two gaze direction detecting units to monitor the gaze direction of both eyes of the user A 5 , i.e. the a gaze direction detecting device may be stereoscopic. The distance between the user A 5  and the sound source A 2  may be determined from the signals provided by a stereoscopic gaze direction detecting device. 
     Sounds emitted from the sound source A 2  may be captured, coded, and sent to a decoder  200  of the listener A 4  such that the sounds of the source A 2  may be reproduced via the transducers SPK 1 , SPK 2 . In particular, the listener A 4  may wear headphones SPK 1 , SPK 2 . 
     The estimated coordinates of the sound source A 2  may be sent as side information to the decoder  200  of the listener A 4 . The direction angle δ 2  of the sound source A 2  with respect to the orientation of the listener A 4  may be determined based on the orientation angle δ 1  of the listener and based on the coordinates (x 2 ,y 2 ) and (x 4 ,y 4 ). 
     A virtual sound field may now be created for the listener A 4  by rendering the processed audio signal S AUDIO1  by using the angle δ 2  as an angle of arrival. 
     The listener A 4  may be physically present at the coordinates (x 4 ,y 4 ), wherein the audio image may be formed of actual ambient sounds augmented with sounds transmitted via the transmission path. 
     The audio field experienced by the listener A 4  may also be augmented by adding virtual sound-emitting objects at the actual locations of real sound sources even when the real sound sources are not active. In other words, transducers SPK 1 , SPK 2  may be arranged to reproduce previously recorded sounds. 
     The distance between the listener A 4  and the audio source A 2  may be used to control the volume of the rendered sound. Thus, if the listener A 4  is farther away from the sound source A 2  than the user A 5 , when the sounds were recorded, then the listener A 4  may hear the sounds at a lower volume than the user A 5  originally did. 
     Audio source enhancement according to detected gaze direction may be utilized in a TV or radio studio in order to rapidly select the most relevant audio source for limited-bandwidth transmission. 
     Audio source enhancement according to detected gaze direction may be applied to e.g. telephony, audio sharing, or free viewpoint video services. Said services may be provided e.g. via internet or via mobile telephone network. 
     For the person skilled in the art, it will be clear that modifications and variations of the devices and the method according to the present invention are perceivable. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.