The present disclosure relates to a sound signal processing apparatus and a sound signal processing method for obtaining a sound from a specific sound source direction.
Japanese Unexamined Patent Application Publication No. 2010-11117 and Japanese Unexamined Patent Application Publication No. 2007-129383 are examples of related art.
For example, a beam forming technique for forming directivity with respect to input sounds from two microphones is known.
FIG. 10 illustrates an example of a noise cancellation headphone (hereinafter, referred to as an NC headphone). Although an NC headphone 100 supplies stereo sounds to a user using left and right speakers 101L and 101R, microphones 102L and 102R that absorb external sounds are provided in order to reduce external noises.
The NC headphone 100, for example, reproduces and outputs the sounds of reproduction music from a portable media player or the like.
In brief, in order to cancel noises, reversed phase components of sound signals absorbed by the microphones 102L and 102R are generated, combined with respective music signals, and then output from the speakers 101L and 101R. Therefore, the sounds of the music signals are listened to by a user in a state in which external noises are spatially cancelled.
Here, it is considered that the microphones 102L and 102R are used not only for canceling noise but also for absorbing external sounds which have directivity.
For example, although it is preferable that a user can normally perform conversation or the like even when wearing the NC headphone 100, if a noise cancellation function is turned on, for example, even the sounds of a person who is in front of the user are reduced, so that it is difficult to listen to conversational sounds.
Therefore, for example, when a conversation or the like is performed, a mode that turns off reproduction music and turns off a noise cancellation function is provided.
However, if the noise cancellation function is turned off, surrounding noise is heard to a large degree together with the sounds of other people. Therefore, in a place where there is much traffic, the inside of a plane, or the like, a state in which conversational sounds or the like are difficult to be heard is not changed.
In such a case, it is preferable that a speaker output, in which conversational sounds are easily heard and surrounding noises are suppressed, can be realized.
If it is considered that a user wears the NC headphone 100 and faces the front as in FIG. 10, it can be considered that the sounds of a target that conducts conversation come from the front of the user in most cases. At this time, as shown in FIG. 10, when viewed from the user, the user regards sound sources other than from the front as noises, of which the level thereof should be lowered while boosting the conversational sounds from the front.
In order to realize this, when a necessary sound source direction is temporarily set to the front, directivity can be formed at the time of absorbing sounds using a so-called beam forming method.
FIG. 11A is a conceptual diagram illustrating a beam forming process, and sound signals from left and right microphones 102L and 102R are processed and output by a beam forming process unit 103.
The simplest beam forming process may be a process of adding sound signals from the left and right microphones 102L and 102R as shown in FIG. 11B when the necessary directivity is the front or the back.
In this case, the phases of the sound signal components of left and right channels with respect to sounds from the front or back, that is, sounds from the sound sources at an equal distance from the microphones 102L and 102R, are matched with each other, and boosted by addition. Since the phases of the sound signal components of sounds from other directions are deviated from the phases of the sound signal components of the left and right channels, the sound signal components are reduced by as much as the deviation. Therefore, sound signals having, for example, directivity in the front direction can be obtained.
Meanwhile, the beam forming process itself can boost the sound signals in the directions other than the front direction. In this case, a delay unit is installed on one side channel, with the result that the time difference between the same wave fronts which reach each of the microphones can be absorbed, so that the beam forming can be realized in an oblique direction or in a traverse direction.
In order to increase the accuracy of the beam forming (in this case, the same meaning as the boosting of the front directivity and the reduction in surrounding noises), a noise suppression device that mainly uses band pass filters shown in FIG. 12 is generally used and not the simple device as shown in FIG. 11B.
The sound signal obtained by the microphone 102L is amplified by the microphone amplifier 104L, and then supplied to band pass filters 121L, 122L, and 123L that have central pass frequencies fc1, fc2, and fc3, respectively. In the band pass filters 121L, 122L, and 123L, the sound signal components of the bands BD1, BD2, and BD3 are extracted.
Further, the sound signal obtained by the microphone 102R is amplified by the microphone amplifier 104R, and then supplied to band pass filters 121R, 122R, and 123R that have central pass frequencies fc1, fc2, and fc3, respectively, so that the sound signal components of the respective bands BD1, BD2, and BD3 are extracted.
Meanwhile, the pass band of the band pass filter that has the central frequency fc1 is represented as a band BD1. In the same manner, the pass bands of the band pass filters that have the central frequencies fc2 and fc3 are represented as bands BD2 and BD3.
The sound signal components of the band BD1, which are the outputs of the band pass filters 121L and 121R, are supplied to the sound source directional angle analysis unit 124 and the adder 127.
The sound signal components of the band BD2, which are the output of the band pass filters 122L and 122R, are supplied to the sound source directional angle analysis unit 125 and the adder 128.
The sound signal components of the band BD3, which are the outputs of the band pass filters 123L and 123R, are supplied to the sound source directional angle analysis unit 126 and the adder 129.
The sound source directional angle analysis units 124, 125, and 126 determine the sound source direction of a dominant sound from among the sound signal components of the bands BD1, BD2, and BD3, respectively.
Thereafter, the sound source directional angle analysis units 124, 125, and 126 control the gain of the variable gain amplifiers 130, 131, and 132 based on the determined direction. That is, the sound source directional angle analysis units 124, 125, and 126 perform control such that the gain increases when the determined direction is a target direction, such as the front direction or the like, and such that the gain decrease when the determined direction is the other direction.
Each of the sound signal components of the bands BD1, BD2, and BD3 is added by the adders 127, 123, and 129 for the respective L and R channels, and then supplied to the variable gain amplifiers 130, 131, and 132. Thereafter, the variable gain amplifiers 130, 131, and 132 are controlled by the sound source directional angle analysis units 124, 125, and 126 as described above, so that, for example, a band in which the sound from the front direction is dominant is boosted, and the other bands are reduced. The outputs of the respective bands BD1, BD2, and BD3 in which gains are adjusted as weights for respective bands are added by an adder 133, and become an output sound signal Sout on which a beam forming process has been performed.
When a beam forming process unit 103 using such a noise suppression device is used, the conversation sounds are not easily buried in noises and can be heard in the state as shown in FIG. 10.
Further, as one type of method of boosting sounds and suppressing noises, a method using an FFT that centers on “spectrum subtraction” may be provided as a representative method of analyzing and combining sounds without using the beam forming method in order to remove noises in the related art.
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