1. Field of the Invention
The present invention relates to a headphone system for reproducing a multi-channel acoustic signal.
2. Description of the Related Art
Recently, as multi-media systems such as DVD systems have become commonplace, more and more multi-channel acoustic information has been provided together with video.
Conventionally, systems for virtually reproducing a multi-channel acoustic signal using a headphone have been proposed.
For example, International Publication WO95/20866 discloses a technology for filtering signals which are input to right and left speakers, so as to cause a sound source provided in front of or behind the listener to be recognized or perceived by the listener.
FIG. 19 shows a structure of a headphone system described in International Publication WO95/20866.
The headphone system shown in FIG. 19 includes a right ear speaker 1901 and a left ear speaker 1902. The speakers 1901 and 1902 are fixed at positions distanced from the listener. The headphone system further includes filters 1910 and 1911. Reference numeral 1960 represents a virtual sound source located behind the listener.
In FIG. 19, H1 indicates a transfer function from the virtual sound source 1960 to the right ear of the listener. H2 indicates a transfer function from the virtual sound source 1960 to the left ear of the listener. Transfer function H1 is set in the filter 1910, and transfer function H2 is set in the filter 1911.
An input signal Z is input to the filters 1910 and 1911. The output from the filter 1910 is input to the right ear speaker 1901, and the output from the filter 1911 is input to the left ear speaker 1902.
Thus, the listener can recognize the virtual sound source 1960.
It is relatively easy for a human being to correctly recognize the sound source to his/her right or left. The reason for this is that he/she has ears on the right side and the left side of the head. It is considered that a human being recognizes the sound source to his/her right or left based on a plurality of different types of information, including the time period required by the signal from the sound source to reach his/her left ear, the time period required by the signal from the sound source to reach his/her right ear, and the difference in the signal levels. Therefore, there is little difference among individuals in the ability to recognize or distinguish between a sound source to his/her right or left.
It is relatively difficult for a human being to correctly recognize or distinguish between a sound source in front of or behind him/her. The reason for this is that there is little difference between the time period required by the signal from the sound source to reach his/her right ear and the time period required by the signal from the sound source to reach his/her left ear. It is considered that a human being recognizes or distinguishes between a sound source in front of or behind him/her based on a single piece of information of the absolute value of the frequency spectrum of the sound source. Since the head-related transfer function from the sound source to the ears is different among individuals due to each person's individual shape of the head, there is a relatively large difference among individuals in the ability to recognize a sound source located in front of or behind the listener.
Using the conventional technology shown in FIG. 19, the sound from a sound source located in front of or behind the listener is transferred to him/her by the combination of the right ear and left ear speakers 1901 and 1902 and the filter processing. Using such technology, it is difficult to cause the listener to correctly recognize a sound source located in front of or behind him/her, due to the individual difference in the transfer function. Especially for high frequencies, the difference between different individuals' recognition of front/rear sound direction is undesirably large, because the influence of differences between different individuals' transfer functions on such recognition is greater for high frequencies than it is for low frequencies.