There are transceivers such as mobile phones and teleconference systems. Some transceivers are implemented with a hand-free communication function enabling communication without a handset or headset.
A transceiver includes at least: a speaker for reproducing voice of a talker who is in a communication destination (hereinafter referred to as a far end); and a microphone for receiving voice of a talker who is in a communication dispatch point (hereinafter referred to as a near end) and transmitting the voice to the far end.
FIG. 9 is a diagram which illustrates a configuration of a communication system using a conventional transceiver.
The communication system illustrated in FIG. 9 includes: a transceiver 900 which is located at the near end and is used by a talker of a communication dispatch point; a transceiver 920 which is located at the far end and is used by a talker of a communication destination; and a communication network 930 used for receiving and transmitting sound of the transceiver 900 and the transceiver 920.
The transceiver 900 includes: a decoder 901: a speaker 903; a microphone 904; an echo canceller 905; and an encoder 906. Likewise, the transceiver 920 includes: a decoder 921: a speaker 923; a microphone 924; an echo canceller 925; and an encoder 926.
Here, the case will be described where the echo canceller 905 and the echo canceller 925 are not included in the communication system. In this case, during a communication, the voice of the far-end transceiver 920 is emitted from the speaker 903 of the near-end transceiver 900, and the sound is received by the microphone 904 of the near-end transceiver 900 and transmitted again to the far-end transceiver 920.
At the far-end transceiver 920, the voice uttered by the far-end talker is outputted from the speaker 923 of the far-end talker (the transceiver 920), and this is heard as an echo. This is annoying when the far-end talker talks. The same holds true for the near-end transceiver 900. The voice uttered by the near-end talker is heard as an echo from the speaker 903 of the near-end talker (the transceiver 900).
In addition, the sound received by the microphone 904 of the near-end transceiver 900 returns to the near-end transceiver 900 again and is outputted from the speaker 903 (closed loop), in such a manner as in: the near-end transceiver 900—the far-end transceiver 920—the near-end transceiver 900. When the acoustic frequency characteristics in the closed loop include a frequency of a gain larger than one, a continuous loud sound called howling is emitted from the speaker 903 with the frequency. This not only is tremendously annoying to the near-end talker but also can break the speaker 903 in some cases. For that reason, such transceivers (the transceiver 900 and the transceiver 920) are provided with the echo canceller 905 and the echo canceller 925 for preventing the occurrence of echo or howling.
The echo canceller 905, for example, identifies the audio characteristics in a path from the speaker 903 in the near-end transceiver 900 trough a space to the microphone 904. Then, by utilizing the identified audio characteristics and a signal transmitted to the speaker 903, it is possible to electrically estimate a signal which is to be outputted as a sound from the speaker 903 and reaches the microphone 904. Here, the signal which is estimated is called pseudo echo. By subtracting the pseudo echo from a signal which is actually received by the microphone 904, it is possible to prevent a signal transmitted from the far-end transceiver 920 returns the far end again as echo via the speaker 903 of the near-end transceiver 900 and the microphone 904. It is to be noted that the same holds true for the echo canceller 925, and thus description for that will be omitted.
Non Patent Literature (NPT) 1 discloses the techniques of the echo cancellation and suppressing of howling.
However, the above-described pseudo echo needs to be generated before a sound (audio signal) emitted from, for example, the speaker 903 reaches the microphone 904. This means that, when a sound is received by the microphone 904, for some reason, before pseudo echo is generated, the pseudo echo for the sound cannot be estimated, and thus echo cannot be canceled. One example is that the case where the far-end transceiver 920 and the near-end transceiver 900 receive the same sound. The following describes that with reference to FIG. 10.
FIG. 10 is a diagram for explaining a problem in a communication system in which a conventional transceiver is used. It is to be noted that elements identical to elements in FIG. 9 are provided with the same reference signs, and detailed explanation will be omitted.
FIG. 10 illustrates the case where there are a TV receiver 1000 which is a different casing from the near-end transceiver 900 and is located near the transceiver 900 and a TV receiver 1020 which is a different casing from the far-end transceiver 920 and is located near the transceiver 920, the TV receiver 1000 and the TV receiver 1020 receive broadcast waves, and both of a near-end talker and a far-end talker view the same TV program.
Here, in FIG. 10, a sound emitted from the TV receiver 1020 located near the far-end transceiver 920 is received by the microphone 924 of the far-end transceiver 920. In the same manner as above, a sound emitted from the TV receiver 1000 located near the near-end transceiver 900 is received by the microphone 904 of the near-end transceiver 900.
The sound emitted from the far-end TV receiver 1020 is received by the microphone 924 of the far-end transceiver 920, transmitted to the near-end transceiver 900 as an audio signal of the transceiver 920 together with a sound uttered by the far-end talker, and inputted to the echo canceller 905 of the near-end transceiver 900.
At this time, the same sound is emitted from the TV receiver 1000 in the near-end transceiver 900. However, under normal conditions, by the time a TV sound of the TV receiver 1020 which is received by the microphone 924 of the far-end transceiver 920 reaches the echo canceller 905 of the near-end transceiver 900, a TV sound of the TV receiver 1000 already reaches the microphone 904.
This means that the TV sound of the TV receiver 1000 cannot be canceled by the pseudo echo estimated from the TV sound of the TV receiver 1020 included in the audio signal of the transceiver 920 which is inputted into the echo canceller 905. As a result, the TV sound is transmitted to the far-end transceiver 920, and an annoying echo occurs.
A counter measure against the above-described problem is disclosed by PTL 1, for example. Patent Literature (PTL) 1 discloses in Embodiment 1 a method of performing echo cancellation for each of a TV sound which reaches the near end from the far end, a TV sound generated in the near end, and a sound of a far-end talker, when the TV receiver is a different casing. In addition, Patent Literature (PTL) 1 discloses in Embodiment 2 a method of performing echo cancellation for the TV sound which reaches the near end from the far end, the TV sound generated in the near end, and the sound of a far-end talker, when the TV receiver and the transceiver are integrated.