1. Field of the Invention
The present invention relates to an echo canceler, which cancels echoes generated during hands free speaking on a cellular telephone.
2. Description of the Related Art
Echoes generated during the hands free speaking mean that a speech reception voice generated from a speaker of communication equipment is re-input to a microphone and is transmitted as echoes to a far end talker through a transmission circuit.
Therefore, it is required to prevent from transmitting the speech reception voice through the transmission circuit back to the far end talker at receiving equipment. To purse the requirement, it is necessary to develop an echo canceling technique.
The Japanese laid-open patent application No. 7-297901 describes as one of conventional techniques, voice communication equipment with an echo canceler to cancel echoes generated during the hands free talking.
It is possible to cancel the echoes by estimating and subtracting echoes included in a speech transmission signal from a speech reception voice. In the echo canceling technique, described in the Japanese patent publication No. 7-297901, a variable attenuator is activated to fix the transmission signal level at the hands free speaking but not during the both far end and near end concurrently speak so that noise level is sent and unnatural feeling is given to the far end talker.
Further, the other technique relating to an echo canceler to cancel the echoes generated during the hands free speaking is described in the Japanese laid-open patent application No. 6-216811. It is an object of the technique is to solve a problem such that distortion of the speech voice signal occurs when a speech reception voice signal, of which level is large, is input, thus the echo signal can not be canceled sufficiently at a speech transmission signal path.
To achieve the object, in the description of the Japanese laid-open patent application No. 6-216811, the speech reception voice signal level is limited to a predetermined amplitude level or less before the voice signal is supplied to an echo canceler and a speaker. However, there are some cases where the limitation of signal level gives an influence not preferable for the voice signal output from the speaker.
Accordingly, it is an object of the present invention to provide an echo canceler to improve preferable echo canceling, which is not improved sufficiently by the conventional techniques.
It is another object of the present invention to provide an echo canceler whereby a speech reception voice power can be adaptably controlled and the distortion of voice output from a speaker can be reduced.
It is further object of the present invention to provide an echo canceler to improve a stable echo canceling operation by attenuating a voice input to the echo canceler, varying the attenuation amount, and removing the distortion of the voice data.
It is furthermore object of the present invention to provide an echo canceler for general use, which does not depend on the delay amount and gain of an echo pass by automatically measuring echo pass delay, echo pass gain and clipping state in an analog circuit.
To achieve the above-described objects according to the present invention, an echo canceler comprising, a power control section controlling a power for a speech reception voice, a first filtering section removing a particular frequency component from an output of the power control section, an echo canceling section removing echo components of a speech reception voice, which are added to a speech transmission voice, a second filtering section removing the particular frequency components of the transmission voices which include echo, and a state judgement section judging existences of the speech reception and transmission voices from an output of the second filtering section and controlling a power attenuation amount of the power control section based on the judgement result.
In one preferred mode of the present invention, the state judgement section calculates the result of judging whether or not the speech reception and transmission voices exist and a power of the speech reception voice of N samples, based on N samples of the speech reception voice and the output from the second filtering section, and there is comprised of a delay buffer maintaining the calculated result of the judgement section and the power of the speech reception voice for T blocks, each block having N samples.
In the above-described structure, the state judgement section controls to make the power control of the power control section operable, when both the speech reception and transmission voices exist.
In the other mode of the present invention, the state judgement section makes the attenuation amount obtained by the power control of the power control section at the time when either of the speech reception or transmission voice does not exist smaller than that at the time when both of the speech reception and transmission voices exist.
Additionally, the echo canceling section estimates a echo replica according to an attenuated signal obtained by attenuating an input signal to the echo canceling section, amplifies the estimated echo replica according to the attenuation of the input signal, and subtracts the amplified echo replica amplified from the input signal.