Echo in a communication system is commonly characterized as the return of a part of a transmitted signal from an end user back to the originator of the transmitted signal after a delay period. As is known in the art, a near end user transmits an uplink signal to a far end user. Conversely, the near end user receives a downlink signal from the far end user. Echo at the near end occurs when the near end user originates an uplink signal on the uplink path, and a part of the transmitted signal is reflected at the far end as an echo signal on the downlink path back to the near end. Echo at the far end occurs when the far end user originates a downlink signal on the downlink path, and a part of the transmitted signal is reflected at the near end, as an echo signal on the uplink path back to the far end. Typically, the echo delay period corresponds to the round trip transmission time in the communication system plus the dispersion or group delay of the echo generating source. The reflection of the transmitted signal may occur due to a number of reasons, such as an impedance mismatch in a four/two wire hybrid, or feedback from acoustic coupling in a telephone, wireless device or hands free speaker phone. An echo signal corresponding to the delayed transmitted signal is perceived as annoying to the near end user and in some cases can result in an unstable condition known as “howling.”
Echo cancellers are required at any echo generating source at both the near end and at the far end in an attempt to eliminate or reduce the transmission of echo signals to the far end and near end. Echo cancelers may be employed in wireless devices, such as cellular phones, car phones, two-way radios, car kits for cellular telephones and other suitable devices. Additionally, echo cancelers may be employed in wireline devices, such as hands free speakerphones, video and audio conference phones and telephones otherwise commonly referred to in the telecommunications industry as plain old telephone system (POTS) devices. Hands free speakerphones typically include a microphone to produce the uplink signal, a speaker to acoustically produce the downlink signal, the echo canceler to cancel the echo signal and a telephone circuit.
The hands free speaker phone may be integrated into an in-vehicle audio system. The vehicle may be an automobile, a boat or an airplane, or any suitable vehicle. The in-vehicle audio system may include an amplifier, speakers and an audio source, such as a tuner module, CD/DVD player, tape player, satellite radio, etc. The in-vehicle audio system may be integrated with a communication apparatus, such as a telematics communication module. For example, the telematics communication module may be a component of a General Motors' OnStar system. The telematics communication module typically collects and disseminates data, such as location information and audio, such as speech.
Typically, the downlink audio signal received from the far end through the downlink path is played through the at least one speaker in the in-vehicle audio system. However, the hands free speaker phone installed in the vehicle may experience significant coupling between the at least one speaker and the microphone and is referred to herein as an acoustic coupling channel. As a result, an amplified downlink audio signal transmitted through the at least one speaker will be partially received by the microphone as an echo signal. The amplitude of the echo signal referred to herein as the echo return loss depends on the amount of coupling between the at least one speaker and the microphone.
Echo cancelers are known to attempt to cancel the echo signals produced at the near end when the far end is transmitting by generating echo estimation data corresponding to a portion of an amplified downlink audio signal traveling through the acoustic coupling channel. The echo canceler generates the echo estimation data through the use of an echo canceler adaptive filter. The echo canceler attempts to subtract the echo estimation data from pre-echo canceler uplink data received from the microphone in order to produce post-echo canceler uplink data. The echo canceler attempts to adapt to changes in the echo return loss by dynamically generating the echo estimation data via the echo canceler adaptive filter. Additionally, attenuators in the uplink path and in the downlink path may also be used to reduce the effect of the echo signals.
The echo canceler adaptive filter adapts not only between different calls, but also during a call, due to the nonfixed nature of the acoustic coupling channel between the at least one speaker and the microphone. For example, movement of passengers in the vehicle may affect the acoustic coupling channel and, therefore, the echo canceler attempts to dynamically adapt to the varying echo return loss. However, the pre-echo canceler uplink data may change due to variations in the acoustic coupling faster or beyond the capabilities of the echo canceler adaptive filter. As a result, due to imperfect knowledge of the network medium and the acoustic coupling channel creating the echo signal, the estimated echo data may contain errors.
Additionally, if the amplifier gain is increased, the downlink signal may cause the received microphone signal to be so great that the reduced echo return loss may significantly reduce the effectiveness of the adaptive filter and possibly cause the adaptive filter to become ineffective or possibly unstable. Consequently, the adaptive filter under this condition may actually cause the uplink signal to also become degraded, unstable, or corrupted. As a result, the corrupted post-echo canceler uplink data will cause annoying loud noises at the far end.
According to one method, the echo canceler attenuates the post-echo canceler uplink data based on the amplitude or power level of the post-echo canceler uplink data in an attempt to attenuate an echo signal transmitted on the uplink path before reaching the far end. However, an increase in the amplitude or power level of the post-echo canceler uplink data may occur due to an increase in the downlink data power and not due to a change in the acoustic coupling channel. As a result, the uplink data may be improperly attenuated causing the near end user not to be heard by the far end user because the echo canceler incorrectly interpreted an increase in post-echo canceler uplink data power due to an increase in downlink data power as an increase in acoustic coupling.
According to yet another method, the capabilities of the echo canceler adaptive filter are improved by, for example, increasing a number of coefficients in a finite impulse response filter (FIR). However, since the echo canceler adaptive filter is typically implemented in a processing device, increasing the number of coefficients may result in an increased processor load, and may reduce the rate of adaptation of the echo canceler adaptive filter and increase power consumption. As a result, a more complex and more costly echo canceler adaptive filter is required to satisfy the required processor load.