Cabin communication system without acoustic echo cancellation

Details Cabin communication system without acoustic echo cancellation Cabin communication system without acoustic echo cancellation

2000-10-19

2004-06-08

Ramakrishnaiah, Melur (Department: 2643)

Electrical audio signal processing systems and devices

Acoustical noise or sound cancellation

Within cabin or compartment of vehicle

C381S086000, C381S092000

Reexamination Certificate

active

06748086

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a cabin communication system without acoustic echo cancellation that nevertheless provides echo rejection to enable a voice spoken within the cabin to be increased in volume for improved understanding without creating any unwanted noise amplification. The present invention also relates to a movable cabin that advantageously includes such a cabin communication system for this purpose. In this regard, the term “movable cabin” is intended to be embodied by a car, truck or any other wheeled vehicle, an airplane or helicopter, a boat, a railroad car, turboprop aircraft and indeed any other enclosed space that is movable and wherein a spoken voice may need to be amplified or clarified.
BACKGROUND OF THE INVENTION
As anyone who has ridden in a mini-van, sedan or sport utility vehicle will know, communication among the passengers in the cabin of such a vehicle is difficult. For example, in such a vehicle, it is frequently difficult for words spoken by, for example, a passenger in a back seat to be heard and understood by the driver, or vice versa, due to the large amount of ambient noise caused by the motor, the wind, other vehicles, stationary structures passed by etc., some of which noise is caused by the movement of the cabin and some of which occurs even when the cabin is stationary, and due to the cabin acoustics which may undesirably amplify or damp out different sounds. Even in relatively quiet vehicles, communication between passengers is a problem due to the distance between passengers and the intentional use of sound-absorbing materials to quiet the cabin interior. This communication problem is frequently compounded by the simultaneous use of high-fidelity stereo systems for entertainment, whether those systems are traditional audio systems or, as is coming into prevalence now, television and/or computer systems that provide an audio output.
To amplify the spoken voice, it may be picked up by a microphone and played back by a loudspeaker. However, if the spoken voice is simply picked up and played back, there will be a positive feedback loop that results from the output of the loudspeaker being picked up again by the microphone and added to the spoken voice to be once again output at the loudspeaker.
To avoid an echo due to the reproduced voice itself, prior art approaches have generally used acoustic echo cancellation, such as for example in U.S. Pat. No. 5,602,928, which includes active noise control (ANC), or in U.S. Pat. No. 5,706,344, which does not include ANC. In this type of system, an acoustic echo cancellation apparatus can be coupled between the microphone and the loudspeaker to remove the portion of the picked-up signal corresponding to the voice component output by the loudspeaker. This is possible because the audio signal at the microphone corresponding to the original spoken voice is theoretically highly correlated to the audio signal at the microphone corresponding to the reproduced voice component in the output of the loudspeaker.
Any reproduced noise components may not be so highly correlated and need to be removed by other means, for example by means of an appropriate noise filter.
These systems attempt to selectively receive speech from one location (e.g. by a microphone from where a first person in the vehicle is talking), enhance that speech by removing noise and entertainment stereo interference, amplify the enhanced speech and reproduce it in another location (i.e. from a loudspeaker directed at a second person in the vehicle). It is highly desirable not to degrade either the quality of the speech or the quality of the entertainment stereo output.
In an enclosed cabin with such prior art systems, the microphone and the loudspeaker are not acoustically isolated, i.e. the microphone is in a position to pick up the output of the loudspeaker. As noted above, the acoustic echo cancellation system is intended to estimate and remove from the microphone signal that part which is due to speech, music, noise etc. that was just played out of the loudspeakers. It is particularly important that the music (or other sounds) from the high-fidelity stereo system be cancelled. Any uncancelled music will be reproduced in the cabin from a loudspeaker at a location spaced from the original stereo system loudspeaker. This results in the detriment of stereo separation, fade and balance. It may introduce unpleasant reverberation due to processing delays, where the delay in picking up the music by the microphone, processing it and then re-outputting it by the loudspeakers may be long enough to be detectible by the human ear. Therefore, the connection of the high fidelity stereo system to the acoustic echo cancellation system contributes to the cabin communication system cost.
FIG. 1
is a simplified block diagram of a conventional cabin communication system (CCS)
100
using a microphone
102
and a loudspeaker
104
. As shown in the figure, a conventional echo canceller
106
and a conventional speech enhancement filter (SEF)
108
are connected between the microphone
102
and loudspeaker
104
. A summer
110
subtracts the output of the echo canceller
106
from the input of the microphone
102
, and the result is input to the SEF
108
and used as a control signal therefor. The output of the SEF
108
, which is the output of the loudspeaker
104
, is the input to the echo canceller
106
. In the echo canceller
106
, on-line identification of the transfer function of the acoustic path (including the loudspeaker
104
and the microphone
102
) is performed, and the signal contribution from the acoustic path is subtracted at summer
110
.
As disclosed in a commonly-assigned U.S. patent application, a further development of such a system, illustrated in
FIG. 2
, includes a novel and unobvious echo canceller
112
and a novel and unobvious speech extraction filter in the form of a special Wiener filter
114
, and further includes a specific input from the car's (vehicle's) audio system. This input is fed to a stereo gain estimator
116
, such as a conventional single tap LMS, which estimates a cancellation signal to be fed to the echo canceller
112
through summer
118
under the control of the output of summer
110
.
However, there are certain problems associated with the use of an acoustic echo cancellation (AEC) system in a cabin communication system. First, the AEC must be adaptive to accommodate changes in the acoustic environment, and second it provides cancellation proportional to the length of its filters. An AEC will always have some residual, i.e. partially uncancelled, signal that may be perceptually unpleasant or contribute to unintelligibility. Long AEC filter length and adaptivity impose a significant computational cost.
The requirement of adaptation on the AEC also poses a problem in system stability. The AEC must adapt to changes in the environment, e.g. passenger movement, such that it always provides enough cancellation. Insufficient cancellation results in a loop gain greater than one and unpleasant acoustic screeching.
A further problem with known cabin communication systems (CCS) is the need for microphone independence. The microphones of known CCS's are often responsive to not only the desired location, but also to other locations as well. This has the undesirable effect that a person hears an amplified version of his own speech. Ideally, each person should hear only the amplified versions of the other passengers' speech.
One approach to solve this problem is found in the above-mentioned U.S. patent application using acoustic echo cancellation, wherein a plurality of microphones are used at each location and each plurality is beamformed to improve the signal to noise ratio of the person speaking, to reject some of the stereo signal from the stereo system loudspeakers and to provide some microphone independence. There are many well known beamforming techniques and adaptive microphone arrays, although some do not relate to spatial filtering and no conventional system is known to

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