Transmission system for correlated signals

Electrical audio signal processing systems and devices – Noise or distortion suppression – Using signal channel and noise channel

Reexamination Certificate

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Details

C381S056000

Reexamination Certificate

active

06256394

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a signal transmission system comprising processing means for isolating an estimate for at least one wanted signal contained in at least one mixed signal, at least one sensor for detecting the mixed signal, the mixed signal comprising at least the wanted signal and at least two correlated interference signals which are produced by two sources of the system in response respectively to two correlated electric signals.
This signal transmission system may in turn relate to an audio signal broadcasting system present, for example, in a motor car or in a room. The system comprises a sound source formed, for example, by a car radio, a compact disc reader, a television receiver, a hifi system or by other stereophonic sound sources. The system may include voice recognition which permits a user to give voiced commands for controlling notably the sound source.
This signal transmission system may in turn relate to a teleconference system which comprises a transmitting station which communicates with a receiving station for which stations the conversations captured in the transmitting station are to be recovered in the receiving station without degradation.
This signal transmission system may also relate to systems for which radio broadcast signals arrive by radio link in the form of mixtures on antennas, the radio broadcast signals being locally interfered by noise sources.
2. Description of the Related Art
By way of example, let us consider the case where the wanted signal is a speech signal coming from a person.
A first situation appears in the case of the transmission of conversations via teleconferencing. A microphone installed in a transmitting station captures the voices as well as the ambient noise, and all the sounds thus captured are transmitted to the receiving station. Evidently, the sounds broadcast by loudspeakers situated in the transmitting station and coming from the receiving station, will also be captured and then broadcast to the receiving station and cause undesirable echoes. A solution restricted to certain types of signals is revealed in the document entitled: “Stereophonic Acoustic Echo Cancellation—An Overview of the Fundamental Problem” by M. M. Sondhi, D. R. Morgan, J. L. Hall, IEEE Signal Processing Letters, Vol. 2, No. 8, 1995, pp. 148-151.
None the less, when the loudspeakers broadcast stereophonic sounds, no satisfactory technique is known which permits correctly isolating the person's voice expressed in the microphone.
Another situation occurs in the case where the voice to be captured is that of a driver who expresses himself in a microphone installed in an automobile over the past few years, there have been developed possibilities for the driver to have voice control of equipment inside an automobile. The object of this is to set the driver free from movements he has to make to effect certain settings or to have certain controls in the automobile itself. It is thus necessary, in a first period to recognize the voice message pronounced by the driver and then, in a second period, to decode this voice message and extract therefrom commands intended to influence the equipment. By placing several microphones inside the driver's compartment, there is achieved that the driver's voice is isolated and the commands it contains are decoded to take appropriate action. But the automobile is a considerably noisy environment where known techniques are not satisfactory, notably, when the driver's compartment contains loudspeakers which broadcast stereophonic sounds. Each time, mixed signals contain mutually correlated signals, it is very difficult to separate them and also to separate other signals that form the mixed signal.
SUMMARY OF THE INVENTION
It is a main object of the invention to propose a signal transmission system which is suitable for separating signals contained in mixed signals comprising correlated signals and which is more robust to interference than prior-art techniques.
A particular object of the invention is to check the sound volume returned to the user of the system on the basis of voice messages pronounced by the user.
SUMMARY OF THE INVENTION
Receives on the input, the detected mixed signal and the two correlated electric signals wherefrom, the processing means extracts the estimate of the wanted signal contained in the detected mixed signal by decorrelating, via multiple shifts, the estimate relative respectively to the correlated electric signals.
The voice message is thus correctly separated from all the other sound signals present in the sound environment, these other signals coming from whatever sound source is present in the vehicle. The invention provides an effective solution to the processing of stereophonic signals, that is to say, correlated signals, which is impossible with known processings.
The correlated electric signals which give rise to correlated interference signals may be obtained from the loudspeakers of a car radio, a television receiver, a hi-fi system or other sound sources.
In the cases where the sensor is a microphone, where the mixed signal is an ambient sound signal captured at the listening end by the microphone, where the wanted signal is a voice message sent by a speaker at the listening end and, where the voice message is interfered by stereophonic signals broadcast by loudspeakers which form the sources, the system is such that the processing means extracts the estimate of the voice message contained in the ambient sound signal by decorrelating the estimate of the voice message relative, respectively, to the stereophonic signals.
According to a particular embodiment, converting means permits to converting the estimate of the voice message into at least one voice control. The voice controls may be used for controlling in return the sound source from which the correlated signals come. Thus, a voice control may request the modification of the sound volume produced by the car radio. When the system detects such a voice control, it subsequently applies this control to the car radio.
But the use of voice controls is not restricted to the control of the sound source from which the correlated signals are taken. The voice controls may also be used for controlling the other sound sources or for acting on actuators at the listening end, in the car or in the room, for example. Thus, a first voice control may request a lowering of the sound volume broadcast by the car radio, after which a second voice control may request the windows of the car to be closed. The means producing the voice controls are therefore connected to the respective actuators via the voice controls provided to this effect.
In the case of a teleconference system comprising a transmitting station and a receiving station interconnected by at least an up channel and at least a down channel, the stations comprising each at least two microphones and at least two loudspeakers broadcasting two stereophonic signals, the system is characterized in that the processing means undesirable echoes generated by the stereophonic signals arriving at the transmitting station coming from the receiving station, the transmitting station transmitting in stereo only the estimates of the local voice message to the loudspeakers of the receiving station.
The speech signals pronounced by the speaker may thus be perfectly separated from the correlated signals broadcast by the loudspeakers and coming from the other station. The transmitting station can thus transmit solely the speaker's signals from the transmitting station to the receiving station. This makes it possible to avoid the phenomena of echoes which manifest themselves if the signals produced by the loudspeakers were retransmitted in a loop to the station that has broadcast them.
In the case where the sensor is an antenna which receives a radio broadcast signal, the system permits separation of the radio broadcast signal by clearing it of all the correlated signals coming from sources that transmit interference signals.
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