Multiplex communications – Duplex
Reexamination Certificate
1998-03-20
2001-08-28
Ton, Dang (Department: 2732)
Multiplex communications
Duplex
C370S286000, C379S413020
Reexamination Certificate
active
06282176
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to telecommunication circuits. More specifically, the present invention relates to full-duplex speakerphone control circuits including a supplementary echo suppressor.
2. Description of the Related Art
Most modern speakerphones use half-duplex operation which switches transmission between the far-end talker and the near-end speakerphone user. System designers resort to half-duplex operation because the acoustic coupling between the speaker and microphone is much higher in speakerphones than in a handset where the coupling is mechanically suppressed .
Hands-free communication through a microphone and speaker typically results in acoustic feedback or howling because the loop gain of the system exceeds unity when audio amplitudes are adjusted to a reasonable level.
Howling is a condition occurring in full-duplex operation in which both the microphone and speaker are active at the same time so that, in conjunction with the reflection off the hybrid, a closed loop is created. The signal coupling between the speaker and the microphone causes feedback oscillation or howling when the coupling between the speaker and microphone is strong enough to increase the system closed loop gain above unity.
The solution to the howling problem has typically been half-duplex operation, in which either the transmit channel or the receive channel is active with both channels never active at the same time. Half-duplex operation prevents howling but diminishes the overall communication quality by clipping words and forcing the speaker at each end to wait for the speaker at the other end to stop talking.
In full-duplex conversation, both transmit and receive channels are active simultaneously. Telephone handsets allow full-duplex conversation quality.
Another technique for addressing acoustic howling that results from the loop becoming unstable due to a high loop-gain is to utilize an “echo suppressor” that operates in combination with an echo canceller to substantially reduce residual echo. The echo suppressor includes a first attenuator in the transmit path and a second attenuator in the receive path. If either attenuator is activated, the loop-gain is reduced to prevent acoustic howling. Control logic for the attenuators is typically designed so that at least one attenuator is on at one particular time. In echo conditions, the attenuator in the path from the near-end microphone to the far-end is designed to have at least enough attenuation to prevent echo that is objectionable to the far-end user. The control logic is designed so that the attenuator is continuously active when potentially objectionable echo is present.
Conventional systems including echo cancellers in combination with echo suppressors typically do not adequately accommodate full-duplex communication. Echo cancellers and suppressors that do support full-duplex communication generally have difficulty determining which signals are speech signals and which are echo. These echo suppressors therefore have difficulty in determining which signals to suppress.
SUMMARY OF THE INVENTION
In accordance with the present invention, a full-duplex communication circuit including a supplemental echo suppressor is provided for full-duplex, hands-free communication. The communication circuit includes a first signal path for carrying a first signal from a first-end input terminal to a second-end output terminal, and a second signal path for carrying a second signal from a second-end input terminal to a first-end output terminal. An adaptive filter is coupled to the first signal path and to the second signal path. The adaptive filter can access the second signal, determine a compensation signal from the second signal, and form a comensated first signal using the compensation signal. A first signal path variable gain supplemental echo suppressor is coupled between the first end and the second end. The first signal path echo suppressor includes a controllable attenuator, a power estimator, a noise estimator, a slow noise estimator, a normalizer, and a suppression controller. The power estimator is coupled to the first signal path to receive the first signal, determine a power level of the first signal, receive the compensated first signal, and determine a first power level of the compensated first signal. The noise estimator is coupled to the first signal path power estimator to receive the power level of the compensated first signal and generate a first background power estimate. The slow noise estimator is coupled to the first signal path noise estimator to receive the first background power estimate. The slow noise estimator is also coupled to the first signal path power estimator to receive the power level of the first signal. The slow noise estimator generates a lower variance first background power estimate. The normalizer is coupled to the power estimator and the slow noise estimator to normalize the first signal power level to the lower variance first background power estimate in order to determine a normalized first power level. The suppression controller is coupled to the controllable attenuator and the normalizer, and controls the first signal path controllable attenuator as a function of the normalized first power level.
In accordance with another aspect of the present invention, a full-duplex communication circuit includes a first signal path for carrying first signals from a first-end input terminal to a second-end output terminal and a second signal path for carrying second signals from a second-end input terminal to a first-end output terminal. A first-end speech detector and a double-talk detector are coupled to the first signal path. A variable gain echo suppressor is coupled between the second-end and the first-end. The echo suppressor includes a controllable attenuator coupled to the second signal path and a suppression controller coupled to the controllable attenuator. A normalizer is coupled to the suppression controller. The suppression controller controls the controllable attenuator in a default_on mode for the second signal path. In the default_on mode, the suppression controller disengages on an occurrence of first-end speech or double-talk.
In accordance with still another aspect of the present invention, a method of controlling a full-duplex communication circuit is disclosed. A receive signal is accessed from a receive signal path carrying receive signals from a far-end input terminal to a near-end output terminal. A transmit signal is accessed from a transmit signal path carrying transmit signals from a near-end input terminal to a far-end output terminal. Acoustic echo is canceled by determining an echo canceling signal from the receive signal and echo canceling the transmit signal using the echo canceling signal to form an echo-canceled transmit signal. A power level of the transmit signal is determined, and a transmit power level of the echo-canceled transmit signal is determined. A transmit background power estimate is generated from the transmit power level of the echo-canceled transmit signal. A lower variance transmit background power estimate is generated from the transmit background power estimate. The transmit signal power level is normalized to the lower variance transmit background power estimate to determine a normalized transmit power level. The controllable attenuator is controlled as a function of the normalized transmit power level.
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Cirrus Logic Inc.
Lin Steven
Sam Phirin
Skjerven Morrill Macpherson
Ton Dang
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