This invention relates to circuits for reducing acoustic feedback in electro-acoustic systems such as public address systems, telephones, and hearing aids and, in particular, relates to reducing feedback by precisely matching an echo in phase and amplitude.
Sound waves are slight variations in air pressure that a microphone converts into an electrical signal of varying amplitude. In a public address system and other electro-acoustic communications systems, the electrical signal is amplified and converted back into sound waves by one or more speakers.
In theory, a signal passes through a system once, never to return. Outdoors and in well designed auditoriums or concert halls, this is essentially true. In other situations, there can be a significant level of acoustic coupling between the speakers and the microphone, e.g., in a "speaker phone." When the output of an amplifier is coupled to the input of the amplifier, one has feedback, a closed loop with the potential to oscillate.
Acoustic feedback can cause a mild echo or a self-sustaining ring, depending upon the loudness of the sound returning to the microphone. The cause of the feedback can be poor placement of a speaker relative to the microphone, walls that reflect sound, and/or simply having the volume set too high on an amplifier. Electronic echo can be caused by discontinuities in the transmission medium of a telephone system or by unintended coupling between lines. Components, such as speakers and microphones, and electronic circuits introduce random variations in phase and amplitude because no two components are actually identical even if the components are the same brand and model. For example, substituting one speaker for another can affect the amplitude and phase of the feedback, as can changing the placement of a speaker or of a microphone.
There are two difficulties to eliminating feedback in an acoustic system. One difficulty is determining whether the signal passing through the amplifier is from an echo or from an original sound. A second difficulty is determining the travel time of the echo.
U.S. Pat. No. 5,412,734 (Thomasson) discloses an system for eliminating feedback by tagging the original sound with an inaudible replica of that sound, wherein the replica is a frequency modulated (FM) high frequency carrier. U.S. Pat. No. 5,649,019 (Thomasson) discloses a similar system, wherein the replica is a pulse width modulated (PWM) high frequency carrier. The replica is used to reconstruct the original signal, which is then subtracted from a composite signal, i.e. a signal representing an echo combined with other sounds. The contents of the Thomasson patents are incorporated herein by reference.
Although these patents describe effective techniques, it is desired to provide a system that does not require tagging but is compatible with tagging. Further, it is desired to provide a method and apparatus for precisely matching the phase and amplitude of a second signal and an echo in order to cancel the echo. Attempts to match phase precisely are either absent or are ineffective in systems of the prior art. Without precise phase matching, one has a positive feedback system that can oscillate.
The term "phase" is often understood implicitly as a function of frequency, i.e., phase as a fraction of a single cycle of a signal. As used herein, phase can have any value, e.g. 750.degree.. Thus considered, phase becomes more of a question of time than of frequency. As used herein, "phase" or "phase difference" and "delay" are considered synonymous.
Signals can be phase shifted by a number of different techniques, e.g., reactive networks, transmission lines, sampling and storing for later readout, and analog to digital (A/D) conversion and storing in digital memory for later readout.
The sampling rate of A/D converters in telephone systems is typically 8,000 samples per second. This number was chosen because of the relatively narrow bandwidth of a telephone system, 300-3,400 Hz, and because of the speed limitations of digital signal processing (DSP) devices. At 8,000 samples per second, the samples are separated by 125 microseconds and a 3.4 kilohertz signal is sampled only 2.3 times per cycle. This is not particularly good resolution. Stated another way, phase can be matched to within only 157.degree. at 3.4 kilohertz.
In order to increase resolution, one must increase the number of samples, which causes a corresponding increase in the number of storage sites. The number of storage sites is limited by the cost of manufacturing suitable integrated circuits and the complexity of addressing the sites in real time.
In an analog system, a signal can be sampled and the samples stored in a plurality of switched capacitors, typically the gate structure of a field effect transistor (FET). However, the storage time for the samples is presently limited by the characteristics of the storage node to approximately one half second without some sort of refreshing. For longer storage times, A/D conversion and memory storage are necessary.
A large number of storage sites adversely affects the time for the system to lock onto the delay, referred to herein as convergence. In a constantly changing environment, such as a telephone, electronic delays can change during a call and acoustic delays can change during a call because a person moves about a room. In the prior art, the settings for an echo canceling circuit are not changed during a call, largely due to a long convergence time.
Thus, the problem exists of measuring large differences in phase accurately and precisely and matching the differences between two signals to enable an effective cancellation of an echo.
In view of the foregoing, it is therefore an object of the invention to provide apparatus for accurately finding and matching phase with an echo.
A further object of the invention is to improve the ability to match amplitude by precisely matching the phase of two signals or portions of two signals.
Another object of the invention is to provide a method for precisely matching the phase of two signals or portions of two signals even when one of the signals is delayed by as much as 1,500 milliseconds relative to the other signal.