Abstract: A low pass limited, low frequency pseudo-random noise signal is added to the input signal for tagging the input signal in order to facilitate correlation with an echo.

Abstract: An echo is cancelled from a composite signal by providing a coarse phase adjustment, a fine phase adjustment, an amplitude correction, and then subtracting the phase shifted signal from the composite signal. In the coarse adjustment, the phase is slightly under-corrected, thereby allowing the fine adjustment to proceed in a single direction and decreasing convergence time. Correlation is provided by an amplitude inverting multiplying circuit and phase adjustment is provided by an all-pass phase shift network combined with an impedance multiplier coupled to the phase shift network.

Abstract: A band pass filter includes a pair of notch filters having different notch frequencies and a difference amplifier coupled to the outputs of the notch filters. The notch filters preferably have response curves that intersect at −3 dB. The separation of the notch frequencies determines, in part, the ripple in the response curve of the band pass filter.

Abstract: A filter includes an FET and a capacitor in a phase shift network wherein the FET operates as a variable resistor. An impedance multiplier is coupled to the FET for increasing the range of resistance of the FET.

Abstract: Sound is converted into an electrical signal by a microphone and is converted into an inaudible, digitally modulated signal that is combined with the electrical signal from the microphone, amplified, and converted into sound waves by a speaker. Any sound traveling from the speaker back to the microphone includes the inaudible component representing the original sound. The inaudible component is separated from the audible components, and the original sound is reconstructed in a digital demodulator. The reconstructed original sound is subtracted from the signal from the microphone, thereby reducing any echo and canceling feedback. Digital modulation includes any form of shift keying, including coherent and noncoherent techniques for modulating frequency, phase, or amplitude.

Abstract: A high resolution delay line includes a coarse delay having a minimum period of delay and a fine delay having a total delay, wherein the total delay is equal to or greater than half the minimum period. Each delay can be implemented in analog or digital form and the delay line can be implemented with one portion in analog form and the remainder in digital form. The digital delay can provide a delay upward of 1,500 milliseconds. The fine delay provides a resolution of ten microseconds or less. An unknown delay is measured by coupling a signal into two channels, wherein the first channel includes the unknown delay and the second channel includes the coarse delay and the fine delay. The output signals from the channels are correlated while adjusting the coarse delay for maximum correlation and then adjusting the fine delay for maximum correlation.

Abstract: The signal in each channel of a stereo is modulated at an inaudible frequency by a replica of the original signal. The modulated signal is broadcast into a room by a loudspeaker and is picked up by a microphone. The microphone is coupled to the stereo, which includes a demodulator for separating the replica from the signal as received at the microphone. By comparing the replica with the demodulated signal, data is extracted to compensate for the acoustic characteristics of the loudspeaker and the room in which the loudspeaker is located.

Abstract: Sound is converted into an electrical signal by a microphone and is converted into an inaudible, pulse width modulated signal that is combined with the electrical signal from the microphone, amplified, and converted into sound waves by a speaker. The pulse width modulator includes an A/D converter coupled to a shift register in a digital encoder. Any sound travelling from the speaker back to the microphone includes the inaudible component representing the original sound. The inaudible component is separated from the audible components, and the original sound is reconstructed in a pulse width demodulator including a shift register in a digital decoder coupled to a D/A converter. The reconstructed original sound is subtracted from the signal from the microphone, thereby reducing any echo and cancelling feedback. The apparatus includes amplitude correction circuitry for flattening the frequency response of the apparatus and includes phase correction circuitry for eliminating phase shifts in the apparatus.

Abstract: An electronic stethoscope includes a microphone mounted in a resilient collar in a hand-held case. The microphone is coupled to an active filter having independently and continuously adjustable bandwidth and center frequency. A switch bypasses the active filter when the user wishes to hear all of the sounds detected by the microphone. The controls for bandwidth and center frequency are located on one side of the case and can be manipulated by the hand holding the stethoscope. The volume or gain of the stethoscope is adjusted by a third control located on the one side of the stethoscope. The controls can be linear or rotary or combinations thereof. The output signal from the active filter passes through a filter having a frequency response inverse to that of the human ear, is amplified, and is coupled to headphones which plug into a socket on the rear of the case.

Abstract: The AM signal from a microphone is converted into an inaudible, FM signal which is combined with the AM signal to form a composite signal. The composite signal is amplified and converted into sound waves by a speaker. Any sound returning to the microphone from the speaker has an FM component and the FM component is demodulated to reconstruct the original AM sound. The reconstructed AM signal is subtracted from the signal from the microphone, thereby removing the echo and cancelling feedback.