An echo detection circuit that detects an echo by detecting the magnitude of a digitally mixed representation of the received acoustic signal and reference sine and cosine signals. That magnitude is then compared against an echo threshold to verify the presence or absence of an echo signal. A low pass filter with a configurable cut-off frequency may be used to define the selectivity of the echo detector.
1. An echo detection circuit comprising:
an acoustic sensor configured to convert sensed acoustic pressures into a pre-processed analog electrical signal;
analog processing circuitry configured to transform the pre-processed analog electrical signal into a post-processed analog electrical signal suitable for providing to an analog-to-digital converter;
an analog to digital converter configured to convert the post-processed analog electrical signal into a digital signal;
a sine mixer configured to digitally mix the digital signal provided by the analog to digital converter with a sine wave of approximately the same frequency as a carrier wave of a transmitted acoustic signal whose echo is being detected;
a cosine mixer configured to digitally mix the digital signal provided by the analog to digital converter with a cosine wave of approximately the same frequency as the carrier wave of the transmitted acoustic signal whose echo is being detected;
a vector magnitude detection mechanism that detects a magnitude of the vector represented by at least a derivative of a signal received from the sine mixer, and at least a derivative of a signal received from the cosine mixer; and
a threshold detection circuit that compares the magnitude of the vector with one or more thresholds to determine whether or not an echo is detected.
2. An echo detection circuit in accordance with claim 1 , further comprising:
a sine low pass filter positioned between the sine mixer and the vector magnitude detection mechanism; and
a cosine low pass filter position between the cosine mixer and the vector magnitude detection mechanism.
3. An echo detection circuit in accordance with claim 2 , wherein the sine low pass filter and the cosine low pass filter are each configured as a series of one or more first order Infinite Impulse Response (IIR) filters.
4. An echo detection circuit in accordance with claim 3 , wherein the sine low pass filter and the cosine low pass filter are each configured as a series of two first order Infinite Impulse Response (IIR) filters.
5. An echo detection circuit in accordance with claim 2 , wherein the vector magnitude detection mechanism detects the magnitude of the vector represented by the filtered signal received from the sine mixer and the filtered signal received from the cosine mixer.
6. An echo detection circuit in accordance with claim 2 , wherein the sine low pass filter has a configurable cut-off frequency.
7. An echo detection circuit in accordance with claim 6 , wherein the cosine low pass filter has a configurable cut-off frequency.
8. An echo detection circuit in accordance with claim 7 , wherein the cut-off frequency of the sine low pass filter is configured to approximately be the same as the cut-off frequency of the cosine low pass filter.
9. An echo detection circuit in accordance with claim 1 , wherein the cosine wave represents a digital signal that represents digital values generated by a look up table.
10. An echo detection circuit in accordance with claim 1 , wherein the threshold detection circuit compares the magnitude of the vector with one or more thresholds by performing the following:
an act of determining whether a sine component of the vector is above a sine threshold value;
an act of determining whether a cosine component of the vector is above a sine threshold value;
an act of determining whether the vector exceeds a diagonal threshold.
11. An echo detection circuit in accordance with claim 10 , wherein the three acts of determining are performed in parallel.
12. An echo detection circuit in accordance with claim 10 , wherein the threshold detection circuit detects an echo if any of the acts of determining result in the corresponding threshold being exceeded.
13. An echo detection circuit in accordance with claim 1 , wherein the analog to digital converter samples the post-processed analog electrical signal at an oversampling rate of eight compared to the frequency of a transmitted acoustic signal.
14. An echo detection circuit in accordance with claim 1 , wherein the one of more thresholds used by the threshold detection circuits are configurable.
15. An echo circuit comprising:
a resonator;
an acoustic transmitter that is configured to transmit an acoustic signal at a particular frequency;
an echo detection circuit that uses the resonator to convert detected acoustic pressures and convert such pressures into an analog signal that may be used to detect echoes from the acoustic signal transmitted by the acoustic transmitter, the echo detection circuit comprising:
an analog to digital converter configured to convert at least a derivative of the analog signal into a digital signal;
a sine mixer configured to digitally mix the digital signal provided by the analog to digital converter with a digital sine wave;
a cosine mixer configured to digitally mix the digital signal provided by the analog to digital converter with a digital cosine wave;
a vector magnitude detection mechanism that detects a magnitude of the vector represented by at least a derivative of a signal received from the sine mixer, and at least a derivative of a signal received from the cosine mixer; and
a threshold detection circuit that compares the magnitude of the vector with one or more thresholds to determine whether or not an echo is detected; and
a clock configured to control the transmission frequency of the acoustic transmitter, the oversampling ratio of the analog to digital converter, and the mixing rate of the sine mixer and the cosine mixer.
16. An echo circuit in accordance with claim 15 , wherein the resonator is shared by the acoustic transmitter and the echo detection circuit.