Control of a microphone
View Patent ↗A microphone circuit has a clip detection circuit ( 30 ) which detects when an analog to digital converter (ADC, 12 ) output has reached a threshold. A variable capacitance ( 34 a, 34 b, 34 c, 34 d ), which functions as a variable input load associated with the microphone ( 11 ), is controlled based on the clip detection circuit output, the feedback is thus based on the ADC output level, and the processing of this signal can be implemented without requiring baseband processing of the signal—it can simply be based on a state of the ADC output.
1. A microphone circuit comprising:
a microphone;
an analogue to digital converter at an output of the microphone, the analogue to digital converter including a sigma delta converter configured to generate a pulse density modulation output;
a clip detection circuit for detecting when the pulse density modulation output of the analogue to digital converter has reached a threshold, the clip detection circuit configured to generate an output in response to the pulse density modulation output;
a variable capacitance which functions as a variable input load associated with the microphone; and
a control circuit for controlling the variable capacitance based on the clip detection circuit output, wherein the control circuit includes a counter circuit, which is controlled to increase or decrease a count in response to the clip detection circuit generating a detection signal indicative of whether clipping has been detected.
2. A circuit as claimed in claim 1 , wherein the variable capacitance comprises an array of capacitors in parallel between the microphone output and a control terminal, wherein the capacitors of the array are individually switchable into or out of the parallel circuit.
3. A circuit as claimed in claim 2 , wherein the variable capacitance comprises a binary weighted array of capacitors.
4. A circuit as claimed in claim 1 , wherein the counter circuit is controlled to increase a count in response to one of a clip-detection signal and a no-clip-detection signal, and to decrease the count in response to the other of the clip-detection signal and the no-clip-detection signal.
5. A circuit as claimed in claim 1 , wherein the sigma delta converter comprises a 1 bit sigma delta converter configured to generate the pulse density modulation output, the pulse density modulation output provided to the clip detection circuit as feedback and provided to a baseband processor.
6. A circuit as claimed in claim 5 , wherein clip detection signal is based on the pulse density modulation output reaching a threshold ratio of: number of bits in a bit stream that are a common polarity in a time period versus a total number of bits in the bit stream in the time period.
7. A method of processing a microphone output signal, comprising:
converting the microphone output signal to a pulse modulation output, in digital form, using an analogue to digital converter, wherein the analogue to digital converter includes a sigma delta converter;
providing the pulse modulation output as feedback to a clip detection circuit;
implementing clip detection by detecting when the pulse modulation output has reached a threshold using the clip detection circuit; and
controlling a variable capacitance which functions as a variable input load associated with the microphone based on the clip detection including operating a counter by increasing or decreasing a count in response to the output from the clip detection circuit including a detection signal indicative of whether clipping has been detected.
8. A method as claimed in claim 7 , wherein the variable capacitance comprises an array of parallel capacitors and controlling the variable input load comprises individually switching capacitors of the array into or out of circuit.
9. A method as claimed in claim 7 , wherein operating the counter includes increasing a count in response to one of a clip-detection signal and a no-clip-detection signal, and decreasing the count in response to the other of the clip-detection signal and the no-clip-detection signal.
10. A method as claimed claim 7 , wherein the converting comprises using a 1 bit sigma delta converter configured to generate the pulse density modulation output.
11. A method as claimed in claim 10 , outputting a clip detection signal by the clip detection circuitry, in response to the pulse density modulation output reaching a threshold ratio of: number of bits in a bit stream that are a common polarity in a time period versus a total number of bits in the bit stream in the time period.
12. A circuit as claimed in claim 1 , wherein the analogue to digital converter is configured to directly provide the pulse modulation output to the clip detection circuit.
13. A circuit as claimed in claim 1 , wherein the sigma delta converter comprises a multi-bit sigma delta converter.
14. A method as claimed in claim 7 , wherein implementing the clip detection includes detecting the pulse density modulation output reaches a threshold ratio of bits that are one in a time period.
15. A method as claimed in claim 7 , wherein providing the pulse modulation output as feedback to the clip detection circuit includes directly providing the pulse modulation output from the analogue to digital converter to the clip detection circuit.
16. A microphone circuit comprising:
a microphone;
an analogue to digital converter at an output of the microphone, the analogue to digital converter including a sigma delta converter configured to generate a pulse density modulation output;
a clip detection circuit configured to detect the pulse density modulation output of the analogue to digital converter reaching a threshold ratio of number of bits in a bit stream that are a common polarity in a time period versus a total number of bits in the bit stream in the time period, the clip detection circuit further configured to generate and output a clip detection signal in response to the pulse density modulation output reaching the threshold ratio;
a variable capacitance which functions as a variable input load associated with the microphone; and
a control circuit configured to control the variable capacitance based on the clip detection signal, wherein the control circuit includes a counter circuit, which is controlled to increase or decrease a count in response to the clip detection circuit generating the detection signal indicative of whether clipping has been detected.
17. A circuit as claimed in claim 16 , wherein the control circuit is further configured to control the counter circuit by increasing a count in response to one of a clip-detection signal and a no-clip-detection signal, and by decreasing the count in response to the other of the clip-detection signal and the no-clip-detection signal.
18. A circuit as claimed in claim 16 , wherein the analogue to digital converter is further configured to provide the pulse modulation output as feedback to the clip detection circuit.
19. A circuit as claimed in claim 16 , wherein the analogue to digital converter is further configured to provide the pulse modulation output as feedback directly from the analogue to digital converter to the clip detection circuit.
20. A circuit as claimed in claim 16 , wherein the sigma delta converter comprises a 1 bit sigma delta converter configured to generate the pulse density modulation output, the pulse density modulation output provided to the clip detection circuit as feedback and provided to a baseband processor.