Control of a microphone
View Patent ↗A microphone circuit has a clip detection circuit which detects when an analogue to digital converter output has reached a threshold. A variable attenuator 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. An overrule function is also provided so that the clip detection AGC control can be inhibited or controlled differently.
1. A microphone circuit comprising:
a microphone;
an analog to digital converter at the output of the microphone;
a clip detection circuit configured to detect when the analog to digital converter output has reached a threshold;
a variable attenuator coupled to the microphone;
a control circuit configured to control the variable attenuator based on output of the clip detection circuit; and
an overrule circuit configured to overrule the control circuit.
2. The circuit as claimed in claim 1 , wherein the variable attenuator comprises a variable load.
3. The circuit as claimed in claim 1 , wherein the microphone circuit further comprises:
a format circuit configured to set the microphone during initialization as a left microphone or a right microphone based on a left/right control input to the microphone.
4. The circuit as claimed in claim 3 , wherein the overrule circuit comprises:
a timing circuit; and
a controller, wherein the left/right control input is used by the controller to control the overruling operation after a delay set by the timing circuit.
5. The circuit as claimed in claim 4 , wherein the left/right control input is used by the format circuit before the end of the delay set by the timing circuit.
6. The circuit of claim 4 , wherein the delay set by the timing circuit is longer than the initialization.
7. The circuit as claimed in claim 1 , wherein the overrule circuit inhibits the control of the variable attenuator.
8. The circuit as claimed in claim 1 , wherein the overrule circuit is configured to provide the control of the variable attenuator taking into account parameters in addition to or instead of the output of the clip detection circuit.
9. The circuit as claimed in claim 1 , wherein the control circuit comprises:
a counter, which 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.
10. The circuit as claimed in claim 1 , wherein the analog to digital converter comprises:
a 1 bit sigma delta converter with a pulse density modulation signal.
11. The circuit as claimed in claim 10 , wherein the clip detection is based on the pulse density modulation signal reaching a threshold.
12. The circuit of claim 1 , wherein automatic gain control (AGC) is switched off after the overrule circuit overrules the control circuit.
13. The circuit of claim 1 , wherein a host processor is configured to take over control of automatic gain control (AGC) after the overrule circuit overrules the control circuit.
14. The circuit of claim 1 , wherein the overrule circuit comprises a dedicated input terminal for an AGC overrule command.
15. The circuit of claim 1 , wherein the variable attenuator comprises a programmable gain amplifier.
16. A method of processing a microphone output signal, comprising:
converting an analog output signal of a microphone to digital using an analog to digital converter;
implementing clip detection by detecting when the analog to digital converter output has reached a threshold;
controlling a variable attenuator coupled to the microphone based on the clip detection; and
detecting if an overrule command is present, and if present, overruling the controlling of the variable attenuator based on the clip detection.
17. The method as claimed in claim 16 , wherein the controlling comprises:
operating a counter, which is controlled to increase the 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.
18. The method as claimed in claim 16 , wherein the converting comprises:
using a 1 bit sigma delta converter with a pulse density modulation output.
19. The method as claimed in claim 16 , further comprising:
using the left/right control input to control the overruling operation after the initialization.
20. The method as claimed in claim 16 , wherein the overrule function inhibits control of the variable attenuator or provides control of the variable attenuator taking into account parameters other than the clip detection.