| 摘要 |
An automatic squelch is provided within an intercom to control the gating of a microphone signal path. The automatic squelch includes an analog-to-digital converter coupled to receive an analog microphone input signal which is configured to convert the analog microphone input signal into a digital input signal. A peak detector is further provided to maintain a peak value indicative of peaks in the digital input signal. The peak value maintained by the peak detector reacts relatively quickly to changes in the digital input signal. In one implementation, when the digital input signal is larger than the peak value, the peak value is set at the level of the digital input signal. When the digital input signal is less than the peak value, the peak value is "decayed" or reduced at a relatively fast decay rate. A background noise detector is further provided for maintaining a background value indicative of background noise in the digital input signal. In one implementation, the background value is adjusted by comparing the peak value to the background value. When the peak value is greater than the background value, the background value is increased at a relatively slow attack rate. When the peak value is less than the background value, the background value is decreased at a relatively slow decay rate. The background noise detector thus varies the background value relatively slowly in response to changes in the peak value. A comparator is further provided to determine whether the peak value exceeds a predetermined threshold value. In one implementation, the predetermined threshold value is obtained through a table lookup function wherein the background value is used as an index to index into a lookup table including a plurality of corresponding threshold values. If the peak value exceeds the threshold value, a one-shot timer is triggered to activate a microphone signal path. In one implementation, hysteresis is further provided to lessen the threshold required to retrigger the one shot timer while it is active. The functionality of the peak detector, background noise detector, and one shot timer may be implemented by software code executed within a microcontroller.
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