Alarm system audio interface tamper and state detection

1. An alarm system comprising:

an audio interface comprising:

a microphone for generating an electrical signal of audio sensed at said audio interface;

a state sensor for sensing tampering at said audio interface;

a signal generator for generating an analog state signal indicative of said tampering;

a combiner for forming a combined signal from an electrical signal allowing audio sensed at said microphone to be reproduced, and said state signal so that said state signal does not interfere with reproduction of audio from said electrical signal;

a control panel for monitoring sensed conditions at said audio interface and reproducing audio sensed at said audio interface at said control panel;

a conductor interconnecting said audio interface to said control panel carrying said electrical and said state signal as combined from said audio interface to said control panel.

2. The alarm system of claim 1 , wherein said state sensor comprises a tamper switch for sensing tampering with said audio interface.

3. The alarm system of claim 1 , wherein said signal generator comprises a low frequency oscillator producing a frequency below the frequency of audible sound.

4. The alarm system of claim 3 , wherein said low frequency oscillator generates a signal having a frequency less than 50 Hz.

5. The alarm system of claim 3 , wherein said low frequency oscillator generates a signal having a frequency less than 25 Hz.

6. The alarm system of claim 1 , wherein said combiner comprises an analog multiplexer.

7. The alarm system of claim 1 , further comprising two power conductors, for providing electrical power to said audio interface.

8. The alarm system of claim 7 , wherein said two power conductors interconnect said control panel to said audio interface.

9. The alarm system of claim 1 , wherein absence of said state signal on said conductor indicates an alarm condition to said control panel.

10. The alarm system of claim 1 , wherein said signal generator comprises a oscillator producing a frequency above the frequency of audible sound.

11. The alarm system of claim 1 , wherein said combiner comprises a summing amplifier.

12. A method of operating an alarm system comprising a control panel and an audio interface:

sensing an audio signal at an audio transducer at said audio interface;

sensing a state at said audio interface panel, and generating an analog electric state signal in response thereto;

mixing at an analog mixer said audio signal and said state signal to form a combined signal from which audio sensed at said audio transducer may be reproduced, and said state signal does not interfere with reproduction of audio from said combined signal; and

providing said combined signal to said control panel, for sensing said state at said control panel and for reproducing said audio signal at said panel.

13. The method of claim 12 , wherein said sensing a state comprises sensing tampering with said audio interface.

14. The method of claim 12 , wherein said mixing comprises summing said audio signal and said state signal on a single conductor.

15. The method of claim 12 , wherein said state signal has a frequency of less than 50 Hz.

16. An audio interface comprising:

a microphone for generating an electrical signal of audio sensed at said audio interface;

a state sensor for sensing a state at said audio interface;

a signal generator for generating an analog state signal indicative of said state;

a combiner for forming a combined signal from an electrical signal corresponding to audio sensed at said microphone, and said state signal so that said audio may be reproduced from said combined signal at a control panel and said state signal does not interfere with reproduction of audio from said electrical signal.