At 11:20 AM 6/24/2012, enkitec@gmail.com wrote: >I want to build a FM radio squelch using a microprocessor with an=20 >ADC. The ADC will be sampling the FM radio audio output and turn on=20 >a LED when there is only noise present. The LED will be off when=20 >someone is using the channel (no noise, just voice). > >What is the best way of doing this in software? I don't have the faintest idea of how to do this in software=20 (yet). What I can tell you is how I'd be doing this in hardware -=20 perhaps that might give you an idea of how to adapt that to a software meth= od. Although there are many different squelch detect circuits, the one=20 that I like the best is a noise gate. What this does is look for=20 large amounts of high-frequency signal. This occurs where there is=20 no carrier. When the receiver detects a carrier, the noise floor drops. This is=20 called 'quieting'. In fact, one of the measurement specifications=20 for FM receivers is the amount of RF signal required for 20dB of quieting. Anyway, the hardware consists of a simple hi-pass filter right after=20 the FM detector stage. The filter frequency is set to be=20 significantly higher than the highest expected signal. The filter=20 then feeds a detector and comparitor. Large amounts of=20 high-frequency noise means that there is no carrier - and you should=20 mute the audio path. You can wrap a micro into this scenario - the output of the filter=20 can feed a simple rectifier-type detector, then into an a/d input on the mi= cro. dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .