On Jul 26, 2008, at 7:36 PM, Joeles Baker wrote:
Hi,
i wonder if anyone of you ever tried decoding POCSAG (http://en.wikipedia.org/wiki/POCSAG
) sounds or 5-tone alarm sounds on the Mac.
POCSAG is used for public radio transmissions (digital firefighter
alarming system etc) and private radio transmissions (pagers) as well.
Hello Joeless,
Yes, people have used Cocoa/MacOS X to decode analog phone and radio
transmissions. In particular, 2600 hz ;) You can write a CoreAudio
AudioUnit to accomplish this. In general audio parlance, you would use
an FFT to determine the frequency of the sound. Then, account for rate
(as in how many times it comes across the wire) to determine it's more
specific meaning in the protocol (i.e. POCSAG, DTMF, etc.) The Mac OS
X Audio API is called CoreAudio. Here are a few links to get you
started...
http://developer.apple.com/audio/
http://developer.apple.com/documentation/MusicAudio/Conceptual/AudioUnitProgrammingGuide/
http://developer.apple.com/technotes/tn2007/tn2200.html
http://developer.apple.com/documentation/Performance/Reference/vDSP_2D_FFTransforms_Reference/vDSP_2D_FFTransforms_Reference.pdf
(dated but useful)
Also, if you want to go the non-Cocoa route, there is code in the
Asterisk VoIP project that handle DTMF (essentially, what you need.
You could learn from that...
http://www.asterisk.org
As you delve into this, it's probably best to move the thread to the
CoreAudio-API mailing list.
hope that helps,
Jaime Magiera
Sensory Research
http://www.sensoryresearch.net
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