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Re: Decoding digital POCSAG / analog 5-tone (=selective call) sounds

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Re: Decoding digital POCSAG / analog 5-tone (=selective call) sounds

Subject: Re: Decoding digital POCSAG / analog 5-tone (=selective call) sounds
From: Jaime Magiera <email@hidden>
Date: Sat, 26 Jul 2008 19:58:58 -0400

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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