Hello Rud, You do not need to perform FFT for signal processing. You can use two vasic methods to demodulate FSK. I'd like say more, the FFT has a very slow responce, so it can be used for low baudrate modes only.
The first is creating two filters for two frequencies and two amlitude detectors, the demodulatiob result is the difference between the two above mentioned amplitudes. The second is creating a common filter which passes all the needed spectru, and a phase detector after that. See http://www.DSPguide.com for more details. Let me know in case of more questions. Saturday, January 28, 2006, you wrote: Rud> I want to experiment with digital protocols in ham radio Rud> by starting with the PC soundcard. I understand some of the Rud> basics like filters and FFT. I even have some of them working in Rud> code (C++). Rud> ═ Rud> I do not know how to proceed from the basics into Rud> demodulating an audio signal into bits. Rud> ═ Rud> For instance, I am playing with the VHF packet protocol Rud> which use 1200 and 2200 hz signalling. I enter the samples for a Rud> single 1200 hz cycle into the FFT array. The FFT shows a peak at Rud> around 1000 hz. The FFT array═is 2048 samples long filled with Rud> 0s except for the 1 cycle of 1200 hz which is 40 samples at the Rud> rate of 48000 samples per second. If I put 4 cycles in the FFT Rud> array I get 1195 hz which is correct. Hanning or Hamming Windows Rud> do not help. Rud> ═ Rud> I have looked at some of the available source but am Rud> missing the insight I need for proceeding further. Any Rud> suggestions on how to proceed further? Any suggestions on books Rud> or web sites that address communications DSP at this level? How Rud> about an email reflector where I can ask questions of DSP gurus? Rud> ═ Rud> -73- Rud> Rud K5RUD Rud> ARES AEC Rud> Montgomery County, TX Rud> ═ Rud> ═ Rud> Need a Digital mode QSO? Connect to═ Telnet://cluster.dynalias.org Rud> Other areas of interest: Rud> The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ Rud> DigiPol: http://groups.yahoo.com/group/Digipol═ (band plan policy discussion) Rud> YAHOO! GROUPS LINKS Rud> ═Visit your group "digitalradio" on the web. Rud> ═ ═To unsubscribe from this group, send an email to: Rud> [EMAIL PROTECTED] Rud> ═ ═Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service. -- Best regards, Nick mailto:[EMAIL PROTECTED] Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
