Re: [soft_radio] RF Digital Protocol Education

[Andy]

>-----Original Message----- >From: Rud Merriam <[EMAIL PROTECTED]> >To: Undisclosed-Recipient:@one.surfree.co.uk; <Undisclosed-Recipient:@one.surfree.co.uk;> >Date: 2006/01/27 11:16 >Subject: [soft_radio] RF Digital Protocol Education > >I want to experiment with digital protocols in ham radio by starting with the PC soundcard. I understand some of the basics like filters and FFT. I even have some of >them working in code (C++). ========================================================== FFTs and similar are not suitable for narrow sgift FSK wher ethe spectra of each tone keyed on-off overlap eachother.   I relayed your question to a 'guru' of DSP and data demodulation, Charles G4GUO.  His response was : "-------------------------- mix IQ down to baseband, filter, take the arctan of the result, subtract from the previous result to get the phase change, watch for +/- pi transitions here is some code I have used (this was for 300 baud FSK, but I have used it for 1200 and 9600 as well as plain old FM audio demod). Anything prefixed with m_ is a global variable (actually it is a member variable in C++ speak).   float CFsk::discriminate( float real, float imag ) {  float  change,phase;    // discriminator  phase = atan2( imag, real );    change      = phase - m_old_phase;  m_old_phase = phase;    if( fabs( change ) > M_PI )  {   if(change > 0 )    change -= (float)T_PI;   else    change += (float)T_PI;  }  return change; }   void CFsk::demodulate_high_baud_samples( float *in, unsigned int length ) {    unsigned int    i;  float real,imag,delta;    m_mag        = 0;  m_out_length = 0;    delta = m_mid_delta;    for( i = 0; i < length; i++ )  {   //   // Mix to base band   //   real   =  cos(m_mid_acc)*in[i];   imag = -sin(m_mid_acc)*in[i];     real = matched_filter( mfb[0], hi300, real, HI300LEN );   imag = matched_filter( mfb[1], hi300, imag, HI300LEN );     m_out_mem[m_out_length++] = discriminate( real, imag );     //   // Update the accumulators   //   m_mid_acc += delta;   if( m_mid_acc >= T_PI ) m_mid_acc -= (float)T_PI;  } }   --------------"   Hope that helps - Andy  G4JNT www.scrbg.org/g4jnt/     YAHOO! GROUPS LINKS  Visit your group "soft_radio" on the web.    To unsubscribe from this group, send an email to:  [EMAIL PROTECTED]    Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.