exploring effect of cont phas ein FSK modulator on tx spectrum

git-svn-id: https://svn.code.sf.net/p/freetel/code@2682 01035d8c-6547-0410-b346-abe4f91aad63

diff --git a/codec2-dev/octave/fsk_cont_phase.m b/codec2-dev/octave/fsk_cont_phase.m
new file mode 100644 (file)
index 0000000..d6b4e9d
--- /dev/null
+++ b/codec2-dev/octave/fsk_cont_phase.m
@@ -0,0 +1,85 @@
+% fsk_cont_phase.m
+% David Rowe Feb 2016
+%
+% Looking at tx spectrum of FSK signal with and without continuous phase in the
+% modulator.
+
+1;
+
+function tx  = fsk_mod(states, tx_bits)
+
+    M  = states.M;
+    Ts = states.Ts;
+    Fs = states.Fs;
+    ftx = states.ftx;
+    phase_cont = states.phase_cont;
+
+    num_bits = length(tx_bits);
+    num_symbols = num_bits;
+    tx = zeros(states.Ts*num_symbols,1);
+    tx_phase = 0;
+    s = 1;
+
+    for i=1:num_bits
+
+      % map bits to tone number
+
+      tone = tx_bits(i) + 1;
+ 
+      if phase_cont
+        tx_phase_vec = tx_phase + (1:Ts)*2*pi*ftx(tone)/Fs;
+        tx_phase = tx_phase_vec(Ts) - floor(tx_phase_vec(Ts)/(2*pi))*2*pi;
+      else
+        tx_phase_vec = (1:Ts)*2*pi*ftx(tone)/Fs;
+      end
+
+      tx((s-1)*Ts+1:s*Ts) = 2.0*cos(tx_phase_vec);
+      s++;
+      %printf("phase_cont: %d tx_phase_vec(Ts): %f\n", phase_cont, tx_phase_vec(Ts));
+
+    end
+endfunction
+
+
+states.Fs  = 8000;
+states.Rs  = 100;
+states.Ts  = states.Fs/states.Rs;
+states.M   = 2;
+states.ftx = [1200 1305];  % need to choose these carefullto get disc phase
+
+Nbits = 1024;
+
+tx_bits = rand(1,Nbits) > 0.5;
+%tx_bits = [0 1 0];
+
+states.phase_cont = 1;
+tx_cont  = fsk_mod(states, tx_bits);
+
+states.phase_cont = 0;
+tx_disc  = fsk_mod(states, tx_bits);
+
+figure(1)
+clf
+plot(tx_cont(1:states.Ts*4))
+hold on;
+plot(tx_disc(1:states.Ts*4),'g')
+hold off;
+title('Time Domain')
+
+figure(2)
+clf
+subplot(211)
+Tx_cont = fft(tx_cont);
+Tx_cont_dB = 20*log10(abs(Tx_cont));
+plot(Tx_cont_dB)
+axis([1 length(Tx_cont_dB)/2 0 100]);
+grid
+title('Cont Phase Spectra')
+
+Tx_disc = fft(tx_disc);
+Tx_disc_dB = 20*log10(abs(Tx_disc));
+subplot(212)
+plot(Tx_disc_dB,'g')
+axis([1 length(Tx_cont_dB)/2 0 100]);
+grid
+title('Disc Cont Phase Spectra')