--- /dev/null
+% trellis.m
+% David Rowe June 2015
+%
+% Testing ideas for trellis decoding of codec parameters. Uses
+% soft decision information, probablility of state transitions,
+% and left over redundancy to correct errors on codec parameter
+% reception.
+%
+% [ ] Add probability of transition
+% [ ] Will this mess up a perfectly received signal, i.e. all
+% codewords perfectly received. This should dominate prob
+% calc (with small No)
+
+graphics_toolkit ("gnuplot");
+more off;
+
+function [tp codewords] = build_tp(bitstream_filename)
+ bits_per_frame = 28;
+ bit_fields = [1 5 3 3 2 4 3 3 2 2];
+ nfields = length(bit_fields);
+
+ % load encoded speech file (one float per bit)
+
+ fbitstream = fopen(bitstream_filename, "rb");
+ bitstream = fread(fbitstream, "float32");
+ fclose(fbitstream);
+ nframes = floor(length(bitstream)/bits_per_frame);
+ bitstream = bitstream < 0;
+
+ % extract LSPs codewords
+
+ codewords = zeros(nframes,nfields);
+ for f=1:nframes
+ aframe = bitstream((f-1)*bits_per_frame+1:f*bits_per_frame);
+ field = 1;
+ st = bit_fields(field);
+ for l=1:nfields
+ nbits = bit_fields(field);
+ %printf("st: %d %d\n", st, st+nbits-1);
+ codeword = aframe(st:st+nbits-1)';
+ %printf("codeword: %d\n", codeword);
+ codewords(f,l) = sum(codeword .* 2.^(nbits-1:-1:0));
+ %printf("nbits: %d lsp(%d, %d) = %d\n", nbits, f, l, codewords(f,l));
+ st += nbits;
+ field++;
+ end
+ end
+
+ % determine transition probablilities of each codeword
+ % state at time n, state a time n+1
+ % prob must add to 1
+ % so for every state, of every codeword, compute histogram of
+ % transition probabilities
+
+ tp = zeros(32,32,nfields);
+ for f=1:nframes-1
+ for l=1:nfields
+ acodeword = codewords(f,l);
+ bcodeword = codewords(f+1,l);
+ %printf("%d %d %d %d\n", f, l, acodeword, bcodeword);
+ tp(acodeword+1,bcodeword+1,l)++;
+ end
+ end
+endfunction
+
+
+% y is vector of +/- 1 soft decision values for 0,1 transmitted bits
+
+function lnp = ln_prob_of_tx_codeword_c_given_rx_codeword_y(y)
+ nbits = length(y);
+ np = 2.^nbits;
+
+ % work through all possible received codeworks and determine probability
+ % given a number of bits
+
+ lnp = zeros(1,np);
+ for i=0:np-1
+
+ % convert to binary
+
+ c = zeros(1,nbits);
+ for j=0:nbits-1
+ mask = 2.^j;
+ if bitand(i,mask)
+ c(j+1) = 1;
+ end
+ end
+
+ % map to +/- 1
+
+ c = 1 - 2*c;
+
+ % probability calculation for this i
+
+ lnp(i+1) = sum(y .* c);
+ end
+
+endfunction
+
+
+% y is the received soft decision codedwords, each row is one codeword in time
+% tp is the transition probabilities, each row is the start state
+% returns the most likely transitted codeword c
+
+function c = find_most_likely_codeword(y, tp)
+ [nstates nbits] = size(y);
+ tp
+ y_n = y(1,:); % codeword received at time n
+ y_n_1 = y(2,:); % codeword received at time n+1
+
+ lnp_n = ln_prob_of_tx_codeword_c_given_rx_codeword_y(y_n);
+ lnp_n_1 = ln_prob_of_tx_codeword_c_given_rx_codeword_y(y_n_1);
+
+ % determine probability of a path
+ % (prob of start state)(prob of transition)(prob of next state)
+ % cycle through all possibilities of states
+ % state variable for possible state
+ % step through them exhaustively
+
+ max_prob = 0;
+ for n=1:nstates
+ for n_1=1:nstates
+ prob = lnp_n(n);
+ prob += tp(n, n_1);
+ prob += lnp_n_1(n_1);
+ printf("state: %d %d %f\n", n, n_1, prob);
+ if (prob > max_prob)
+ max_prob = prob;
+ max_n = n; max_n_1 = n;
+ end
+ end
+ end
+
+endfunction
+
+% for a given parameter
+% weight for No
+% given received symbols y(n) and y(n+1)
+% calculate all possible paths
+% select most likely
+
+
+printf("loading training database and generating tp .... ");
+[tp codewords] = build_tp("hts.bit");
+printf("done\n");
+
+y = [-1 -1; -1 -1];
+find_most_likely_codeword(y,tp(1:4,1:4,5))
+
+% TODO: normalise tp (is this rqd? Just a const?), ln(tp), add No to calculation
+% then test! Try to insert in channel and correct errors
f->rx_bits = (int*)malloc(nbit*sizeof(int));
if ((f->tx_bits == NULL) || (f->rx_bits == NULL))
return NULL;
+ f->evenframe = 0;
}
if (mode == FREEDV_MODE_700) {
int recd_codeword, codeword1, data_flag_index, n_ascii;
short abit[1];
char ascii_out;
+ int reliable_sync_bit;
assert(f->nin <= f->n_max_modem_samples);
nout = f->n_speech_samples;
if (f->mode == FREEDV_MODE_1600) {
- int reliable_sync_bit;
bits_per_fdmdv_frame = fdmdv_bits_per_frame(f->fdmdv);
f->sync = f->fdmdv->sync;
f->snr_est = f->stats.snr_est;
+ if (reliable_sync_bit == 1) {
+ f->evenframe = 1;
+ }
+
if (f->stats.sync) {
- if (reliable_sync_bit == 0) {
+ if (f->evenframe == 0) {
memcpy(f->rx_bits, f->fdmdv_bits, bits_per_fdmdv_frame*sizeof(int));
nout = 0;
}
/* squelch if beneath SNR threshold or test frames enabled */
if ((f->squelch_en && (f->stats.snr_est < f->snr_squelch_thresh)) || f->test_frames) {
+ //fprintf(stderr,"squelch %f %f !\n", f->stats.snr_est, f->snr_squelch_thresh);
for(i=0; i<f->n_speech_samples; i++)
speech_out[i] = 0;
}
nout = f->n_speech_samples;
+
}
+
+ /* note this freewheels if reliable sync dissapears on bad channels */
+
+ if (f->evenframe)
+ f->evenframe = 0;
+ else
+ f->evenframe = 1;
+ //fprintf(stderr,"%d\n", f->evenframe);
+
} /* if (sync) .... */
else {
/* if not in sync pass through analog samples */
/* this lets us "hear" whats going on, e.g. during tuning */
- if (f->squelch_en == 0) {
+ //fprintf(stderr, "out of sync\n");
+
+ if (f->squelch_en == 0) {
for(i=0; i<nin_prev; i++)
speech_out[i] = FDMDV_SCALE*demod_in[i].real;
}
}
+ //fprintf(stderr,"f->stats.sync: %d reliable_sync_bit: %d evenframe: %d nout: %d\n", f->stats.sync, reliable_sync_bit, f->evenframe, nout);
return nout;
}
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