RTL-FM SSB bug: both bands are audible at same time

Software Defined Radio
1

Hi, I think I found a bug with the USB/LSB mode of RTL-FM. I can hear either type of transmission correctly, but it doesn’t matter which one I use - as both are audible through the same settings. I’m quite sure that it’s due to the low-pass (which is a rectangular window) not applying a hilbert transform to either channel. Since the hilbert transform of the sinc sin(t)/t is (1-cos(t))/t, that should be easy enough to fix by actually doing the lowpass step correctly, however I would probably suspect we’d only like to use the hilbert form of the sinc when necessary?

void full_demod(struct demod_state *d)
{
        int i, ds_p;
        int sr = 0;
        ds_p = d->downsample_passes;
        if (ds_p) {
                for (i=0; i < ds_p; i++) {
                        fifth_order(d->lowpassed,   (d->lp_len >> i), d->lp_i_hist[i]);
                        fifth_order(d->lowpassed+1, (d->lp_len >> i) - 1, d->lp_q_hist[i]);
                }
                d->lp_len = d->lp_len >> ds_p;
                /* droop compensation */
                if (d->comp_fir_size == 9 && ds_p <= CIC_TABLE_MAX) {
                        generic_fir(d->lowpassed, d->lp_len,
                                cic_9_tables[ds_p], d->droop_i_hist);
                        generic_fir(d->lowpassed+1, d->lp_len-1,
                                cic_9_tables[ds_p], d->droop_q_hist);
                }
        } else {
                low_pass(d);
        }
        /* power squelch */
        if (d->squelch_level) {
                sr = rms(d->lowpassed, d->lp_len, 1);
                if (sr < d->squelch_level) {
                        d->squelch_hits++;
                        for (i=0; i<d->lp_len; i++) {
                                d->lowpassed[i] = 0;
                        }
                } else {
                        d->squelch_hits = 0;}
        }
        d->mode_demod(d);  /* lowpassed -> result */
1 Like
2

I’ve received some feedback from the rtl-sdr main author and maintainer steve-m (who is not on discourse so far). It seems that he knows nothing about rtl_fm, as that was entirely contributed by Kyle Keen. It might be best to reach ot to him, you can find the e-mail address in the git commit log (e.g. git log ./src/rtl_fm.c and then scroll to the bottom to see the first series of commits from 2012 onwards)

3

Sure thing. Just composed and sent an email right then.