BTW, besides using FM this waveform is also perfectly useable with a simple double sideband modulator..
On 04/10/2020 10:58 PM, Jeroen Vreeken wrote: > So here are some first BER experiments. > I made a few small improvements to the demodulator (thanks to the > measurements). > But basicly it is still the same. > I used the fm.m modulator and demodulator. > > ber > > The amount of measurement points is still a bit limited, and since I > only use about 24seconds of audio data the numbers are not realy > accurate as of 16dB (from then on the BER is 0, since there are around > 1e+5 bits in the file that makes sense). > Since fm.m introduces noise compared to the carrier this is not a true > Eb/N0 value. > If I am correct the defaults for fm.m limit the bandwidth to 16kHz so > the Eb/N0 should be 4.2dB higher for a 6000 bps rate. > > A funny coincidence: shifting this plot 4.2 db more or less gives the > same plot as David got for his Bell 202 simulation. > Except ofcourse that this is 5 times more bits going through the same > channel. > > ber ebn0 > > I am going to do some more measurements: I want to see the effects of > scrambling and I also want to try some different pulse shapes. > The current shape is a raised cosine in the time domain, but a raised > cosine in the frequency domain should allow a smaller bandwidth. Since > the fm.m demodulator already cuts of most of the high frequencies it > seems we don't really need them and not sending them would allow more > energy to go into the usefull data. > Btw, the article 'The Shape of Bits to Come' > (https://www.qsl.net/n9zia/a108/index.html) was some great inspiration > to me years ago when I was trying to decode cubesat data. > > 73, > Jeroen > > > On 04/08/2020 12:00 AM, David Rowe wrote: >> This simulation: >> >> https://github.com/drowe67/codec2/blob/master/octave/fsk.m >> >> Was used to compare non-coherent 2FSK to a AFSK over FM, it includes an >> analog FM modem simulation fm.m (equivalent of a VCO/discrimator). The >> fm.m module has optional voice band filters. >> >> mancyfsk.m is similar, but with manchester encoder baseband signal, >> which I think is similar to what is being used in this work. >> >> Reason I mention this is that the best way to compare the effect of any >> signal processing step (like a scrambler) is to measure the bit error rate. >> >> Cheers, >> David >> >> On 8/4/20 7:12 am, Jeroen Vreeken wrote: >>> Indeed, I use the packet port of my ft-817, so the signal goes almost >>> directly to the oscilator and almost directly from the discriminator. >>> It might also work over the audio ports as the bandwidth might fit if >>> the filters are not to tight. >>> But then you would have to compensate for the pre/de-emphasis. I haven't >>> tried that yet. >>> >>> The bit stuffing is indeed to make sure we don't go near DC, and to make >>> sure there are enough 'flips' to recover the bit timing. >>> The scrambler looks nice, but we would have to find a balance between >>> lots of zeroes (easy to sync) and a perfect spectrum. >>> I'll try some variants on the air soon. >>> Also note that the frames generated with the test program are not >>> varying much, a stream with real-world data might look different. But >>> the pictures you showed both don't look that bad... >>> >>> >>> On 04/07/2020 11:24 PM, Steve wrote: >>>> I believe it is just direct FSK of the FM oscillator. +/- some >>>> deviation. Using the data port of most modern VHF radios. >>>> So on receive you just slice the ratio detector (discriminator output). >>>> >>>> The bit-stuffing keeps from long runs of a high or low. My experiment >>>> is to replace bit stuffing with a scrambler, but I haven't found all >>>> the places to remove the bit-stuffing :-) >>>> >>>> On Tue, Apr 7, 2020 at 4:08 PM David Rowe <[email protected] >>>> <mailto:[email protected]>> wrote: >>>> >>>> Hi Steve, >>>> >>>> >From what I understand (please feel free to correct me Jeroen) >>>> this is a >>>> baseband PAM waveform over analog FM. I'm haven't seen any >>>> theoretical >>>> analysis of these schemes, but we did measure some a few years back: >>>> >>>> http://www.rowetel.com/wordpress/?p=4663 >>>> >>>> Cheers, >>>> David >>>> >>>> On 7/4/20 5:58 pm, Steve wrote: >>>> > What would be expected here? Say about 9.5 dB Eb/N0 at 10E-5? I >>>> think >>>> > that's what the G3RUH modem got. >>>> > >>>> > It will definitely fill up the channel! :-) >>>> > >>>> > On Tue, Apr 7, 2020 at 2:43 AM David Rowe <[email protected] >>>> <mailto:[email protected]> >>>> > <mailto:[email protected] <mailto:[email protected]>>> wrote: >>>> > >>>> > Steve and Jeroen - it would be interesting to measure the >>>> modem BER (or >>>> > Packet Error Rate) performance. Steve - we could use >>>> cohpsk_ch to >>>> > inject measured amounts of noise. >>>> > >>>> > Cheers, >>>> > David >>>> >>>> >>>> >>>> >>>> >>>> _______________________________________________ >>>> Freetel-codec2 mailing list >>>> [email protected] >>>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>> >>> _______________________________________________ >>> Freetel-codec2 mailing list >>> [email protected] >>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>> >> _______________________________________________ >> Freetel-codec2 mailing list >> [email protected] >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >> > > > > > > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2
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