Hi Brady, Thank you very much for the detailed.description of the code. This was very helpful. It appears that there is no difference between 2400b and my modem except for the pulse shaping filters. I think the performance should also.be very similar. Unfortunately I no longer have the time to keep playing with this. Eagerly awaiting the sm2000, I keep hitting refresh every day on David's website :)
@chris , Adrian On 7 July 2016 01:33:27 GMT+01:00, Brady O'Brien <brady.obrien...@gmail.com> wrote: >Hi Adrian, > >About 2400B: > >-Pulse Shaping: None on the TX end, it's just a manchester encoded bit >stream >-RX Symbol Filtering: Just a simple integrate-and-dump aligned with the >symbols >-Frequency tracking: None. >-Phase tracking and clock offset correction: This is done by the fine >timing estimator > >The job of the fine timing estimator is to estimate the location of >symbols >in the incoming sample stream. It first takes the incoming sample >stream >and applies a nonlinarity, in our case s^2. This causes a line to >appear in >the frequency domain of the incoming signal. We then extract the >average >phase of this nonlinearity over the buffer of samples. This phase is >proportional to the offset of the symbols within the sample stream. >This is >all done on lines 186 to 218 of fmfsk.c . In order to handle clock >offset, >the demod may consume a variable number of samples for a fixed number >of >bits. A sample clock offset can be though of as a drift in the symbol >offset over time. To compensate for this, the demod tries to keep the >symbol offset within a window. If the symbol offset is outside of this >window, it will change the number of samples consumed in the next frame >to >either skip or re-process a few samples. This is done in lines 233 to >242 >of fmfsk.c. > >Thanks, > >Brady O'Brien > >On Thu, Jul 7, 2016 at 4:50 AM, Adrian Musceac <kanto...@gmail.com> >wrote: > >> Hi Brady, >> >> Regarding using Python scripts instead of GRC, I am aware of the >power of >> Python, I use it daily, but I still find the "visual" design tools >(like >> the filter designer and the embedded docs) much more easy when >prototyping. >> My usual workflow is to come up with a flowgraph with labels, >buttons, >> sliders and GUI sinks using GRC, and when I am ready to test live >(for >> example on my phone) I translate the graph to C++ (by replacing >certain >> blocks with their C++ counterparts if necessary). It is very easy and >> powerful. Also allows me to switch on the fly between an audio sink >to a >> USRP sink at the press of a button. >> >> David, regarding the actual numbers, I think that I might create a >"mega >> graph" which calculates all the numbers and displays them along with >BER >> figures live, while you send data over radio and adjust sliders for >various >> values. I like seeing the numbers change even as I run the flowgraph >(I >> have two soundcards, one of them for the radio and the other for >> monitoring) and the USRP and SDR stick are connected via USB but to a >> different graph. I would like to unify that if possible, but I still >> struggle with certain blocks, like the trellis-viterbi, FEC or BER >tools. >> >> Regarding theory versus practice, I am not trying to compare my >numbers >> with theory, because I can do that with a real SDR connected. >Instead, I am >> very interested in this idea of using a cheap FM handheld as a data >modem. >> There are various challenges which make it a completely different >exercise >> compared with SDRs. >> For example, the pre-emphasis and de-emphasis slopes can sometimes be >> different between radios and this will affect the bit stream, or you >can >> get frequency or phase errors introduced by various components of the >> chain, radios might not be tuned to the exact same frequency, you >symbol >> rate might actually not be exactly the same due to CPU limitations >and you >> have to correct all that. I find this a very challenging exercise. >> >> I have read your blog posts about the 2400B modem, but I would really >like >> to understand more, like what kind of filters do you use for pulse >shaping >> and how do they affect performance/bandwidth, how do you track the >carrier >> frequency and correct frequency and phase errors. This does not seem >to be >> explained anywhere (I know I could just look at the code, but reading >an >> explanation seems easier :) ) >> Also I have some questions regarding your numbers, like what figures >do >> you use for channel bandwidth (Eb/No), given that there are actually >two >> channels, one through the audio path of the radio and the other one >> whatever width you set your filters to. >> >> Thanks anyway for making all this possible. If I had not read your >blog >> posts, I probably would never have become interested in this subject. >> >> Cheers, >> Adrian >> >> >> On Tue, Jul 5, 2016 at 9:47 PM, David Rowe <da...@rowetel.com> wrote: >> >>> Hi Adrian, >>> >>> Sure, it's not always possible to get the output we need from the >tools >>> we are using. >>> >>> However the main point I was making is to compare your GR results to >a >>> known baseline, like the theoretical Eb/No versus BER performance of >the >>> modem you are simulating. OK to use another tool for that (e.g. a >>> spreadsheet or pencil and paper). >>> >>> Re filter design it may affect performance of non-ideal modems, like >>> those that pass symbols through an analog modem (e.g. legacy FM >radios), >>> but filter design won't impact ideal modem performance. Ideal modem >>> performance is set by the laws of physics. Fortunately we can get >to >>> within a few fractions of a dB with real world implementations - if >we >>> choose the right modem design. >>> >>> Cheers, >>> >>> David >>> >>> On 05/07/16 23:02, Adrian Musceac wrote: >>> > Hi David, >>> > >>> > I wish I was able to generate BER plots similar to those in your >>> images. Unfortunately, while Gnuradio has powerful real time >plotting >>> blocks, I haven't figured out how to use them yet. >>> > For this reason, I have to use two flowgraphs, one for the analog >radio >>> and one for the USRP, get the SNR figures from one and use pen and >paper to >>> compare with the known bits received on the other. This allows me to >change >>> the block parameters live and watch the result. Hopefully some day I >will >>> be able to learn Octave. >>> > >>> > For reference, the pulse shaping filters I am experimenting with >are >>> derived from this paper: >>> > http://www.ochiailab.dnj.ynu.ac.jp/pdf/tanahashi_tcom06_final.pdf >>> > >>> > Cheers, >>> > Adrian >>> > >>> > On 5 July 2016 03:21:44 GMT+01:00, David Rowe <da...@rowetel.com> >>> wrote: >>> >> Nice work Adrian. A good way to present this work is plots of >BER >>> >> against Eb/No for the various modulation schemes you are >experimenting >>> >> with. See my blog posts on FSK etc for examples. >>> >> >>> >> It's useful to include the theoretical curves as comparisons to >your >>> >> real world implementations, these are well known for BPSK, FSK, >and >>> >> GMSK. >>> >> >>> >> You can also plot BER against SNR if the bit rate Rb and Noise BW >N are >>> >> >>> >> equal. Eb/No is convenient as everything is normalised. >>> >> >>> >> Cheers, >>> >> >>> >> David >>> >> >>> >> On 05/07/16 08:16, Adrian Musceac wrote: >>> >>> Hi, >>> >>> >>> >>> Some more news from the VHF modem front. First of all, I did >some >>> >>> performance comparisons between GMSK waveforms and regular AFSK >as >>> >> used >>> >>> by a typical APRS modem. Obviously, there is a reason why Bell >AFSK >>> >>> was chosen: the Gaussian pulse shaping filter does introduce >ISI as >>> >> a >>> >>> cost for reducing the signal bandwidth. >>> >>> The next question was "how much". Luckily, using the excellent >filter >>> >>> design tool from Gnuradio, we can change the filter parameters >and >>> >> then >>> >>> test the result against a channel with AWGN noise. The >difference >>> >> seems >>> >>> to be around 2 dB in favour of AFSK. >>> >>> >>> >>> However, my earlier experiments showed that DBPSK will do even >>> >> better. >>> >>> Unfortunately, sending PSK over a narrow FM channel requires >some >>> >> tuning >>> >>> of the modem parameters. 3 dB of improvement at the same BER is >not >>> >>> surprising, what was surprising to me though was the fact that >adding >>> >> a >>> >>> pulse shaping filter immediately before the frequency conversion >>> >>> improved the signal decoding by another 1-2 dB. A 9 dB SNR >signal has >>> >>> the same BER as a 14 dB SNR GMSK modem. At this point we are >almost >>> >>> doing better than analog voice already. >>> >>> To add further gain we can always decrease the bitrate to 1200 >bit/s. >>> >>> >>> >>> And since I was focused on filters, I had to take another look >at the >>> >>> data. What we want is a signal that can pass 1300 bit/s Codec2 >>> >> through a >>> >>> narrow mode FM transceiver (wide mode is beginning to be phased >out >>> >> at >>> >>> this point). We still have bandwidth to spare, so it seemed to >me >>> >> that >>> >>> using the same filter bandwidth for TX and RX is wasteful. >Instead, >>> >> the >>> >>> TX filter is 20% wider than the RX filter for (very little) >added >>> >>> performance. >>> >>> >>> >>> The bad news is that the new modem is computationally about >twice >>> >> more >>> >>> expensive. Using it on my phone will be a big stretch. We will >see if >>> >> I >>> >>> can make it work on a 1.2 GHz CPU. >>> >>> >>> >>> Finally, there are some people out there who criticise Gnuradio >for >>> >>> being slow, bloated and too high level for this kind of stuff. I >just >>> >>> want to say that for me as a student, it is a very valuable tool >for >>> >>> learning. Maybe some day I will join the ranks of the Octave >gurus, >>> >> but >>> >>> not today. >>> >>> >>> >>> A video and a screenshot of my flowgraph: >>> >>> https://www.youtube.com/watch?v=YOX7H3Fcxe4 >>> >>> >>> >>> Cheers, >>> >>> Adrian >>> >>> >>> >>> On Fri, Mar 25, 2016 at 10:17 AM, Adrian Musceac ><kanto...@gmail.com >>> >>> <mailto:kanto...@gmail.com>> wrote: >>> >>> >>> >>> Hi Brady, David, >>> >>> >>> >>> I have started transitioning my GRC flowgraphs to C++ and I >have >>> >> put >>> >>> the code up on Github, just in case you might find it >useful in >>> >> your >>> >>> tests for the 2400B mode. My choice of a modem is AFSK, >2000 >>> >> bit/sec >>> >>> (1200 bit voice or 1300 bit data). It has no FEC, no >scrambling >>> >> and >>> >>> no interleaving (yet). The audio is framed for the standard >40 >>> >> msec >>> >>> Codec2. It works through a handheld FM transceiver and can >be >>> >>> decoded easily with Gqrx. >>> >>> >>> >>> The perfomance is 5 dB worse than analog FM, measured for >12 dB >>> >> SNR. >>> >>> I would be very curious to see a comparison with 2400B. My >next >>> >> step >>> >>> is to transition the DBPSK modem which is 3 dB worse than >analog >>> >> FM >>> >>> based on my USRP captured data. >>> >>> >>> >>> Link to the code repo: >>> >> https://github.com/kantooon/gnuradio_audio_modems >>> >>> Video of operation: >https://www.youtube.com/watch?v=2FZBQvVm5Co >>> >>> >>> >>> Cheers, >>> >>> Adrian (YO8RZZ) >>> >>> >>> >>> >>> >>> On Thu, Mar 17, 2016 at 12:04 AM, Brady O'Brien >>> >>> <brady.obrien...@gmail.com ><mailto:brady.obrien...@gmail.com>> >>> >> wrote: >>> >>> >>> >>> Hey Adrian, >>> >>> >>> >>> The gnuradio-companion pipelines I used for the test >are >>> >>> attached. These grc pipelines are only meant to get >16-bit >>> >> 48k >>> >>> samples in and out of the HackRF and rtlsdr for >testing. The >>> >>> meat of the FSK is done with the C fsk modem, used in >>> >> freedv_rx, >>> >>> freedv_tx, fsk_mod, and fsk_demod. We are also running >our >>> >> FSK >>> >>> modem directly from SDR samples instead of through FM >radios. >>> >> We >>> >>> will be running 2400B over legacy FM radios, but don't >yet >>> >> have >>> >>> the same degree of test coverage. >>> >>> >>> >>> Thanks, >>> >>> >>> >>> Brady O'Brien >>> >>> >>> >>> On Wed, Mar 16, 2016 at 3:34 PM, Adrian Musceac >>> >>> <kanto...@gmail.com <mailto:kanto...@gmail.com>> wrote: >>> >>> >>> >>> Hi David, >>> >>> >>> >>> Is it possible to get our hands on the Gnuradio >code? >>> >> I've >>> >>> been trying >>> >>> to optimize modems running at 1200 bits/second and >the >>> >> best >>> >>> I could do >>> >>> was 100% copy at 8 dB above analog FM for DBPSK and >12 dB >>> >>> above for >>> >>> GMSK. Your modem seems to be 16-20 dB better than >that! >>> >> If I >>> >>> could see >>> >>> the Gnuradio graph maybe I could figure out what >I'm >>> >> doing >>> >>> wrong. >>> >>> >>> >>> Thanks, >>> >>> Adrian >>> >>> >>> >>> On 3/16/16, Steve <coupaydevi...@gmail.com >>> >>> <mailto:coupaydevi...@gmail.com>> wrote: >>> >>> > Ha, I wish it was through a radio! >>> >>> > >>> >>> > I'm trying to think what I have laying around >here >>> >> that >>> >>> might make a good >>> >>> > transmitter for it. >>> >>> > >>> >>> > On Wed, Mar 16, 2016 at 4:02 AM, glen english >>> >>> <g...@cortexrf.com.au ><mailto:g...@cortexrf.com.au>> >>> >> wrote: >>> >>> > >>> >>> >> ah. good stuff. nice progress. >>> >>> >> >>> >>> >> >>> >>> >> On 16/03/2016 7:50 PM, Steve wrote: >>> >>> >> >>> >>> >> Just using the script: >>> >>> >> >>> >>> >> freedv_tx 2400A ve9qrp_10s.raw modem.raw >>> >>> >> >>> >>> >> Which produces a RAW audio file sampled at 48 >kHz and >>> >>> importing that into >>> >>> >> audacity. >>> >>> >> >>> >>> >> On Wed, Mar 16, 2016 at 3:46 AM, glen english >>> >>> <g...@cortexrf.com.au ><mailto:g...@cortexrf.com.au>> >>> >>> >> wrote: >>> >>> >> >>> >>> >>> Hi Steve >>> >>> >>> >>> >>> >>> Nice >>> >>> >>> >>> >>> >>> is that through a PA- if so, linear or linear >in say >>> >>> 3dB compression? >>> >>> >>> >>> >>> >>> >>> >>> >>> On 16/03/2016 7:42 PM, Steve wrote: >>> >>> >>> >>> >>> >>> Thought I'd upload a spectrum snapshot. You >can't >>> >>> really see the >>> >>> >>> carriers >>> >>> >>> until you drop the number of samples to about >256, >>> >> but >>> >>> this shows how >>> >>> >>> wide >>> >>> >>> it is. >>> >>> >>> >>> >>> >>> I noticed that the freedv_tx was outputting >+/- 1.0 >>> >>> signals and a couple >>> >>> >>> of clips going on. Maybe need to knock it down >to >>> >> +/- >>> >>> .5 like the old >>> >>> >>> FDM >>> >>> >>> modes. >>> >>> >>> >>> >>> >>> FYI >>> >>> >>> >>> >>> >>> Steve >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >> >>> >------------------------------------------------------------------------------ >>> >>> >>> Transform Data into Opportunity. >>> >>> >>> Accelerate data analysis in your applications >with >>> >>> >>> Intel Data Analytics Acceleration Library. >>> >>> >>> Click to learn >>> >>> >>> >>> >>> >>> >> >more.http://pubads.g.doubleclick.net/gampad/clk?id=278785231&iu=/4140 >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >_______________________________________________ >>> >>> >>> Freetel-codec2 mailing >>> >>> >>> >>> >>> >>> >> listFreetel-codec2@lists.sourceforge.nethttps:// >>> lists.sourceforge.net/lists/listinfo/freetel-codec2 >>> >>> >>> >> <http://lists.sourceforge.net/lists/listinfo/freetel-codec2> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >> >>> >------------------------------------------------------------------------------ >>> >>> >>> Transform Data into Opportunity. >>> >>> >>> Accelerate data analysis in your applications >with >>> >>> >>> Intel Data Analytics Acceleration Library. >>> >>> >>> Click to learn more. >>> >>> >>> >>>>> >http://pubads.g.doubleclick.net/gampad/clk?id=278785231&iu=/4140 >>> >>> >>> _______________________________________________ >>> >>> >>> Freetel-codec2 mailing list >>> >>> >>>Freetel-codec2@lists.sourceforge.net >>> >>> <mailto:Freetel-codec2@lists.sourceforge.net> >>> >>> >>> >>>>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>> >>> >>> >>> >>> >>> >>> >>> >> >>> >>> >> >>> >>> >> >>> >> >>> >------------------------------------------------------------------------------ >>> >>> >> Transform Data into Opportunity. >>> >>> >> Accelerate data analysis in your applications >with >>> >>> >> Intel Data Analytics Acceleration Library. >>> >>> >> Click to learn >>> >>> >> >>> >> >more.http://pubads.g.doubleclick.net/gampad/clk?id=278785231&iu=/4140 >>> >>> >> >>> >>> >> >>> >>> >> >>> >>> >> _______________________________________________ >>> >>> >> Freetel-codec2 mailing >>> >>> >> >>> >>> >>> >> listFreetel-codec2@lists.sourceforge.nethttps:// >>> lists.sourceforge.net/lists/listinfo/freetel-codec2 >>> >>> >>> >> <http://lists.sourceforge.net/lists/listinfo/freetel-codec2> >>> >>> >> >>> >>> >> >>> >>> >> >>> >>> >> >>> >>> >> >>> >>> >>> >> >>> >------------------------------------------------------------------------------ >>> >>> >> Transform Data into Opportunity. >>> >>> >> Accelerate data analysis in your applications >with >>> >>> >> Intel Data Analytics Acceleration Library. >>> >>> >> Click to learn more. >>> >>> >> >>> >>> >>> >> http://pubads.g.doubleclick.net/gampad/clk?id=278785231&iu=/4140 >>> >>> >> _______________________________________________ >>> >>> >> Freetel-codec2 mailing list >>> >>> >> Freetel-codec2@lists.sourceforge.net >>> >>> <mailto:Freetel-codec2@lists.sourceforge.net> >>> >>> >> >>> >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>> >>> >> >>> >>> >> >>> >>> > >>> >>> >>> >>> >>> >> >>> >------------------------------------------------------------------------------ >>> >>> Transform Data into Opportunity. >>> >>> Accelerate data analysis in your applications with >>> >>> Intel Data Analytics Acceleration Library. >>> >>> Click to learn more. >>> >>> >>> >> http://pubads.g.doubleclick.net/gampad/clk?id=278785231&iu=/4140 >>> >>> _______________________________________________ >>> >>> Freetel-codec2 mailing list >>> >>> Freetel-codec2@lists.sourceforge.net >>> >>> <mailto:Freetel-codec2@lists.sourceforge.net> >>> >>> >>> >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>> >>> >>> >>> >>> >>> >>> >>> >>> >> >>> >------------------------------------------------------------------------------ >>> >>> Transform Data into Opportunity. >>> >>> Accelerate data analysis in your applications with >>> >>> Intel Data Analytics Acceleration Library. >>> >>> Click to learn more. >>> >>> >>> >> http://pubads.g.doubleclick.net/gampad/clk?id=278785231&iu=/4140 >>> >>> _______________________________________________ >>> >>> Freetel-codec2 mailing list >>> >>> Freetel-codec2@lists.sourceforge.net >>> >>> <mailto:Freetel-codec2@lists.sourceforge.net> >>> >>> >https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> >> >>> >------------------------------------------------------------------------------ >>> >>> Attend Shape: An AT&T Tech Expo July 15-16. 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