Re: [Freetel-codec2] Legacy modems

[Brady O'Brien]

Hi Adrian,

Just for completeness, the modem we are planning on using for
FreeDV-over-analog-FM, is more like ASK+Manchester Encoding over FM. We're
calling it FMFSK or MEFSK because it ends up looking like a dirty FSK over
the air. The mod works by Manchester encoding the bits and firing them over
the radio. The demod is a bit more complex. It can be examined in
octave/fmfsk.m and src/fmfsk.c . Like the other FreeDV modems, neither of
those have been wrapped up as modules for Gnuradio, but it shouldn't be too
difficult.

Thanks,

Brady O'Brien

On Tue, Mar 8, 2016 at 4:12 PM, Adrian Musceac  wrote:

> Hi David,
>
> To answer your questions, I am measuring SNR at the same bitrate for
> all modems (1200 bit/sec) using the AWGN channel model that is
> available in Gnuradio. I'm afraid my math is a bit iffy, so the
> figures should be taken with a grain of salt.
>
> However, for practical purposes, I settled on the following
> modem/bitrate combinations:
> DBPSK 1200 - best low signal modem
> GMSK 2000 - average modem
> DQPSK 4000 - about the same performance as GMSK 2000
>
> I intend to code logic to switch between them as needed, depending on
> the quality of the channel and the amount of data to be transfered.
>
> Performance is below analog FM by about 8 dB in the best case
> scenario. However the data capability and the fact that we can use a
> cheap handheld wins here. I believe these digital waveform might also
> pass undisturbed through a repeater as well.
>
> I will make a video and upload it to Youtube, to demonstrate my setup.
> Real mobile performance data would be nice, but I have yet to finish a
> suitable case for my board+LCD+battery contraption. Also the software
> is still in the infancy.
>
> Cheers,
> Adrian (YO8RZZ)
>
> On 3/8/16, David Rowe  wrote:
> > Hi Adrian,
> >
> > Great work - you should write it up (or YouTube).  What noise (channel)
> > bandwidth are your SNRs measured in?  Are all the modems tested below
> > measured using the same bit rate?
> >
> > That's why Eb/No is useful - it's normalized to a bandwidth of 1Hz and
> > bit rate of 1 bit/s.  It's like the "SNR of one bit".
> >
> > How does analog FM sound at the same SNR?
> >
> > I also found AFSK over FM to be particularly bad:
> >
> >http://www.rowetel.com/blog/?p=3799
> >
> > Getting modems to work over FM analog modems is an interesting area.
> > Brady is getting good results from manchester encoder symbols.  I guess
> > that's putting BPSK over analog FM, it's a pulse train of +ve and -ve.
> > In our simulations it's only 2dB off 2FSK which is surprising (or
> > suspicious!).
> >
> > Looks a lot like the GMSK modem Kristoff uses, but the manchester
> > encoding moves the energy away from DC, so a "data mode" radio is not
> rqd.
> >
> > Cheers,
> >
> > David
> >
> > On 09/03/16 06:07, Adrian Musceac wrote:
> >> Hi all,
> >>
> >> Motivated by David's work with VHF modems, I have started to
> >> investigate various Tx-Rx chains that might work with legacy narrow FM
> >> radios. My goal is to develop a completely portable system composed of
> >> a development board running open-source software paired with an FM
> >> transceiver. The Linux board could be any ARM board capable of running
> >> a supported Linux distribution and providing a decent interface to a
> >> 7-9 inch LCD screen with touch controller. My setup is fully described
> >> in a Youtube video I mentioned before.
> >>
> >> At the moment I have all the hardware components available and have
> >> run the first tests on the air. The software was first based on
> >> Kristoff's Codec2-GMSK modem and a Qt user interface designed by me.
> >> Although it had worked fine, it required a radio with a data port,
> >> which meant portable ops were restricted to a very narrow list of
> >> handheld radios.
> >>
> >> Since then, I thought of using the huge power of the Gnuradio
> >> ecosystem to design my own modems that could go through the audio
> >> passband of a handheld. I have learned many things during this
> >> process, especially that I can very quickly prototype and test a modem
> >> without going too deep in the maths, which has never been my strong
> >> point.
> >>
> >> My findings are somewhat surprising: I can use any of the modems over
> >> a narrow FM channel, but there are a few which perform well enough for
> >> my purposes.
> >> Considering a RF chain composed of one laptop running Gnuradio, one
> >> Yaesu handheld, one Yaesu mobile transceiver and one Linux board with
> >> soundcard ports also running Gnuradio, here is the performance of my
> >> modems in descending order:
> >> 1. DBPSK (~10 dB S/N)
> >> 2. DQPSK ( ~12 dB S/N)
> >> 3. GMSK (~14 dB S/N)
> >> 4. 4FSK (~15 dB S/N)
> >> 6. AFSK (~16 dB S/N)
> >>
> >> I am especially fond of the GMSK modem which gives me 2 kbit/sec with
> >> a reasonable SNR.
> >>
> >> Since I have full control over the modems, I can select the signal
> >> width, samples per second and 

Re: [Freetel-codec2] Legacy modems

[David Rowe]

Hi Adrian,

Great work - you should write it up (or YouTube).  What noise (channel) 
bandwidth are your SNRs measured in?  Are all the modems tested below 
measured using the same bit rate?

That's why Eb/No is useful - it's normalized to a bandwidth of 1Hz and 
bit rate of 1 bit/s.  It's like the "SNR of one bit".

How does analog FM sound at the same SNR?

I also found AFSK over FM to be particularly bad:

   http://www.rowetel.com/blog/?p=3799

Getting modems to work over FM analog modems is an interesting area. 
Brady is getting good results from manchester encoder symbols.  I guess 
that's putting BPSK over analog FM, it's a pulse train of +ve and -ve. 
In our simulations it's only 2dB off 2FSK which is surprising (or 
suspicious!).

Looks a lot like the GMSK modem Kristoff uses, but the manchester 
encoding moves the energy away from DC, so a "data mode" radio is not rqd.

Cheers,

David

On 09/03/16 06:07, Adrian Musceac wrote:
> Hi all,
>
> Motivated by David's work with VHF modems, I have started to
> investigate various Tx-Rx chains that might work with legacy narrow FM
> radios. My goal is to develop a completely portable system composed of
> a development board running open-source software paired with an FM
> transceiver. The Linux board could be any ARM board capable of running
> a supported Linux distribution and providing a decent interface to a
> 7-9 inch LCD screen with touch controller. My setup is fully described
> in a Youtube video I mentioned before.
>
> At the moment I have all the hardware components available and have
> run the first tests on the air. The software was first based on
> Kristoff's Codec2-GMSK modem and a Qt user interface designed by me.
> Although it had worked fine, it required a radio with a data port,
> which meant portable ops were restricted to a very narrow list of
> handheld radios.
>
> Since then, I thought of using the huge power of the Gnuradio
> ecosystem to design my own modems that could go through the audio
> passband of a handheld. I have learned many things during this
> process, especially that I can very quickly prototype and test a modem
> without going too deep in the maths, which has never been my strong
> point.
>
> My findings are somewhat surprising: I can use any of the modems over
> a narrow FM channel, but there are a few which perform well enough for
> my purposes.
> Considering a RF chain composed of one laptop running Gnuradio, one
> Yaesu handheld, one Yaesu mobile transceiver and one Linux board with
> soundcard ports also running Gnuradio, here is the performance of my
> modems in descending order:
> 1. DBPSK (~10 dB S/N)
> 2. DQPSK ( ~12 dB S/N)
> 3. GMSK (~14 dB S/N)
> 4. 4FSK (~15 dB S/N)
> 6. AFSK (~16 dB S/N)
>
> I am especially fond of the GMSK modem which gives me 2 kbit/sec with
> a reasonable SNR.
>
> Since I have full control over the modems, I can select the signal
> width, samples per second and tone frequencies in such a way that they
> provide the maximum bitrate for a bandwidth limited to 300 Hz - 3 kHz.
> Now that I have some real results, I am thinking of starting to move
> the modems to C++ space and integrate them with the Qt GUI application
> destined to run on the Linux board. This application will be based on
> qradiolink v0.2 and will have support for VOIP via the 2.4 GHz WiFi on
> board, enabling the station to be quickly transformed into a portable
> repeater.
>
> You can find my test modems on Github if you want to play with them:
> https://github.com/kantooon/gnuradio_audio_modems
>
> Obviously David's modem will probably be more robust and performant,
> however I am providing a huge selection of GRC graphs here :)
>
> Cheers,
> Adrian (YO8RZZ)
>
> --
> Transform Data into Opportunity.
> Accelerate data analysis in your applications with
> Intel Data Analytics Acceleration Library.
> Click to learn more.
> http://makebettercode.com/inteldaal-eval
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>

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[Freetel-codec2] Legacy modems

[Adrian Musceac]

Hi all,

Motivated by David's work with VHF modems, I have started to
investigate various Tx-Rx chains that might work with legacy narrow FM
radios. My goal is to develop a completely portable system composed of
a development board running open-source software paired with an FM
transceiver. The Linux board could be any ARM board capable of running
a supported Linux distribution and providing a decent interface to a
7-9 inch LCD screen with touch controller. My setup is fully described
in a Youtube video I mentioned before.

At the moment I have all the hardware components available and have
run the first tests on the air. The software was first based on
Kristoff's Codec2-GMSK modem and a Qt user interface designed by me.
Although it had worked fine, it required a radio with a data port,
which meant portable ops were restricted to a very narrow list of
handheld radios.

Since then, I thought of using the huge power of the Gnuradio
ecosystem to design my own modems that could go through the audio
passband of a handheld. I have learned many things during this
process, especially that I can very quickly prototype and test a modem
without going too deep in the maths, which has never been my strong
point.

My findings are somewhat surprising: I can use any of the modems over
a narrow FM channel, but there are a few which perform well enough for
my purposes.
Considering a RF chain composed of one laptop running Gnuradio, one
Yaesu handheld, one Yaesu mobile transceiver and one Linux board with
soundcard ports also running Gnuradio, here is the performance of my
modems in descending order:
1. DBPSK (~10 dB S/N)
2. DQPSK ( ~12 dB S/N)
3. GMSK (~14 dB S/N)
4. 4FSK (~15 dB S/N)
6. AFSK (~16 dB S/N)

I am especially fond of the GMSK modem which gives me 2 kbit/sec with
a reasonable SNR.

Since I have full control over the modems, I can select the signal
width, samples per second and tone frequencies in such a way that they
provide the maximum bitrate for a bandwidth limited to 300 Hz - 3 kHz.
Now that I have some real results, I am thinking of starting to move
the modems to C++ space and integrate them with the Qt GUI application
destined to run on the Linux board. This application will be based on
qradiolink v0.2 and will have support for VOIP via the 2.4 GHz WiFi on
board, enabling the station to be quickly transformed into a portable
repeater.

You can find my test modems on Github if you want to play with them:
https://github.com/kantooon/gnuradio_audio_modems

Obviously David's modem will probably be more robust and performant,
however I am providing a huge selection of GRC graphs here :)

Cheers,
Adrian (YO8RZZ)

--
Transform Data into Opportunity.
Accelerate data analysis in your applications with
Intel Data Analytics Acceleration Library.
Click to learn more.
http://makebettercode.com/inteldaal-eval
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