RTTY (AFSK) demodulation
in [DSP]
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From: sven98de on 2 Mar 2010 04:43 > > >Tauno Voipio wrote: > >> On 27.2.10 4:02 , sven98de wrote: >> > Hi, >> > has somebody here implemented a RTTY (AFSK) decoder. Any suggestions how to >> > proceed ? >> > I've found some technical abstracts about FSK demodulation in the WEB >> > like: >> > - frequency discriminator with IIR resonator >> > - frequency discriminator with FIR bandpass filter >> > - digital PLL >> > >> > Any idea how complex it is to implement one of these methods ? >> > What processor is sufficient for this, does a microcontroller the job ? >> >> Do you like to use it on real band conditions, or is this just >> an exercise. >> >> When there is QRM and QRN on the channel, a PLL is not a good >> idea, as it will pretty easily grab an interfering signal. >> >> The traditional TU (terminal unit) has two analog filters, >> one for mark and one for space. If you like to use it as a >> model, make a single pole-pair IIR resonator for both >> frequencies and compare the amplitudes. >> > >I implemented one with a single simple filter and >used the phase shift across the filters center frequency >to decode 0/1 > >The same trick also works for simple 300baud modems > >The phase discriminator was D latch with filter input going in >the D input and filter output clocking the latch. > >Regards, > > >Walter.. >-- >Walter Banks >Byte Craft Limited >http://www.bytecraft.com > > > > > > Do you mean something like this ? .............. . . .>>..... Filter ............ . . . . . .............. . . . . . . . . . . ............. . .... . D Q....>> 0/1 . . Latch . . ...... C /Q..... . . ............ . . . . . ..............................
From: Tauno Voipio on 2 Mar 2010 06:39 On 2.3.10 10:44 , sven98de wrote: >> On 27.2.10 4:02 , sven98de wrote: >>> Hi, >>> has somebody here implemented a RTTY (AFSK) decoder. Any suggestions how > to >>> proceed ? >>> I've found some technical abstracts about FSK demodulation in the WEB >>> like: >>> - frequency discriminator with IIR resonator >>> - frequency discriminator with FIR bandpass filter >>> - digital PLL >>> >>> Any idea how complex it is to implement one of these methods ? >>> What processor is sufficient for this, does a microcontroller the job ? >> >> Do you like to use it on real band conditions, or is this just >> an exercise. >> >> When there is QRM and QRN on the channel, a PLL is not a good >> idea, as it will pretty easily grab an interfering signal. >> >> The traditional TU (terminal unit) has two analog filters, >> one for mark and one for space. If you like to use it as a >> model, make a single pole-pair IIR resonator for both >> frequencies and compare the amplitudes. >> >> -- >> >> Tauno Voipio, OH2UG >> > > Yes I thought to use it on real band conditions, so there will be more or > less interferences. > > Sven OK. If you have not yet done it, get an ARRL handbook and read the chapter on RTTY. My experience is that just two band-pass filters, rectification on both and comparator works best on actual HF conditions. There are plenty of tricks that can be done in setting the comparator thresholds, look for ATC and DTC. -- Tauno Voipio, OH2UG
From: Vladimir Vassilevsky on 2 Mar 2010 09:07 Tauno Voipio wrote: > On 27.2.10 4:02 , sven98de wrote: > >> Hi, >> has somebody here implemented a RTTY (AFSK) decoder. Any suggestions >> how to >> proceed ? >> I've found some technical abstracts about FSK demodulation in the WEB >> like: >> - frequency discriminator with IIR resonator >> - frequency discriminator with FIR bandpass filter >> - digital PLL >> >> Any idea how complex it is to implement one of these methods ? >> What processor is sufficient for this, does a microcontroller the job ? > > > Do you like to use it on real band conditions, or is this just > an exercise. > > When there is QRM and QRN on the channel, a PLL is not a good > idea, as it will pretty easily grab an interfering signal. > > The traditional TU (terminal unit) has two analog filters, > one for mark and one for space. If you like to use it as a > model, make a single pole-pair IIR resonator for both > frequencies and compare the amplitudes. QRN... Long ago a friend of my was demodulating RTTY by PC. I remember how he cursed all computers in the World. So, it may be not such a great idea to demodulate in the software. VLV
From: ok1iak on 3 Mar 2010 20:21 > QRN... Long ago a friend of my was demodulating RTTY by PC. I remember > how he cursed all computers in the World. So, it may be not such a great > idea to demodulate in the software. I don't see any reason why the DSP system would not do at least as well as an analog system. Neither processing speed nor dynamic range is an issue on short waves. gMFSK and my PocketDigi use the two filter method and matched boxcar filter on baseband signal. It works reasonably, but RTTY is an asynchronous protocol. One may gain about 1 or 2dB when decoding RTTY synchronously. But this would require to guess the stop bit length. Actually a wide bandwidth digital noise blanker has a great benefit compared to analog systems when dealing with QRN aka atmospheric impulse noise. SM5BSZ wrote plenty of DSP code for weak signal processing and noise blanking. http://www.sm5bsz.com/ http://www.sm5bsz.com/linuxdsp/qex/030910qex029.pdf Vojtech OK1IAK, AB2ZA
From: Vladimir Vassilevsky on 3 Mar 2010 20:34 ok1iak wrote: >>QRN... Long ago a friend of my was demodulating RTTY by PC. I remember >>how he cursed all computers in the World. So, it may be not such a great >>idea to demodulate in the software. > > > I don't see any reason why the DSP system would not do at least as > well as an analog system. Neither processing speed nor dynamic range > is an issue on short waves. The issue is EMI created by digital board which effectively jams the receiver. No problems other then that. Of course, it could be mitigated, however this sets entirely different circle of problems. gMFSK and my PocketDigi use the two filter > method and matched boxcar filter on baseband signal. It works > reasonably, but RTTY is an asynchronous protocol. One may gain about 1 > or 2dB when decoding RTTY synchronously. But this would require to > guess the stop bit length. > > Actually a wide bandwidth digital noise blanker has a great benefit > compared to analog systems when dealing with QRN aka atmospheric > impulse noise. > SM5BSZ wrote plenty of DSP code for weak signal processing and noise > blanking. > http://www.sm5bsz.com/ > http://www.sm5bsz.com/linuxdsp/qex/030910qex029.pdf Amateurish sandbox :)))))) Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com > Vojtech OK1IAK, AB2ZA
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