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From: sven98de on 27 Feb 2010 09:02
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 ?

greetz
Sven


From: Tauno Voipio on 27 Feb 2010 15:48
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
From: Walter Banks on 1 Mar 2010 16:29


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





From: Tim Wescott on 1 Mar 2010 20:25
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.

Or I/Q demodulate at both the mark and space frequency, do a running
average (a _real_ running average, not an exponential decay) for one bit
period, and choose the I/Q pair with the best magnitude. If you deal
with the synchronization issue, you can just sum-and-dump instead of
doing the running average.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Jerry Avins on 1 Mar 2010 20:31
Tim Wescott wrote:
> 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.
>
> Or I/Q demodulate at both the mark and space frequency, do a running
> average (a _real_ running average, not an exponential decay) for one bit
> period, and choose the I/Q pair with the best magnitude. If you deal
> with the synchronization issue, you can just sum-and-dump instead of
> doing the running average.

By "real running average" as opposed to "sum and dump", Do you mean
forever and ever amen? The sensitivity would tank after a few bit times
if you did that.

Jerry
--
"I view the progress of science as ... the slow erosion of the tendency
to dichotomize." --Barbara Smuts, U. Mich.
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