Reply by Vladimir Vassilevsky●March 3, 20102010-03-03
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
Reply by ok1iak●March 3, 20102010-03-03
> 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
Reply by Vladimir Vassilevsky●March 2, 20102010-03-02
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
Reply by Tauno Voipio●March 2, 20102010-03-02
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
Reply by sven98de●March 2, 20102010-03-02
>
>
>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 ?
..............
. .
>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
Reply by Jerry Avins●March 2, 20102010-03-02
Tim Wescott wrote:
> Jerry Avins wrote:
>> Tim Wescott wrote:
>>> Jerry Avins wrote:
>>>> 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.
>>>
>>> Boxcar filter, running average, y(t) = integral from (t - T0) to t
>>> dt, etc.
>>
>> How dos a boxcar averager that is consulted only just before it is
>> reset differ from sum-and-dump, leaving aside its more complex
>> implementation?
>
> Not at all
>
>> What am I missing?
>>
> Without synchronization to the data clock, sum-and-dump doesn't work.
Ah so! Thanks.
Jerry
--
"I view the progress of science as ... the slow erosion of the tendency
to dichotomize." --Barbara Smuts, U. Mich.
�����������������������������������������������������������������������
Reply by Tim Wescott●March 2, 20102010-03-02
Jerry Avins wrote:
> Tim Wescott wrote:
>> Jerry Avins wrote:
>>> 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.
>>
>> Boxcar filter, running average, y(t) = integral from (t - T0) to t dt,
>> etc.
>
> How dos a boxcar averager that is consulted only just before it is reset
> differ from sum-and-dump, leaving aside its more complex implementation?
Not at all
> What am I missing?
>
Without synchronization to the data clock, sum-and-dump doesn't work.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Reply by Jerry Avins●March 2, 20102010-03-02
Tim Wescott wrote:
> Jerry Avins wrote:
>> 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.
>
> Boxcar filter, running average, y(t) = integral from (t - T0) to t dt, etc.
How dos a boxcar averager that is consulted only just before it is reset
differ from sum-and-dump, leaving aside its more complex implementation?
What am I missing?
Jerry
--
"I view the progress of science as ... the slow erosion of the tendency
to dichotomize." --Barbara Smuts, U. Mich.
�����������������������������������������������������������������������
Reply by Tim Wescott●March 1, 20102010-03-01
Jerry Avins wrote:
> 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.
Boxcar filter, running average, y(t) = integral from (t - T0) to t dt, etc.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com