Re: pre-processing signals for digimode #Software


Kristoff Bonne
 

Bonjour Patrick,


OK, I understand it.
In essence, it does not make a difference where you place the "correction" algorithm, at the transmitter side or in the receiver.

In the audio-example, it must be placed on the transmitting-side as the goal is that the "receiver" (i.e. the listener) should not have to do any "correction" but for HF communication, I guess placing it in the receiver is better as it does not require a return-channel.


Sometimes, signal-processing can logical, ... if you just think about it :-)


Question. Do you really need these "known patterns"?
Once you have a 100 % correctly decoded signal, you should be able to recreate the signal as it was transmitted no? Why not use that as a reference?


(I know, coding this is probably a lot more difficult than just discussing it) :-)


Thx
Kristoff - ON1ARF

On 29/10/18 13:48, Patrick Lindecker wrote:
Hello Kristoff,

Equalization is used for several professional modes (HFDL for example), i.e quick PSK modes used in HF for long distances. Note that without equalization the decoding would be almost impossible.

For this, known sequences are sent regularly. From these sequences, using complex equalization algorithms, you can determine the best corrections to do under the form of a filter, more or less the inverse of the ionosphere effects transformed in the form of a filter: i.e Amplitude=f(t, t+dt, t+2*dt...). These corrections will apply to the next unknown sequence, as it is implicitly supposed that the ionosphere will not change while transmitting the unknown sequence.

As far as I know, equalization is not used for Ham communication.

73
Patrick


-----Message d'origine-----
De : main@digitalradio.groups.io [mailto:main@digitalradio.groups.io] De la part de Kristoff Bonne
Envoyé : lundi 29 octobre 2018 13:12
À : main@digitalradio.groups.io
Objet : [digitalradio] pre-processing signals for digimode #Software

Hi all,


A very basic "digital" question.
Yesterday, I was doing some basic DSP 101 (trying to create a FIR-filter based on the measured impulse response of an analog filter) and was watching this video by Dave Gunness for some more background information. It talks on how measuring the acoustics of a loudspeaker, turn that is a mathematical model and finally a FIR-filter. The goal is to "pre-process" the audio-signal send to the loudspeaker in such a way to "undo" the effects the physical build of the loudspeaker has on the audio-signal before it is send to the loudspeaker.

https://www.youtube.com/watch?v=jL_1DwUMD2w


This got me wondering.
In essences, this is basic signal-processing, .. in a way that also digimode communication is basically signal-processing techniques: you send out a signal over RF, the radio-propagation does all kind of weird things to that signal, and -at the receiving end- the digimode software must try to "undo" these changes so to extract the original data.

So, my question.
Are there ham-radio digimodes that use the same idea as explained in the video and "pre-process" the signal before transmitting.
I know this requires a communication return-path from the receiver to the transmitter as the receiver must be able to tell the transmitter how it receives the transmission.
Or does this not make sense for HF? (perhaps the conditions on HF change to fast)?



Anybody any idea on this?


73
kristoff - ON1ARF





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