Re: pre-processing signals for digimode #Software


Kristoff Bonne
 

Mariano,



You know. This is funny.
For some reason, every time I discuss digital-modes, I end up with a link to astronomy.

One of my side-projects is the promotion of amateur-radio to the open-source / hacker / maker community (*). That is why I describe digimodes as basically "digital signal-processing techniques that happens to be applied to radio, but can be applied everywhere".

One of the nice examples of this is also related to astronomy (or more precisely, astrophotography).
When taking pictures of very weak objects (like deep-sky objects, outside the Milky Way), the problem is that the amount of light they receive is actually less then the inherent noise of the CCD-chip of a camera. For that reason, astronomy uses a technique called "photo-stacking": instead of taking one photograph of on object, take a multiple of them and for every pixel calculate the average of that pixel in all these photos.
This "averaging out noise" is exactly one of the techniques used by the amateur-radio weak-signal modes to decode signals with negative S/N values.

And; in fact, a professor who was visiting our ham-radio infobooth at FOSDEM explained to me that in his field he used electron-microscopes, and they have exactly the same problem ... and use the same technique to solve it.


If you don't mind, I will use your remark during my next presentation on amateur-radio. :-)



73
kristoff - ON1ARF

(*) And the other way around, promoting open-source, hacker/maker spaces and fablabs to the amateur-radio clubs, which turns out to a lot more difficult then the former!

On 29/10/18 16:05, Mariano Sanchez Pont wrote:
Hello to everyone!

In astronomy, the use a similar approach to effectively cancel the atmospheric turbulence. They use sodium lasers to project an artificial star in the upper atmosphere, all the characteristics of this star are known, and a special detector is used to see this star. The differences between the original and the observed star are calculated and with this data a small deformable mirror is used to correct the atmospheric turbulences in the real images. The results are awesome.
I think that it could be possible to do something similar with radio.

73s

El lun., 29 oct. 2018 15:56, Rick Muething <rmuething@cfl.rr.com <mailto:rmuething@cfl.rr.com>> escribió:

Kristoff/All,

That is a good question and you have highlighted the major
obstacle in
preforming lets call it "DSP pre emphasis or post processing" of
signals
to offset distortions caused in propagation.  Here are the major
issues:

1) Propagation changes fairly rapidly...sometimes as fast as every
100
ms (ever hear of fast flutter on a signal).  Normally HF has
significant
changes every 1- 5 seconds or so.  Its rare to have a stable channel
with just random Gaussian noise.

2) Some of these changes can be reduced with post reception DSP
processing....PROVIDED: You have a recent (within the last few
seconds)
reference (known signal) to use as a template from which to "measure"
the channel distortion.   This is done to at least some degree on
some
modes (Pactor 4 I think) but requires significant real time DSP
horsepower (basically implementing something like a reverse HF
propagation simulator to try and partially null the effects of the
channel changes).  Of course sending the reference signal also
contains
little or no message information so this itself degrades the net
throughput of the channel. (part of the no free lunch Theorem!)

3) Some of the most common channel degradation is deep fading due to
multipath.  We have all heard this and seen this on HF. If the
desired
signal (or reference signal in 2 above) fades deep into the noise
it is
not practical to recover it. True random (near Gaussian) noise
cannot be
filtered out of a very weak signal ....its random.

4) Much of the interference we hear on HF (as opposed to
VHF/UHF/Microwave) is atmospheric and NOT random Gaussian noise.
Things
like QRM, Static crashes or motor/arc interference (noise but not
Gaussian noise) and these demand much different approaches and are
not
easy to implement and often are not very effective. This is why
normally
on digital modes turning on a radio's "noise blanker" seldom helps
and
often hurts reception.  The same is true with things like notch
filters.

One other worthwhile (but sometimes hard to implement
...especially for
remote mobile stations) is diversity reception (two antennas using
different polarization or spaced from each other).  With the right
combination diversity reception can significantly improves things
like
deep fading but this is not really normally a DSP type function.
(Though I guess you could have two receivers and use DSP to
combine the
audios emphasizing the one with the highest S:N.)

So that's my thoughts. I have a lot of experinece in DSP and have and
MSEE degree but I think there are always room for good ideas to
try and
this is the way we make progress (but sometimes SLOW progress!!)
Perhaps others may have some experience or suggestions.

73,

Rick Muething, KN6KB Winlink Development Team, Author of WINMOR
and ARDOP

On 10/29/2018 8:12 AM, Kristoff Bonne wrote:
> 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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