Re: pre-processing signals for digimode #Software


Kristoff Bonne
 

Hi Rik, all,


First, thanks for the reply :-) (and also everybody else who replied)


On 29/10/18 15:56, Rick Muething wrote:
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.
Well, I did not realise that a HF radio-channel could change that fast. (but then again, I guess that is something you only notice when you deal with digital communication at low-level).


I guess this is also what Patrick explained.

(...)

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.
I guess that where interleaving comes in.


But interleaving requires a error-correction scheme on top of it, which requires more bits for the same data, .. which then reduces the overall net bitrate. ..
I know, "no free lunch".



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.
This in interesting. Then how do you deal with this?
Or do you just throw more error-correction bits at this hoping that sufficient of them make it to the receiver so the original data can be reproduced?



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.

As I am just starting of at this, I will surely not be the one to teach you how to do your job.


But I must say that, for somebody who is interested in this but has never had any experience with this or learned this at university, the learning-curve is very steep. I haven't found that much information on how to learn this step by step.

Learning and understanding general principles and ideas (modulation-schemes, error-correction, interleaving, basic signal-processing techniques) is one thing. But actually learning how to actually implement this, is a very different beast.

As we now have tools like gnuradio, it would be nice if there would be some flow-graphs of basic digimode receivers to help introduce newcomers to the basic ideas of digital signal-processing, to experiment with it and wrap their head around it.


I think that the radio-hobby -as a whole- as everything to gain from having more people actually learn, understand and -perhaps- develop digimodes, instead of just being a user / operator, .. but I really think that, without some basic tools to get people started, this is a very difficult thing to do.


73
kristoff - ON1ARF

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