Re: research problem

Rune Allnor wrote: 
Steve Underwood skrev:

porterboy76@xxxxxxxxx wrote:

Yes, thats one of the reasons I like digital comms so much. It is very
easy to tell a good algorithm from a bad algorithm objectively, unlike
for example in audio or imaging, where there is a lot of subjectivity
(I've stared blankly at a lot of similar Lena jpgs and not really been
able to tell the difference). I think that's why ended up in digital
comms, I originally started a PhD in imaging and switched almost
immediately. Are you saying that seismic (seismological?) signal
processing is a more objectively defined field than image and audio
processing? What kind of algorithms are involved... wait, OK, time for
Google ;-)


Interesting view.

Those Lena images, they always miss the best parts of the original :-) . 


Does anybody know the history of those images? Is it as simple as
some geek wanted an excuse to read playboy at work? Or was caught
reading it and had to come up with an explanation...?

Who knows for real. The guys say they needed a picture at short notice that would scan nicely on the old drum scanners of the day. They needed something glossy to work well, and the only thing to hand was Playboy. They couldn't use the whole picture, for obvious reasons, so they tore it in half.

You just get used to picking out these differences. I found similar
issues with colour work, and speech coding. Differences that were very
hard to detect on the first day stood out like sore thumbs pretty soon.


Would those differences be perceptible by a layman end user?

In what sense? In the sense that they too subtle to be important? In that sense they certainly do matter. The imperfections gradually grate at you, and drive you nuts.

I don't agree with your notion about the ease of recognising good comms
alorithms. The behaviour or radio in complex environments like dense
city areas is still poorly understood.



Imagine to provide a radio com link in a densely populated area
where

- the speed of light is unknown and varies between blocks
- the reflection coefficient from the ground and walls of
  buildings are unknown
- signals propagate through buildings and experience
  internal reflections inside rooms before they propagate
  to the reciever, all transmission and reflection coefficients
  being unknown, as well as the speed of light inside each room
- you have only nominal control of the source signal,
  i.e. you control the time at witch it will send pulse and
  the general frequency band, but that's it
- the width of the roads vary from block to block
- the length of the blocks vary

and you start approaching the world of the seismic signal analyst.

I did a little work on hunting submarines at depth. Water is really nasty. I imagine your seismic work is comparatively easy. :-)

To take a pretty simple radio
system as an example - 1200bps POCSAG paging generally worked better in
dense cities than the original 512bps. That surprised most radio systems
researchers, so they clearly had a weak idea of what works well. :-)



Was the bandwidth proportinal to the bit rate? Were the two radios
working in different frequency bands? Did they use the same
modulation scheme?

Same 25kHz channel. Same frequency band. Same simple FSK modulation. A pretty simplistic bit stream, with no interleaving.

If the 1200bps system has a higher bandwidth, everything else being
equal, it might be expected to work better than the 512bps system,
since the relative bandwidth of nulls to signal bandwidth in the
Lloyd mirror equivalent (the superposition of direct and reflected
signal) is lower in the 1200bps system. neither would work perfectly,
but the higher bandwidth one would work better.


If the 1200bps system works at a higher carrier frequency, the
transmision band will be more diffuse than the lower-carrier 512bps
system. Again, it has to do with the effects of reflected and
direct signals adding up.


Modulation schemes are difficult to analyse, but there could be
different robustness to reflectied signals that arrive at a
given delay. If so, what works well in test scenario A needs not
work in test scenario B.


I once used the Lloyd mirror to explain why a radio link at sea
had a very short range, only 1/4th of what the same radio achieved
over land. It made the day for the guy whos neck was on the line
for not getting that link to work.

Well, you said "if the 1200bps system has a higher bandwidth". I turned out that even using the same bandwidth, the real world nulling behaviour of people moving about in dense city areas actually damaged less wordwords beyond the point where the BCH code could recover them.

Here's another oddity. Almost all pagers supplied in Hong Kong were of a slightly modified design, sold nowhere else in the world. Their senitivity had to be crippled, to get better performance. The reason stems from a problem caused purely by a regulatory quirk. Not enough people understood the quirk, so the regulations were never fixed. :-)

It doesn't take much complexity for a propagation channel to become
seriously messed up. 

The last paragraph sums it up nicely. :-)

Regards,
Steve
.

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