Re: research problem


Steve Underwood skrev:
> 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.

Like some of the MP3 effects, or audio tracks on DVDs, where rainfall
and applause sounds like an electric saw?

> >>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. :-)

Well, oil doesn't shoot back at you, shallow gas might leak to the
surface to blow you up. On the other hand, you don't have to go
stealthy in oil exploration, at least within reason, and the oil or
gas don't try to hide.

I have done some of the ASW stuff myself, and I think the main
difference is that oil explorers actually get to test their method
in 'live' situations. The ASW guys might get a surprise or two
once some of the recent methods are put to use outside the test
ranges and the labs.

> >>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.

How does FSK behave with respect to nulls? Qould it be that one
of the ferquencies was too close to a null?

> > 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.

Same bandwidth, higher capacity, that would mean higher transmission
power, right? Would the added power compencate for nulls?

> 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. :-)

Rune

.

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