On Wed, 25 Mar 2009 11:11:48 -0700, "Joel Koltner"
wrote:

Hi Richard,

"Richard Clark" wrote in message
.. .
In other posts related to deep space
probe's abilities to recover data from beneath the noise floor, much
less cell phones to operate in a sea of congestion, I encountered the
economic objection that such methods cost too much - expense of
bandwidth.

I don't think anyone stated they cost "too much," just that there is a cost in
increased bandwidth, and bandwidth isn't free.

Um, this last statement seems to be hedging by saying the same thing
in reverse order.

Well, not having seen anything more than yet another qualification -
how much is "too much?"

Definitely depends on "the market."

It would be more compelling if you simply stated the cost for ANY
market. Qualified statements are suitable for Madison Avenue to sell
cheese, but it doesn't make for an informed cost-based decision.

That being said, back in the analog broadcast TV days (oh, wait, not all of
them are gone yet, but they will be soon), I believe that "studio quality"
NTSC is considered to be 50dB SNR (for the video), whereas people would start
to notice the noise if the received signal's SNR had dropped below 30ish dB,
and 10dB produces an effectively unwatchable pictures. This reinforces your
point that "good enough" is highly subjective depending on how the
"information" transmitted is actually used.

I would suspect that "studio quality" observes other characteristics
of the signal. A multipath reception could easily absorb a
considerable amount of interfering same-signal to abyssmal results. It
would take a very sophisticated "noise" meter to perform the correct
S+N/N.

You make a good point that the Shannon limit gives a good quantitative measure
of how you go about trading off bandwidth for SNR (effectively power if your
noise if fixed by, e.g., atmospheric noise coming into an antenna). Shannong
doesn't give any hint as to how to achieve the limits specified, although I've
read that with fancy digital modulation techniques and "turbo"
error-correcting codes, one can come very close to the limit.

The "Turbo" codes are achievable in silicon with moderate effort. A
work going back a dozen years or more can be found at:
http://sss-mag.com/G3RUH/index2.html
(consult the adjoining pages for fuller discussion)

73's
Richard Clark, KB7QHC

Hi Richard,

"Richard Clark" wrote in message
...
I don't think anyone stated they cost "too much," just that there is a cost
in
increased bandwidth, and bandwidth isn't free.
Um, this last statement seems to be hedging by saying the same thing
in reverse order.

No, they really are different. What costs too much for me might very not cost
too much for the military or NASA, for instance.

It would be more compelling if you simply stated the cost for ANY
market.

The original example was meant to be more of a "textbook" problem, hence the
lack of elaboration on the specifics of the "market" involved.

I would suspect that "studio quality" observes other characteristics
of the signal.

Agreed, I would too.

A multipath reception could easily absorb a
considerable amount of interfering same-signal to abyssmal results. It
would take a very sophisticated "noise" meter to perform the correct
S+N/N.

Yep, very true -- I think this is why you see people legtimately complaining
about the quality of their cable TV even though the cable installation tech
whips out his SINAD meter and verifies it meets spec; the quality of a
transmission can't always be boiled down to just one number.

The "Turbo" codes are achievable in silicon with moderate effort. A
work going back a dozen years or more can be found at:
http://sss-mag.com/G3RUH/index2.html

Great link, thanks!

---Joel

The "Turbo" codes are achievable in silicon with moderate effort.

And the payment of a suitable fee to the folks who OWN the turbo codes
at France Telecom
http://www.francetelecom.com/en_EN/i...y/turbo_codes/
http://www.spectralicensing.com/licfaq.htm


Note also that turbo and LDPC are really suited to longer block lengths
(1000 bits and bigger). For small block lengths, codes like Hamming
might be better.

Reed-Solomon combined with Viterbi decoders of convolutional codes are
also popular.

Note that in deep space, at a bit rate of 8 bps, you might not want to
use a code with a 1000 bit codeblock..

A
work going back a dozen years or more can be found at:
http://sss-mag.com/G3RUH/index2.html
(consult the adjoining pages for fuller discussion)

73's
Richard Clark, KB7QHC