binary data is two state data from its very state of being, on or off
normalize your noise level (to a value which will always be subtracted
from the "on markers") then there is only two states in question, an
on and an off
next, you are NOT actually transmitting ones and zeros (on's and
off's), but are transmitting "markers", the length (time)between the
"markers" is what determines if it a one or a zero (or a sting of two
or more ones or zeros, under proper compression techniques.)
If those markers are above the noise level--you have uncorrupted
data--if not, you do have corrupted data, since data is transmitted in
"packets", and since each and every (say in this case) 1024 bit packet
is checked against a CRC (Cyclic Redundancy Check "value"), corrupted
packets are tossed away and a request to resend is initiated. Packets
are sequentially numbered so as to keep their display sequence in
proper sync.
My experience is that most digital transmissions can take place with
acceptable success if cw can... and I expect that statement to fall to
heavy challenge! smirk
John
wrote in message
oups.com...
Mike Coslo wrote:
wrote:
Mike Coslo wrote:
Dave Heil wrote:
LenAnderson@ieeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee eeeeeeeeeeeeeeeeeeeeeeeee.org
wrote:
How we be gonna scale those pictures and live video to fit
into 2.5 KHz?
Two steps:
1) Convert the pictures and video into highly-compressed
digital formats for transmission.
Oooh, there could be a problem there! There are limits to the
compression, and we have exceeded them in some forms already.
Of course! But it all depends what you consider "acceptable
quality"...
Check to
see how many vertical pans there are on video signals lately.
The
compression on the digital signals (note that even if you are
getting
your feed via analog cable, you are still almost certainly
looking at a
digital signal) already calls for some major aliasing.
OTOH, if all you want is B&W ~CGA video....
There are limits, and there are limits. How much more are we
going to throw away?
Always a tradeoff. Hams routinely use 1.8 kHz wide SSB filters for
"communications quality". Hardly hi-fi but it can make the
difference
between QSO and QRJ.
2) Use different modes/modulations/protocols
Shannon's Theorem tells us that we can get very high data
rates through
very narrow bandwidths *if* we have adequate signal-to-noise
ratio.
Note that "noise" takes many forms, not just the thermal
noise we're used to.
What're we going to do when the data rate that we need is darn
near(or above) frequency in use?
Use modes designed for the purpose. See below.
For example, PSK has an advantage over OOK when dealing with
thermal
noise. But when dealing with other types of noise, OOK can
have an
advantage. It all depends on the transmission medium. What
works on a
telephone line may not work on an HF path of the same apparent
bandwidth.
I thought that we were going to be able to send live video
and digital images on HF?
You can do that now - just need enough S/N.
Always?
No mode always gets through.
But if you have enough S/N, all sorts of things are possible.
Here's one way. I apologize if you are way beyond this simplified
example:
Consider how PSK31 works in BPSK mode. There are just two basic
modulation states - 0 degrees and 180 degrees. One bit per unit
time.
But in QPSK mode, there are four basic modulation states - 0
degrees,
90 degrees, 180 degrees and 270 degrees. Two bits per unit time, but
the bandwidth is no greater than with BPSK. Only problem is that you
need a transmitter, receiver and transmission medium whose total
distortion is low enough that you can accurately tell the four
states
apart.
Now consider a theoretical "256PSK" mode, in which there are 256
states: 0 degrees, 360/256 degree, 720/256 degrees, etc., all the
way
to ~359 degrees. 8 bits in one unit time, in the same bandwidth! But
you need a transmitter, receiver and transmission medium whose total
distortion is low enough that you can accurately tell the 256 states
apart.
You can see that if we just keep increasing the number of states,
the
number of bits per unit time in the same bandwidth keeps going up.
But
you need more and more accurate modulator/medium/demodulator - IOW,
better and better signal-to-noise. Or to look at it another way, the
mode carries huge amounts of data in a tiny bandwidth but has very
little tolerance for noise that takes the form of phase distortion.
Now imagine multiple spaced carriers in the 2.5 kHz bandwidth all
carrying data - lotta bits, huh?
Of course you may find that in practice it's not that easy to get a
modulator/medium/demodulator setup that meets the requirements -
particularly if the medium is HF RF with relatively low power.
Simply by hooking our computers to our rigs via the
proper interfaces.
And software.
I really didn't think it was all that simple.
Nobody said it was simple!
Why don't we get together
and pop off a live video system for say the 160 meter band. The
video
would be real time, 30 fps, and otherwise like broadcast video.
Better
yet, Why don't we do it at computer resolution?
Ask the PROFESSIONALS, Mike. Remember, ham radio is a HOBBY,
according
to them....
Now it seems that the *idea* is that we are going to use
DRM, and we're
going to need to get more spectrum in which to use.
There are all sorts of solutions. But there's a world of
difference between people talking theory and actual
application.
I did hear that DRM was capable of doing imagery. I couldn't find
any
examples tho'. And they were very vague about it.
Most of all, some folks confuse the journey and the destination.
The journey beats all.....
Exactly.
Does complex and newer equal better?
Sometimes. Not always.
Is analog simpler than digital?
Sometimes!
Does having a computer that attaches to the Internet
make a person a digital expert?
Some folks think so! I don't. And besides - "digital expert"
doesn't
mean someone knows much about radio.
Ain't that the truff?
I ask for enlightenment, I get invective.
Are you surprised?
Nope. It doesn't make for a very good discussion tho'.
Discussion is not what the invective-hurlers want, Mike.
Appears to be what there is to offer.
Now consider how effective such a person would
be trying to sell amateur radio - with or
without a code test.
They might attract others of their ilk.
You see that happening right here.
I'll bet they like some of the "wonder antennas" that keep cropping
up...
Exactly.
73 de Jim, N2EY