isw wrote:
(Floyd L. Davidson) wrote:
The SIGNALS are electrical or optical.
Really? Nothing could be acoustical? Are you deaf?
I should have put a smiley on that, sorry for missing it.
Regardless, that does not address the incorrectness of
stating that all signals are analog. Morse code is not
analog.
The carrier or tone that is keyed on and off to send the code starts out
Morse code does not necesarily have either a carrier or
a tone involved, but we can ignore that for this
discussion without changing the validity of our
conclusions.
at a certain strength at the transmitter and grows weaker in a
continuous fashion as the receiver moves further and further away, until
at some point it becomes impossible to understand the *message* it is
carrying.
Well, lets take exactly that as an example, because it
is a good one. We could use a tone as the carrier if
you like, and send it down a regular twisted pair cable.
I'm going to describe this for Morse Code signaling, but
I'd like to point out that virtually any FSK modem does
exactly the same thing with exactly the dynamic range
I'm describing here. Instead of on/off though, it uses
two tones. Everything else is the same, except the
modem is many times faster than a human can decode Morse
Code.
If we put it on the cable at 0 dBm, we'll likely have an
SNR of roughly 50 dB or so, plus or minus a few.
The message is sent using on/off keying of a tone, so at
the cable head we have a 50 dB range which is used to
determine on vs. off. If we head down the road several
miles and get to a point where the signal level has
dropped 10 dB (about the maximum that can be used by a
POTS line), we now have a 40 dB SNR range to deal with.
We could go twice that distance again (losing 10 dB of
signal each time) and get to a point where our signal is
-30 dB and we have only a 20 dB SNR.
At 20 dB SNR there is no reason at all that you won't
get perfect copy, with no errors. Clearly the *signal*
has not changed, even though it has dropped 30 dB in
power. That is because the symbols used are discrete.
From perhaps -40 dBm to 0 dBm there is *no* *change* *in*
*the* *value* *of* *the* *symbols*!
That is, the signal is analog. How can it be digital if it can take on
*any* value?
Obviously it does *not* take on any value. The value
for Morse code is either on or off. There is no "on at
-22.4 dB" value, just on.
--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)