| Paul Cardinale 2007-06-27, 3:25 am |
|
j...@specsol.spam.sux.com wrote:
> In sci.physics.electromag Randy Poe <poespam-trap@yahoo.com> wrote:
>
>
> Does a digital signal, i.e. one where all the information is in
> discreate voltage levels, really meet the definition of "modulation"?
>
Yes, it does. In that case, the bits are modulating a DC voltage.
But ethernet (and many other digital signals) is not voltage encoded.
In ethernet, a 1 is represented by a transition, and a 0 by the
absence of a transition. Each cycle carries a minimum of 4 bits.
Interestingly, not all digital transmission is binary: long distance
telephone communications uses hexadecimal; encoded using a combination
of frequency and phase modulation.
> I guess one could make the case that, using 5V logic for example, the
> information varies a DC voltage of about 2.5V at the information
> rate and thus "modulates" a 2.5V DC signal.
>
> It certainly isn't AM as AM contains the multiplicative term sin(Fc)
> which is zero for a DC carrier.
>
There are many kinds of modulation. Amplitude modulation of a sin
wave is just one.
> It isn't FM as the carrier frequency deviation is zero, thus the
> modulation index is zero.
>
It's true that TTL isn't FM, but the frequency is not constant.
Paul Cardinale
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