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From: Don Bruder on 11 Mar 2010 09:54
In article <hn9i18$m3h$1(a)speranza.aioe.org>, RalfM <rm(a)invalid.invalid>
wrote:

> Hi, here's some beginners questions:
>
> digital data transfer on a wired copper medium is done usually by setting
> a DC voltage (for example 5V) for a defined duration to indicate
> a binary 1 value, and say 0V to indicate binary 0.
> I think the duration of such a signal is called "bit period".
>
> Here are some questions & thoughts:
> What are the chips doing this switching for transmitting are called?
> (modulator? DAC?)
> How is it done one the receiving side? (via an ADC ?)
> How many such binary signals can a say 2 GHz CPU generate in real-world per
> second?
>
> Such a bitwise transfer does need only a fast switching between 2 voltage
> levels.
> Is it possible to have a transmitter/receiver that can use more than
> just 2 possible values per "bit period" (for example: a 12-bit ADC or DAC
> can detect 2^12=4096 different values (ie. voltages). Then why use
> only 2 values (0/1) in copper wired data transfers instead of using
> say 256 or 512 or 1024 or 2048 or 4096 bits etc.? (ie. make the cable a "8
> bit cable" :-) or more.
> IMHO one could dramatically increase the speed on wired copper medium
> (for example a 256-fold increase or even more would be possible).
> Are there such chips which can switch fast a voltage source say to 256
> different values? ie. fast DAC and ADCs, DSP maybe?
>
> Just some crazy thoughts of mine... :-)

Congratulations - You've just re-invented analog transmission.

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From: WangoTango on 11 Mar 2010 10:43
In article <hn9i18$m3h$1(a)speranza.aioe.org>, rm(a)invalid.invalid says...
>
> Just some crazy thoughts of mine... :-)

http://en.wikipedia.org/wiki/Constellation_diagram

From: LittleAlex on 11 Mar 2010 11:36
On Mar 10, 5:44 pm, RalfM <r...(a)invalid.invalid> wrote:

> Just some crazy thoughts of mine... :-)

I think "Santayana's Law of Repetitive Consequences" should be applied
here.

AL
From: Mike Paff on 11 Mar 2010 14:01
On Thu, 11 Mar 2010 02:44:52 +0100, RalfM <rm(a)invalid.invalid> wrote:

>Such a bitwise transfer does need only a fast switching between 2 voltage levels.
>Is it possible to have a transmitter/receiver that can use more than
>just 2 possible values per "bit period" (for example: a 12-bit ADC or DAC
>can detect 2^12=4096 different values (ie. voltages). Then why use
>only 2 values (0/1) in copper wired data transfers instead of using
>say 256 or 512 or 1024 or 2048 or 4096 bits etc.? (ie. make the cable a "8 bit cable" :-) or more.
>IMHO one could dramatically increase the speed on wired copper medium
>(for example a 256-fold increase or even more would be possible).
>Are there such chips which can switch fast a voltage source say to 256
>different values? ie. fast DAC and ADCs, DSP maybe?
>

Sounds good in theory, but is likely to fall apart once reality
sets in.

Think a little bit about how noise picked up by the transmission
line will affect the reading at the receiver. Also consider the
case where the transmitter and receiver have different ground
references.
From: Andy on 11 Mar 2010 14:53
On Mar 11, 12:44 pm, RalfM <r...(a)invalid.invalid> wrote:
> Grant Edwards wrote:
> > On 2010-03-11, RalfM<r...(a)invalid.invalid>  wrote:
>
> >> digital data transfer on a wired copper medium is done usually by
> >> setting a DC voltage (for example 5V) for a defined duration to
> >> indicate a binary 1 value, and say 0V to indicate binary 0. I think
> >> the duration of such a signal is called "bit period".
> ...
> >> (for example: a 12-bit ADC or DAC can detect 2^12=4096 different
> >> values (ie. voltages). Then why use only 2 values (0/1) in copper
> >> wired data transfers instead of using say 256 or 512 or 1024 or 2048
> >> or 4096 bits etc.?
>
> > That called a modem.  They use a sometimes large set of
> > phase/amplitude to encode multple bits per symbol.  Using
> > phase/amplitude instead of DC level allows the signal to be
> > transmitted over media that won't carry DC.
>
> Doesn't Ethernet use DC?
> And Ethernet uses bit-serial transfer at the lowest physical level.
> What I mean is to replace the bit-serial part by say a byte-serial transfer.
> Ie. in the same transfer cycle (time) now 8 bits could be transferred (insted of just 1)
> by using more than 2 DC signalling levels (here 8), much like how a DAC
> and ADC do it.- Hide quoted text -
>
> - Show quoted text -

No, Ethernet is transformer coupled, so there is no DC in the data
signal. There is a standard for Power Over Ethernet where DC power is
superimposed on the data signal, but stripped off at a center tap on
the outer winding of the transformer.

I think you are confusing AC with frequency modulation, and DC with a
simple time-varying voltage or signal. A time varying signal contains
at least AC (it alternates), and if it does not alternate evenly and
balanced, then it also has a DC component.

In a binary, single ended (not differential) signal, the transmitter
is a conceptually just a switch that puts either the 1 level voltage
or the 0 level voltage on the line, and can be very fast, very low
power and/or very inexpensive. The receiver is conceptually just a
comparator with a built in reference.

What you are proposing, multilevel discrete signaling (as opposed to
analog signaling or binary signaling), is already used in some
versions of Ethernet, and internally in the latest generation of flash
memory, where multiple bits are stored in each cell by having more
than just two discrete signal levels stored per cell. Most of these
systems that have to operate fast use primitive DAC and flash ADC
circuits for transmitting and receiving the signal.

Compared to binary signaling, multilevel discrete signaling has less
noise margin for a given min-max signal swing (there is less
difference between discrete signal levels).

Andy
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