256-fold (or even more) increase in speed on copper wire possible IMHO

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

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UART, CAN controller, SPI controller, I2C controller, Ethernet controller, etc.

The CPU doesn't generally do it.

Yes. They're called modems.

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.

Modems. Most of which are just specialized DSPs.

Not crazy. Just 30 years late...

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

AL

Wouldn't that be the same as the difference between telephone and telegraph?

...

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.

Isn't this intended for AC signals? I was meaning DC signals like in Ethernet.

Hmm. isn't analog transmission using AC? I mean use DC like in Ethernet but instead of transmitting just 1 bit per cycle use instead say 8 bits per cycle, ie. 8 DC voltage levels much like done with a DAC and ADC.

levels.

cable" :-) or more.

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.

No. "Analog" transmission (really multi-level transmission) just means that you're using multiple levels. There's nothing to say it must be AC, although it's often used over long wire runs, and it's often convenient to AC couple such runs -- which means that it's often convenient to design your signaling with no DC content.

I make the distinction because "real" analog transmission is where you transmit a continuous voltage to mean a continuous quantity -- i.e. 3V =

3 gallons in a tank, or AM radio where the envelope represents the sound you're trying to reproduce.

Ethernet -- at least twisted-pair Ethernet -- is transformer coupled, which means no DC can get through. The signaling is designed for zero DC content, to make it all work.

For that matter, some Ethernet protocols use multilevel signaling. See Wikipedia:

That's been fiddled with. About the only place that it's really popular is over really long stretches of wire. If the wire run is short (e.g. USB, IEEE-1394, LVDS), or if it can be well controlled (e.g. lower speed Ethernet) then the signaling is usually binary with some sort of BEC, or FEC combined with BEC.

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levels.

bit cable" :-) or more.

I think they can be solved easily by (a) using a twisted pair cable per direction, much like in Ethernet, and (b) one of the wires would be the common ground and the other obviously the DC level against the ground wire. The only problem I see is to have a DAC and an ADC which can switch fast enough the DC levels. But I remember having seen in the specifications of these chips that they can well do several Mega or even Giga samples per second, so then it should suffice IMO.

Twisted pair does not equal perfect noise cancellation. It helps, but not infinitely. Then on the far end you've got to have a differential receiver, which isn't going to have a perfect common mode rejection. All the while taking into account that all of the analog signal path has to settle to whatever level you're talking about (256 levels means better than 0.5%) in well under the bit time.

Basically, you're making the common mistake of really really REALLY wanting the real world to look like your undergraduate theory. The real world is hairy, nasty, and complex. If you really want to understand how much so then look into the history of the 56kbaud modem. Realize and understand that the folks working on it were very bright and very experienced, and if there was some easier answer then they wouldn't have gone to all that trouble.

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Yes. That's why I listed "Ethernet controller" as an example of a chip that does the baseband, binary, bit-serial method that was initially described.

10BaseT does. Some of the higher speed versions encode multiple bits per symbol.

Some of the higher speed Ethernet schemes sort of do that. For example, 100BaseT2 sends four bits per symbol. It uses 5 different DC voltage levels.

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Hmm. I don't understand why restrict yourself to use only binary signalling when it can be done byte-wise (or even more) in the same time. This could dramatically increase the speed, regardless of the distance.

Though Ethernet is baseband, it's not really DC. Various encoding schemes are used to make sure there is little or no DC voltage.

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levels.

bit cable" :-) or more.

Go ahead.

I'm sure all those engineers at TI, Bell Labs, DEC, Intel, HP, etc. were all wrong.

You're going to have a lot of problems with waveforms changing too much. Have you ever looked at any eye-plots and and compared what comes out of the far end of a cable with what goes in?

Do you think that everybody goes to such measures to avoid DC signalling just for fun?

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Because it's cheap, reliable, and works well.

_You_ claim it can be done. Everybody else seems to have failed and chosen methods like phase/amplitude modulation and various other schemes.

Again, you seem to be making a claim that contradicts what everybody who has worked in the data communication industry has experienced.

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er.

sted of just 1)

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

For the same reason that most folks drive, ride or walk to work instead of flying there in a helicopter. The helicopter dramatically increases speed, regardless of distance, but it's expensive and takes constant fiddling to keep it working.

Why don't you actually learn what Ethernet does? Then you would know that even your examples don't always restrict themselves to binary signaling, and you wouldn't keep repeating erroneous information.

While you're at it, learn what "baseband signaling" is, as opposed to "DC", and ponder why the statement "Ethernet uses DC" is patently false.

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Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com