Talk:Differential signaling

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I would have included all sorts of other technologies under the topic of differential signaling, including RS-422, RS-485, ethernet and even MIDI. Is there some technical reason these have not been included in these collective topics yet? Is there a better term to use for the technology involved in these other major signalling technologies?

Charlie Richmond 23:05, 5 December 2005 (UTC)

  • Looks like a clear candidate for merging to me. --ragesoss 16:26, 18 December 2005 (UTC)
So what do we do now? Charlie Richmond 11:35, 19 December 2005 (UTC)

Differential signaling just means you have two conductors and that you send your signal as a difference of voltages (or currents) rather than as the voltage (current) on a single wire (single ended singnaling). There are a host of circuit families that fall under the heading of differential signaling, including: current mode logic (CML), emitter coupled logic (ECL), positive supply emitter coupled logic (PECL), low-voltage ecl/pecl (LVECL/LVPECL), LVDS and the others mentioned (though I'm not an expert on RS-422, RS-485, or MIDI). These differences between these families have to do with where the termination is referenced, what the common-mode levels are set to, how large a differential signal amplitude is required, etc. The term "Differential signaling" is a generic topic which I think deserves its own entry. Citing examples like LVDS, ECL, CML, RS-422, etc might be useful. The latter, RS-422, is a signalling standard which describes an electrical interface and communications protocols. I don't see how it would be correct to merge the generic topic of "differential signaling" with any of the specific I/O standards cited.

It would be wrong to merge this article with LVDS. LVDS is a specific technology, whilst this article encompasses all forms of differential signalling. --Heron 17:51, 17 March 2006 (UTC)
As above, I say no merge. A link in the other page to here is fine. -- RevRagnarok 01:32, 3 May 2006 (UTC)
Agreed with Heron... it would be wrong to merge unless every type of differential signal was pulled into the article (also not a wise thing to do). There are lots of different types of differential signals, LVDS is just one type. Mrand 23:20, 7 June 2006 (UTC)

Contents

[edit] Two l's?

Single-ended signalling has two l's while Differential signaling only has one l. We need to make this consistent. 129.162.1.32 13:58, 16 October 2006 (UTC)

UK vs US spelling... This situation is rather contentious on Wikipedia, and it's unlikely there will be a standard any time soon (current policy is just, stick with whatever the article was originally written in unless the article is clearly written about a US or UK subject). -- mattb @ 2006-10-16T15:18Z

[edit] Accuracy

Balanced lines reject noise; not differential signals. As long as the lines are at the same impedance, they'll pick up the same noise, which can be rejected at the other end. The desired signal can be differential, only on one wire, or whatever.

"... only the common-mode impedance balance of the driver, line, and receiver play a role in noise or interference rejection. This noise or interference rejection property is independent of the presence of a desired differential signal. Therefore, it can make no difference whether the desired signal exists entirely on one line, as a greater voltage on one line than the other, or as equal voltage on both of them." - (IEC 602689-3:2001, page 111) — Omegatron 02:03, 6 April 2007 (UTC)

I think I agree. I haven't visited this article for a while, so I haven't noticed its decline. It looks as if it is the result of a merge or two.
If you are right, which seems likely, then most of this article is BS. We are then left with these reasons for using DS:
  • No need for solid ground connection between source and load, since receiver is floating. The only requirement is to keep the signal within the common-mode range of the receiver.
  • The differential voltage is bipolar and the threshold voltage is zero. This makes the system work with high line attenuation, as long as attenuation is similar in both wires.
  • Differential swing is twice the swing in each wire, so you get more noise immunity from a given power supply rail voltage.
Did I miss any? There are lots of people out there claiming that DS cuts down on EMI (incoming and outgoing), e.g. this article by a 'recognized industry expert' (PDF) and this one from Lattice (PDF), but all these claims contradict your IEC reference and, if you read them carefully, are assuming that the line is balanced. --Heron 22:43, 7 April 2007 (UTC)

Yeah, you get 6 dB more signal (and therefore SNR) by sending differential signals, but it isn't the property that allows cancellation of noise. Parallel discussion on Talk:Balanced_line#Shielding. — Omegatron 19:49, 9 April 2007 (UTC)

Agreed. See you at Balanced line. --Heron 20:21, 9 April 2007 (UTC)

See Talk:Balanced_audio#Incorrect and Talk:Balanced_line#Accuracy for parallel discussion. — Omegatron 18:14, 22 May 2007 (UTC)

It says "Differential signaling is commonly transmitted on balanced lines, which offer advantages beyond those of differential signalling itself", but this still kind of implies that the differential signaling is responsible for the rejection. We should state this pretty clearly since it's a common misconception. — Omegatron 18:17, 22 May 2007 (UTC)

I've fixed the statement in this article, but I'm not going to do the same for the audio article because I have little practical experience in that area, so I would just be hypothesising. --Heron 20:11, 22 May 2007 (UTC)
Yeah. In light of this, though, I'm not really sure why audio circuits use differential signalling at all, besides the extra dynamic range allowed. — Omegatron 20:58, 22 May 2007 (UTC)
I suppose it allows rejection of common-mode signals in the output amps? Like if the output amp didn't have good power supply rejection?
And older circuits used (center-tapped?) output transformers, which the differential out tries to emulate for some compatibility reason? — Omegatron 21:00, 22 May 2007 (UTC)
Audio Balancing Issues by Graham Blyth of Soundcraft [1] is a good bit of debunking. He says that the point of differential balancing is that it keeps the source impedances of the two wires equal over the whole band, since they are both driven by similar amps, while a single-ended balanced line would only be accurately balanced at DC. --Heron 09:54, 23 May 2007 (UTC)
That's an excellent reference. Thanks. It seems like filter caps at the output (like RF caps from each output pin to chassis) would keep the impedance balanced at higher frequencies? But I can see it being marginally better in some circumstances.
As he points out, the fully-differential "cross-coupled" outputs will send the same signal level down an unbalanced cable that shorts one pin to ground. Although the "impedance balanced" output can also do this for a standard TS cable, it doesn't work if the hot is shorted, so interfacing with incorrectly-wired cables is also a very marginal benefit. — Omegatron 15:44, 23 May 2007 (UTC)

Can you look at Talk:Balanced_audio#Incorrect? (And also Talk:Faraday_cage#Mesh_size.3B_explanation.3B_magnetic_fields.) I'm not really fluent with things like "wave impedance". — Omegatron 15:16, 24 May 2007 (UTC)

OK. See you there. --Heron 15:20, 26 May 2007 (UTC)

I'd like to point out that for a single signal over long wires, it often doesn't matter if the transmitter starts out with single-ended (signal and ground) or differential signal (S+ and S- swinging in opposite directions). After the wires pass through a braid-breaker or choke (to reduce electromagnetic interference), the signal seen at the far end looks like a differential signal, one voltage swinging in the opposite direction as the other. (For example, many MIDI hardware *drivers* often have one "+5 V" line and another line that is driven single-ended 0 to +5 V. Other MIDI drivers have one "GND" line and a "signal" line that is driven 0 to +5 V. But on the other side of the the resistors and the choke, the voltages at the reciever -- for both kinds of drivers -- look like differential signals swinging in opposite directions).

I wonder if this patent has any relevance to the "differential signal" dispute: "Ground return for high speed digital signals that are capacitively coupled across a DC-isolated interface" United States Patent 6023202

[edit] it can make no difference

"... only the common-mode impedance balance of the driver, line, and receiver play a role in noise or interference rejection. This noise or interference rejection property is independent of the presence of a desired differential signal. Therefore, it can make no difference whether the desired signal exists entirely on one line, as a greater voltage on one line than the other, or as equal voltage on both of them." - (IEC 602689-3:2001, page 111) —

Perhaps I'm taking this statement out of context, or perhaps I've been suckered into believing a myth, but the very last bit of this statement seems very wrong to me: "it can make no difference whether the desired signal exists ... as a greater voltage on one line than the other, or as equal voltage on both of them."

Often we have 2 balanced wires feeding into a receiver. And often the "desired signal" is some analog audio music. And often we get some unwanted outside noise that (because we designed the wires to be balanced) couples equally into the 2 wires.

If we were to hypothetically put "the desired signal ... as equal voltage on both of them", I don't see how it is possible to reject the noise. But if we do the usual thing and make "the desired signal ... as a greater voltage on one line than the other", then we can reject the (common-mode) noise with a standard differential amplifier. So it does make a difference. Right? --76.209.28.72 07:59, 16 June 2007 (UTC)

I assume they mean 'equal in amplitude but opposite in phase', otherwise it doesn't make sense. Does anyone have a full copy of the standard? I wonder if it's a misquote, or a mistranslation from the French, since the standard seems to be bilingual. --Heron 19:42, 16 June 2007 (UTC)

They mean the voltage is equal in amplitude (but opposite in polarity). A "greater voltage on one line than the other" (if the signal were on one wire and the other were connected to ground, for instance) is not typical in balanced systems; but still works as long as the impedances are balanced. — Omegatron 22:57, 17 June 2007 (UTC)

[edit] Diff Sig Diagram

So I had made this diagram for diff signaling because I thought it would help with understanding. Feel free to take out or replace with a better one if needed. Guerberj 20:05, 21 June 2007 (UTC)


Excellent. Thanks. — Omegatron 01:48, 22 June 2007 (UTC)

Clarification on high frequency signals: The article states an advantage of Diff Sig. over Single Ended is greater ability at high frequency. Gives example of coaxial cable as single ended but the link to coax describes it's ability at high frequency. My understanding is coax is superior at high frequency signaling due to such things as smaller variation in impedance. All seems a bit contradictory. Suggest some clarification by an expert (which I'm not). Stuzzo 20:35, 18 July 2007 (UTC)

[edit] Twice the noise immunity

I don't know enough about the area to change anything myself here but it would be useful to say what sort of noise differential signaling provides twice the protection from. Presumably if you include noise in the ground at different points in the device differential signaling provides more than twice the protection against this. It would also be useful to say exactly what this means. These may be obvious to people who know the field but not to me.

For instance maybe we could change the sentence to read something like, "At a given voltage differential signaling can tolerate twice as much noise induced in the lines." However, now that I think about it I would guess that some sort of statement about the shape of the noise (independent Gausians) might need to be made to avoid the situation where both wires experience perfectly negatively correlated noise. Anyway I don't really have a clue hence the reason to ask someone who does. 128.103.187.69 (talk) 19:15, 26 November 2007 (UTC)

[edit] differential signaling in computers

The article currently claims "Differential signaling has to be used in computers".

Certainly some computers do use differential signaling in some places. But my understanding is that many computers don't use differential signaling anywhere.

What should we say about computers in this article? --68.0.124.33 (talk) 21:07, 26 April 2008 (UTC)