Talk:Manchester code

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This article contains material from the Federal Standard 1037C (in support of MIL-STD-188), which, as a work of the United States Government, is in the public domain.


So, what does this code do?

Provides a simple way of encoding arbitary binary sequences without ever having long periods where there is all one voltage, thus losing clock synchronization. Also ensures that the DC component of the encoded signal is zero. -- The Anome 12:13, 18 Sep 2004 (UTC)

Manchester code is commonly refered to as Bi-phase-level or split phase in telemetry documentation including the Range Commanders Council document IRIG 106. I belive it would be constructive to include these as my search from bi-phase-level did not produce any results. --138.64.8.53 14:09, 30 January 2007 (UTC)Thad Sterling


Contents

[edit] Manchester code = bi-phase mark code ?

Several articles that I read (perhaps originating from the same source) use bi-phase code and Manchester code as two equivalent terms for the same encoding. The description of "Biphase Mark Coding" here on Wikipedia (http://en.wikipedia.org/wiki/Biphase_Mark_Code) seems to confirm this. A link "see also ..." on this page might be useful (or an explanation where the difference lies).

Thiadmer Riemersma


I think the Manchester graph is wrong. The correct bits or drawing should be opposite depending on how you look at it. In Manchester Encoding, a 1-bit transits from negative to positive and vice versa for 0-bit.

SA DIP 25 Swl

Actually there are two types of bit representations and the page explains that. To avoid ambiguity, I have specified the type of encoding displayed on the image. Natrij 23:59, 1 February 2006 (UTC)

I think it's important to note here somewhere that the bi-phase mark (and bi-phase space) are commonly referred to in some industries as FM1 and FM0, respectively. It's an important distinction as FM1 or FM0 (bi-phase mark or space) are easily decoded without reference to (absolute) phase at all. This simplicity exists since a transition occurs at the beginning of each bit period. This stands in contrast to Manchester encoded data where the (absolute) change of phase direction must be noted. FM0 and FM1 as common terms probably came about since they don't imply phase sensitity.

Gene Gajewski 11-Mar-2006 71.32.73.216 04:08, 12 March 2006 (UTC)

There is an article on Differential_Manchester_encoding as well as one on BMC. That information would probably be more appropriate there. Though I think this article needs a tad more detail about the differential variation and more obvious link -- maybe it's worth putting this nomenclature there as well. I'd advocate the mention of them here in reference to BMC. I believe differential manchester is subtly different again. Maybe we should merge them all into Biphase Encoding or something?--Ktims 22:24, 31 March 2006 (UTC)

[edit] Baseband

Is it worth mentioning that the codes are only useful on baseband mediums, or would that be implicit? It doesn't seem to me that it's entirely clear in the articles on telecoms how modulation and encoding relate to each other. Probably such a change doesn't belong here, but it's where I thought of it...--Ktims 22:24, 31 March 2006 (UTC)

[edit] Where does the name "Manchester Code" come from?

Does anybody know where the name "Manchester Code" comes from? I have heard several explanantions but none of them did convince me. —The preceding unsigned comment was added by 87.165.10.155 (talk) 20:29, 21 February 2007 (UTC).

Could it be something to do with the Manchester Mark I? Bastie 19:27, 27 February 2007 (UTC)

[edit] Hz per bit/second

There is nothing dubious about this unit; it is simply the reciprocal of spectral efficiency. Spectral efficiency is measured (typically) in bit/s/Hz, i.e. bit/s per Hz. Crudely put, it is the ratio of the bit-rate vs. the spectral bandwidth used by the coding/modulation scheme. The higher this figure, the better, as it means that you're getting a higher information-rate for the same bandwidth.

The reciprocal of spectral efficiency is in units of Hz per bit/s; clearly the lower this figure is, the better. It describes, crudely, the amount of bandwidth required to transmit a given information rate.

Hz per bit, as was proposed as an alternative, is meaningless (in this context, at least). As a unit, it would imply that the bandwidth is related to the total number of bits transmitted.

This is basic comms theory and maths; so consequently, I've removed the dubious tags.

If you're still not happy, then we can re-write that section to talk about spectral efficiency rather than its reciprocal, because I admit that's not a commonly-used dimension. Oli Filth 01:46, 21 April 2007 (UTC)

[edit] Encoding conventions

The difference between the "Thomas" and IEEE conventions is so trivial (inversion of the signal) that its description causes more confusion than illumination. If any conventions are to be compared, perhaps an explanation of "differential" encoding would be more use (if someone can do this concisely). Mike Shepherd 11:36, 12 July 2007 (UTC)

[edit] The generic nature of Manchester code

It's useful in editing to remember that Manchester code is abstract from the means used to encode and decode it. It is often generated and decoded by software, so we should avoid suggesting (for example) that the decoder is "hardware" (or even that it's necessarily part of an electronic system). Mike Shepherd 11:36, 12 July 2007 (UTC)

[edit] Fourier components

Regarding the recent edit with the comment "by definition, a Fourier component is constant", we should distinguish between the "constant" results (the Fourier coefficients) of analysing a given segment of signal and the (typically different) results from analysis of another segment. A communication system exists precisely because these are different. It is in this sense that we might speak of the Fourier components changing in time. Thus (colloquially), we say "the DC component is constant" when we might say (more formally) that "the DC component is the same for any long segment of the signal". But let's remember that there's little point to an article which would satisfy a pure mathematician but which almost no-one else understands.Mike Shepherd 08:16, 25 August 2007 (UTC)

I see your point! However, might I suggest something along the lines of the following as a rewrite, so as to keep "everyone" happy (changes marked in bold):
"The DC component of the encoded signal is not dependent on the data and therefore carries no information, allowing the signal to be conveyed conveniently by media (e.g. radio) that usually do not convey a DC component."
Incidentally, I'm not convinced that radio is the best example here, as it's highly unlikely that anyone would ever try to transmit a baseband signal directly over radio. Any thoughts on this? Oli Filth 10:52, 25 August 2007 (UTC)

I'm easy with either version. I think I was being pedantic when I said that radio does not "usually" convey a DC component. Radio is a good example, but probably we should remove the word "usually". To me, the change from "which" to "that" makes the style colloquial (perhaps because I'm more familiar with British English), so I would never use the second form myself in a formal document. But I think either is OK, particularly if it's OK in American English.Mike Shepherd 14:33, 26 August 2007 (UTC)