Talk:Convolutional code

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Code Rate 1[edit]

Re the recent change to say that code rates could be equal to 1, I don't know of any such codes ever being used in practice, and it certainly doesn't seem very useful...

There *are* codes that when combined with M-ary modulation schemes produce one output symbol per input bit, but the output symbols are non-binary. But I don't think that's what you meant. Can you clarify?? User: Karn

From BrettE: This page doesn't descripte the ENCODING at all. Perhaps I'll add some of that later...

Josh Rubin: I second BrettE. The page for Reed-Solomon codes gives a rough but accurate definition; this page manages to talk about convolutional codes without defining them. A definition of "convolution" as used in mathematics and signal processing is in order too.

Codes of rate 1 are used in practice I'm quite sure. They would be used in storage media usually to satisfy frequency-related constraints. You can think of these codes as simply transforming information into a different format without changing its rate. BourkeM ^Converse! 05:46, 23 September 2015 (UTC)[reply]

Sequential decoding[edit]

I notice there is some confusion on sequential decoding and algorithms. The Fano algorithm (which doesn't currently seem to have a page on Wikipedia) is a low-memory sequential decoding algorithm. Shannon-Fano coding is a data compression technique and AFAIK is just related by having the same inventer. Perhaps we need a page on sequential decoding? Edratzer 22:46, 12 February 2007 (UTC)[reply]

Wrong picture?[edit]

The Img. 2 is said to have a constraint length of 4, but I can only see a length of 3, since reg3 has no effect on any output (it is used twice for output 1 (which cancels out in xor)). Am I wrong?

Yes, reg3 does not affect output 1 in direct way - by the argument you describe. However, reg3 affect reg1 : reg1=input+(reg2+reg3), and hence affect output 1 on the next performance cycle. Alexander Chervov (talk) 18:42, 8 October 2009 (UTC)[reply]

Another way of looking at constraint length is by the z-exponents; reg3 provides the memory for all

z^{-3}

terms in the numerator and denominator. If these dropped out of the final transfer functions, then you could argue for a constraint length of 3. --64.203.138.235 (talk) 20:24, 30 November 2009 (UTC)[reply]

Turbo Code for Planetary Exploration?[edit]

'"However, in the interest of planetary exploration this may someday be done."'?

Can somebody explain this one to me? --74.13.201.210 14:05, 3 July 2007 (UTC)[reply]

I've added a request for citation to the article. However I support of the statement I can say that deep-space channels are mainly characterised by Gaussian noise and are not subject to fading or ISI to the same degree as terrestrial channels. Therefore striving for the theoretical Shannon Limit (traditionally defined in terms of Gaussian noise) is worthwhile for deep-space channels. Edratzer (talk) 19:41, 4 December 2008 (UTC)[reply]

I am curious as well. There is no reason to concatenate a turbo code with a hard decision algebraic code other than to remove a residual error floor. The original rationale for code concatenation was that the convolutional code alone got too high a complexity if its constraint length was too long. If the statement cannot be justified, it should be removed from the article. Nageh (talk) 19:48, 8 October 2009 (UTC)[reply]

Dashed Arrows?[edit]

Please can someone explain what the difference is between solid and dashed arrows in the trellis diagram? Thanks. 190.160.224.58 (talk) 22:46, 16 November 2008 (UTC)[reply]

I've now expanded the caption. Edratzer (talk) 19:33, 4 December 2008 (UTC)[reply]

Convolutional encoding -> Registers[edit]

In example 1 "To convolutionally encode data, start with k memory registers" "The figure below is a rate 1/3 (m/n) encoder with constraint length (k) of 3" However the figure has 2 registers, m1 should not be a register, it's the input bit If m1 was was a register, it would output its input on the _next_ clock tick, with its input being what I observe as an unnamed arrow from the left — Preceding unsigned comment added by 195.251.124.94 (talk) 18:08, 10 February 2012 (UTC)[reply]

You are right, and the description is wrong: that code has only 4 states, not 8. --Cantalamessa (talk) 20:03, 9 May 2016 (UTC)[reply]

Vandalism[edit]

Just removed vandalism from 202.184.111.75 (which seems to have a track record of vandalism on other pages : see https://en.wikipedia.org/wiki/User_talk:202.184.111.75) SN74LS00 (talk) 12:15, 28 January 2014 (UTC)[reply]

MATLAB example code?[edit]

I'm curious as to what the policy is regarding sample code being written in open/free language/tools. The sample code on this page is not useful for people who don't have MATLAB, and for MATLAB users, it's the kind of thing that likely belongs in a user manual. Would a more open language make more sense to provide a demonstration of this kind of thing?

XXV (talk) 04:46, 12 September 2014 (UTC)[reply]

k vs. K confusion[edit]

I had to pull out a textbook to un-confuse myself -- I think later in this article, lower-case k (defined at the top as the output symbol rate) is being re-used at the bottom where it's referred to as 'constraint length'; which was defined as upper-case K at the beginning. Is that correct? 50.205.21.122 (talk) 01:15, 6 July 2017 (UTC)[reply]