Talk:Envelope detector

Why do these things work?

It would be helpful if someone could explain why these things work the way they do. Mathematically it is clear, but what makes the diode do the right thing? - 72.58.19.66 08:09, 7 May 2006 (UTC)

For the benefit of others: LC circuit seems to answer my question. - 72.58.19.66 08:12, 7 May 2006 (UTC)

Article Rating

The article was rated as start-Class because it has several useful sections and a good image, but lacks content in those sections. Steven Hepting 07:26, 12 April 2007 (UTC)

Segregate envelope from the detector

The envelope shouldn't be a subsection of the detector article, it should be a separate one. Although there is little information on the subject currently, it should be split from the detector page, so it can be expanded alone. Indomaster (talk) 23:51, 25 March 2008 (UTC)

As a long lost radio amateur and computer programmer (now moved on to legal briefs), I must disagree with Indomaster. Thirty years ago, I understood these things, but that was a long time ago. I am checking in to refresh (and hopefully update) my knowledge.

As far as separating the discussion, it would be difficult to explain to a duffer like myself what an "envelope detector" is, without explaining what an "envelope" is! If Indomaster is concerned that the envelope discussion is too mathematical for some readers, I say: let those readers skim past sections they can't understand. But don't dumb down the article. Personally, I found the explanation fully satisfactory and mercifully terse. What else would one want to add in a separate article about an envelope?

Further to "as detailed as necessary, as brief as possible" school, I believe it is good style, when throwing around greek letters and assorted gibberish, always to define one's variables. $\omega$ and $x(t)$ could stand a definition, even if as simple as "Any AM or FM signal $x(t)$ can be defined as ... User:WN1YHQ 4/18/08

Modulation envelope

In this diff I reverted the addition of material about the modulation envelope, a signed envelope distinct from the nonnegative envelope being discussed in this article. Perhaps a section could be added to mention this alternative type of envelope, but as an example of what this article is about, it was wrong. Dicklyon (talk) 01:34, 7 April 2012 (UTC)

A modulated waveform in space at a selected time instant composed of two sinusoids of nearly the same wavelength.

A signal in blue and the magnitude of its analytic signal in red, showing the envelope effect

Dick: Thanks for mentioning this on the Talk page; I missed your explanation here. However, it is just nitpicking to say that the example in the left figure is unacceptable because the envelope is a cosine function, compared to the right-hand example that is, as you say, non-negative. Obviously, the left example shows both the upper and lower boundaries while the right-hand one shows only the upper boundary. That is a trivial point. Brews ohare (talk) 01:53, 7 April 2012 (UTC)

It's not a trivial point. See the history of your left-hand figure, in which you had a hard time deciding which definition of envelope and envelope periodicity to use. The definition used in this article is the type of envelope that an envelope detector detects (as opposed to the type that a synchronous demodulator would detect, which usually wouldn't be called an envelope). Besides, there's no reason to complicate this article with space-domain concepts like wavelength. Dicklyon (talk) 04:42, 7 April 2012 (UTC)

There is no reason in principle that this waveform should not be used as an example of an envelope. There will certaily be an output if it is fed into an envelope detector. This explains the characteristic "beating" heard on AM radio due to an interfering station. However. I agree with Dicklyon that is an unnecessarily complication to introduce an analysis in terms of wavelength - in fact, I am not convinced that any analysis at all is needed for the reader's understanding. I also think that the diagram needs to be amended to show the actual envelope output that would be obtained, which it currently does not. The example does not really belong under the "definition" sub-heading, unless it is intended as an example showing that the definition of envelope given breaks down at greater than 100% modulation, but Brews text does not currently make that point. SpinningSpark 09:47, 7 April 2012 (UTC)

The section Definition of the envelope appears to intend to define envelope in the sense of analog modulation, not as the item measured by an envelope detector. In its present form, the article does not point out that the envelope as mathematically defined will not be what is detected because the connection between the output response of a real detector and the defined envelope isn't discussed.

Inasmuch as this article is supposedly devoted to detectors, a general definition of envelope is not really needed, and the appearance of doing so is misguiding. The page Envelope (disambiguation) shows no article that provides a proper definition of envelope for AM modulation. Envelope (mathematics) contains no treatment of cases like those in these figures. Envelope (waves) redirects to Envelope detector. Amplitude modulation shows some relevant waveforms, but doesn't discuss envelopes. So at the moment WP has no appropriate article.

One could adapt this submitted paragraph with its accompanying figure to the article by discussing how a detector would deal with it, but this completely unsourced and sketchy article is not worth fixing up to handle this issue. I propose to drop the matter for this article, and attempt to find a better home for this paragraph. Brews ohare (talk) 11:47, 7 April 2012 (UTC)

It seems that you are often looking for a home for your latest creations, rather than looking for how to improve articles. That's backwards. Dicklyon (talk) 15:53, 7 April 2012 (UTC)

Dick, that remark is most uncollegiate and wrong-headed. These "creations" are intended to improve articles, as you do know. Although you are reluctant to give credit, many of these "creations" of mine have succeeded in adding value to WP. Your collaboration in creating them would be a far more useful undertaking on your part than you now exhibit. Brews ohare (talk) 16:05, 7 April 2012 (UTC)

I have placed this material in the new article I've written called Envelope (waves). Brews ohare (talk) 16:59, 10 April 2012 (UTC)

Dick Lyon's reversion of an example

Dick Lyon removed from this article this material with the one-line explanation:

"the definition of envelope in this example is not compatible with the definition that is the subject of this article."

Reverted example

A modulated waveform in space at a selected time instant composed of two sinusoids of nearly the same wavelength.

A commonly used example in both space x and time t is that of the superposition of two waves of almost the same wavelength and frequency:

$f(x,\ t)=\sin \left[2\pi \left({\frac {x}{\lambda -\Delta \lambda }}-(\omega +\Delta \omega )t\right)\right]$$+\sin \left[2\pi \left({\frac {x}{\lambda +\Delta \lambda }}-(\omega -\Delta \omega )t\right)\right]$

$\approx 2\cos \left[2\pi \left({\frac {x}{\lambda _{mod}}}-\Delta \omega \ t\right)\right]\ \sin \left[2\pi \left({\frac {x}{\lambda }}-\omega \ t\right)\right]\ ,$

which uses the trigonometric formula for the addition of two sine waves, and the approximation Δλ<<λ:

${\frac {1}{\lambda \pm \Delta \lambda }}={\frac {1}{\lambda }}\ {\frac {1}{1\pm \Delta \lambda /\lambda }}\approx {\frac {1}{\lambda }}\mp {\frac {\Delta \lambda }{\lambda ^{2}}}.$

Here the modulation wavelength λ_mod is given by:

$\lambda _{mod}={\frac {\lambda ^{2}}{\Delta \lambda }}\ .$

The modulation wavelength is double that of the envelope itself because each half-wavelength of the modulating cosine wave governs both positive and negative values of the modulated sine wave.

Comparison with the earlier formula for a modulated wave shows that in this case the modulating envelope function R(t) is:

$R(t)=2\cos \left[2\pi \left({\frac {x}{\lambda _{mod}}}-\Delta \omega \ t\right)\right]\ .$

References

As pointed out in this text, this example is a generalization from a a presentation based upon time alone to the case of an analog modulated wave propagating in space as well as varying in time. Personally I see nothing incompatible in this example: it is simply a minor generalization to consider propagation as well a time variation.

Besides its interest as an academic generalization, this example also is an introduction to similar cases used in phenomena ranging from radar and GPS signals to electronic distance measurement. Brews ohare (talk) 01:44, 7 April 2012 (UTC)

Brews, it is perhaps trivial to you that this sort of signed envelope is not at all what an envelope detector detects, nor is it like an analytic envelope, which is a fair description of what an envelope detector attempts to estimate. Personally, I don't see how this adds to the article, but I'll listen to other opinions. Dicklyon (talk) 04:39, 7 April 2012 (UTC)

Dick: I take your point. Although the subsection Definition of the envelope appears to intend a general discussion of envelopes in AM modulation, in fact it doesn't do this, and in fact, a general definition is not needed for the topic of detection. The article Envelope (waves) should be written instead of redirecting here. Maybe this material would be appropriate there? Brews ohare (talk) 11:56, 7 April 2012 (UTC)

I would suggest Beat (acoustics) as a fitting place for this stuff. Olli Niemitalo (talk) 17:57, 8 April 2012 (UTC)

I have placed this material in the new article I've written called Envelope (waves). Brews ohare (talk) 17:01, 10 April 2012 (UTC)

Audio

IMHO, the audio section is wrong. Envelope detectors are of use in audio only in the crudest applications, like wah-wah, in which design does not aim at any preservation of the signal ; one seeks any interessant modification of sound. In all other cases, envelope detection in audio must detect both negative and positive, as the signal is not generally symmetrical ; and use AOP+diode, so as to function down to zero amplitude, or alternatively a much shorter time constant in order to approximate quadratic detection. If however the contributor might cite a reference or source, I'd be glad to reform my opinion. PolBr (talk) 08:57, 22 March 2014 (UTC)

There are plenty of book sources describing audio envelope followers but it is not clear to me what you think is actually incorrect in the article. It does not seem to contradict any of the statements you have made. SpinningSpark 11:11, 27 March 2014 (UTC)

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Figure of envelope is misleading

The red trace of the envelope over the blue signal is misleading, not realizable in real time. The trace of the output of a rectifier is a better, more realistic example of what an envelope is. 12.3.203.132 (talk) 21:42, 8 September 2021 (UTC)