Talk:Frequency modulation

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why the radio based on frequency modulation has very small transmission area?

It's nothing to do with the FM modulation, it's to do with the higher frequency used for FM radio broadcasts, compared with AM radio broadcasts. AM radio is on a lower frequency, so it travels further. The lower longer-range AM radio channels were handed out first, so they use the older (and more frequency-efficient, but less interference-tolerant) modulation. -- The Anome 11:04 18 Jul 2003 (UTC)

Actually, FM is more frequency efficent than AM, except that commercial FM uses wide FM to get more audio bandwidth into the signal. If FM used the same audio bandwidth as AM, it would use the same or less RF bandwidth than AM. Commercial frequencies (i.e., public safety) and amateur frequencies that use FM use narrow FM. --ssd 03:59, 23 Jun 2004 (UTC)

Note: we should have a radio propagation to explain why the lower frequency radio waves travel further. -- The Anome

I've started one, but there may be lots of factual inaccuracies, I'm not sure, I haven't done radio stuff in ages :) Dysprosia 11:26 18 Jul 2003 (UTC)

I started Category:Radio frequency propagation with the intent of writing an article including as many of those terms as sane. Feel free to use it as a seed. --ssd 03:59, 23 Jun 2004 (UTC)

Could we please stop moving the disambiguation page and setting to FM. FM does stand for more than one thing. Thank you. --Numerousfalx 23:51, 15 Dec 2004 (UTC)

Contents 1 Example 2 Too focused on radio 3 english lesson 4 font 5 Formula Error 6 FM recording 7 Wave Images 7.1 AM and FM compared 7.2 FM 8 Theory 9 How to demodulate FM ? 10 Assignments

[edit] Example

Can someone give an example for frequency modulation? let's say there is a test signal low tone, high tone, and a sweep. this composed audio signal is supposed to be put on FM radio - how would the modulated tone look like? how much bandwidth does FM radio need? When I detune my favorite radio station "92.2 MHz Rock" by, let's say +0.2 MHz, will the music and all transmitted acoustics be pitched to higher frequencies (so that radio moderators talk like Mickey Mouse) - or is FM resistant against those mistunings.

It would be great if someone could work my (answered) questions into the article. Thanks, --Abdull 21:41, 17 Jan 2005 (UTC)

I agree this sort of information should be covered by the article, preferably with diagrams. Answering these questions (for FM) is somewhat tricky, though, and involves a lot of math.

I can answer your last question here: no, tuning away from an FM station does not shift the frequency components of the output, so you won't hear "Mickey Mouse" voices like you would with SSB. The capture effect inherent to analog FM receivers means that slight mistunings are "automatically" compensated for. Tune far enough away and you hear static-y, distorted sound; tune further away and you hear only static.

Simon 18:20, 23 Jan 2005 (UTC)

Simon is correct about detuning an FM receiver that has no AFC (Automatic Frequency Control) or has AFC switched off. However most FM receivers do have AFC (because it is quite easy to implement) which truly compensates for mistuning. Try to tune away and the sound remains good, until you reach the limit of the AFC and suddenly lose the signal completely. Then the sound changes to the hiss heard between stations. Some receivers have an additional "squelch" function that mutes the hiss noise. Note: all the above observations are for home VHF FM radios, which the frequency Abdull mentions indicates.Cuddlyable3 18:50, 12 February 2007 (UTC)

[edit] Too focused on radio

The article reads as "FM as used in radio" with a little theory thrown in at the last minute. Shouldn't an article titled "Frequency Modulation" primarily cover the theory and link to separate articles on radio/data transmission/encoding (and FM synthesis)? The FM synth article has almost no theory at all.

That might be because the authors of the FM Synth article are unaware
of that FM-synthesis á la Yamaha is based on phase-modulation. /ja

The first sentence "...represents information as variations in the instantaneous frequency of a carrier wave" makes sense if you're reading about radio, but is hard to "get" if you're coming at this from the FM synth perspective.

Please have a look at the discussion page under "phase modulation" where I
present a few C like oneliners to make the point. /ja 

I suppose it makes sense to add a bit more on the theory, whether the article should have its current focus or not; I'd do it myself, except for the fact that I don't really understand FM all that well yet.

magetoo 14:16, 27 Apr 2005 (UTC)

[edit] english lesson

"The phrase frequency-modulated, an adjective, should have a hyphen when used attributively."

Is this significant to FM for some reason? - Omegatron 21:39, May 25, 2005 (UTC)

[edit] font

Could the author consider changing the font on the maths? I found it very difficult to see on a computer screen whether I was reading "t" for time or "f" for frequency. Otherwise found the article well balanced.

I agree that the font used makes the formulae very hard to read, but the author correctly used TeX to create them; there is nothing to fix on this page.

In my opinion there is a problem with the HTML generation of formulae (it was just as unreadable in IE as Firefox). I will look around and see if there is somewhere I can raise this issue. In the meantime, I recommend changing your wikipedia math preferences to use png for all formulae (the default is to use HTML for 'simple' formulae and png for everything else). Happyharris 18:40, 18 August 2005 (UTC)

After reading some more doc, it seems that individual formula can be forced to PNG. That seems appropriate here so I did it (although I still think the default use of HTML is a wikepedia-wide fault) Happyharris 18:44, 30 August 2005 (UTC)

Happyharris, i really agree and have had some of my PNG converted to HTML like in Plank units at the bottom. but i think that consistency in notation and appearance is important. ironically, i was recently editing and was putting in in-line HTML math instead of the usual PNG. so maybe i got infected with WP orthodoxy. r b-j 05:15, 20 October 2005 (UTC)

[edit] Formula Error

The main theory discussion has a very common mistake in deriving frequency modulation. I picked up on it because I make the same mistake every time I start to review FM, and have to fix it. The formula x(t) = A*cos(2*pi*f(t)*t) is incorrect. It arises because A*cos(2*pi*f*t) is correct when f is a constant - f is the frequency. However, frequency is the rate of change of the cos() argument. If f(t) is time varying, then the rate of change of f(t)*t is not f(t), it is some complex derivative that is no fun at all.

A better derivation is to start with x(t) = A*cos(phi(t)). Frequency is defined as the rate of change of phi(t). To get sinusoidal FM, we need some phi(t) such that the derivative of phi(t) is 2*pi*deviation*cos(2*pi*rate*t). That isn't so hard - we know the derivative of sin(kt) is k*cos(kt), so we postulate phi(t) = deviation/rate *sin(2*pi*rate*t). It is inconvenient to carry the ratio of deviation/rate, so it is often called Beta. The final formula, then, is x(t)=A*cos(Beta * sin(2*pi*rate*t)) where Beta is deviation/rate, and both deviation and rate are in Hz.

This leads nicely to the Bessel functions, which are a formalized way of working out equations like cos(m sin(x)).

References: "Analog and Digital Communications" by Hwei Hsu (Schaum's Outline) and "Fundamentals of Electronics" by Aldo Vieira da Rosa.

[edit] FM recording

"FM is also used at intermediate frequencies by most analog VCR systems, including VHS, to record the luminance (black and white) portion of the video signal."

Has this method ever been tried for audio recording, aside from the HiFi modes on some analog VCR systems such as VHS and Betamax?144.139.87.8

On magnetic tape, the common method of adding a supersonic frequency to the audio signal reduces distortion very effectively, and exploits the dynamic range of the tape to the full. An FM signal with its sidebands would demand a faster tape speed, hence allow less recording time.

FM of an audio-frequency carrier has been useful as a means of recording low-rate digital data on audio tape (cassette) recorders. Such audio equipment could not handle long strings of repeated 0's or 1's directly because of limited low-frequency response.Cuddlyable3 19:01, 12 February 2007 (UTC)

"FM is the only feasible method of recording video to and retrieving video from magnetic tape without extreme distortion, as video signals have a very large range of frequency components — from a few hertz to several megahertz, too wide for equalisers to work with due to electronic noise below -60 dB. This sentence tries to say too many things and needs attention. The megahertz-wide frequency range of video does not force any particular choice of modulation method, nor is it difficult to "equalise" over the video frequency range. I think the subject of FM in video recording belongs in a new section, where the significant issues treated are linearity, pre- and post-emphasis, tape speed, head gaps and noise distribution.Cuddlyable3 18:53, 9 February 2007 (UTC)

[edit] Wave Images

[edit] AM and FM compared

The animation is nice. However the AM example is jerky, gives an impression that carrier cycles are individually modulated, and they don't look sinusoidal. There is also redundant text "modulatie", not even in English (the M's stand for modulation already). I think we have to look extra critically at animated drawings because of the relatively high data overhead they represent.Cuddlyable3 15:19, 15 February 2007 (UTC)

I have fixed the above with a new animation. Besides being accurate and non-language specific, it is a much smaller file, and we all like pages to load quickly, don't we?Cuddlyable3 01:04, 18 February 2007 (UTC)

[edit] FM

I may be wrong, since I'm still studying elementary physics, but does the first graph show a frequency over time (signal) superimposed on an amplitude over time (carrier)? To remain consistent, I would think that the two graphs should have the same two variables. --Pyg 01:09, 11 March 2006 (UTC)

Your question confuses me but the graphs do not. They all have the variables voltage (vertical) and time (horizontal).Cuddlyable3 18:18, 9 February 2007 (UTC)

The red curve shows the signal as advertised, which is a varying voltage input to the modulator. Only after the modulation does the carrier frequency over time take on the same curve (or it could be inverted since there is no fixed convention about the direction of frequency deviation).Cuddlyable3 21:22, 11 March 2007 (UTC)

I think the first image (AM, green waveform) is incorrect - it should be showing a varying signal amplitude (vertical scale) at a fixed frequency. Nogami 00:47, 12 August 2006 (UTC)

It is correct. The carrier drawn in green has neither AM nor FM.Cuddlyable3 18:18, 9 February 2007 (UTC)

[edit] Theory

There seems to be something wrong with the formula. It is different from the one shown in one of the external links (http://www.fas.org/man/dod-101/navy/docs/es310/FM.htm). In this page the formula does not involve an integral at all. It seems to me that the formula as it is now requires the time integral of the signal Xm(t) from 0 to t to be restricted to [-1,1] for all t. This is not necessarily true even if |Xm(t)| < 1. Also it would be interesting to know how to recover the transmitted signal from the modulated carrier wave.

The integral is correct - it comes from the fact that FM is actually a special case of PM (phase modulation), and phase is the integral of frequency w.r.t. time. Also, I don't see why the time integral must be restricted to [-1,+1]. The only reason that the restriction | X_m(t) | < 1 exists is so that the frequency is restricted to . Oli Filth 09:58, 22 January 2007 (UTC)

Could someone expand the theory section significantly? I think it could use more math/physics background and a sub-section on the engineering of actually accomplishing frequency modulation. Ryanluck 15:55, 6 November 2006 (UTC)

[edit] How to demodulate FM ?

There is no explanation given of how the transmitted signal is recovered. That is cheating the reader after promising "The rest of this article...concentrates on the FM modulation [sic] and demodulation process." A solution could be instead to link to descriptions of the established FM detectors (Ratio, Foster-Seeley, Slope and PLL) in the article on Detector (radio).Cuddlyable3 17:54, 9 February 2007 (UTC)

Common methods used in FM receiver are slope detection and phase-locked loops.

The above is not true if by "common" we understand the majority of domestic radios.Cuddlyable3 19:06, 12 February 2007 (UTC)

That's what Carlson mentions. Please update the article if you have a more up-to-date reference. Alinja 21:16, 14 February 2007 (UTC)

Please help me by identifying what you mean by "Carlson". Until I check that, I like the solution I proposed i.e. a link to Detector (radio). BTW I have the impression that Slope detection of FM is too unwieldy for common use and is only kept mentioned in textbooks.Cuddlyable3 15:30, 15 February 2007 (UTC)

By Carlson I mean 'A. Bruce Carlson: "Communication systems, 2nd edition", McGraw-Hill, Inc, 1981'. A link to a more complete article about detectors is of course ok, but we want some kind of mention of how FM can be detected here anyway. Alinja 19:28, 15 February 2007 (UTC)

Thank you Alinja for that book reference. However FM signals having no amplitude variation are rather incompatible with AM detectors. The phenomenon called slope detection must have been an accidental discovery that AM radios nevertheless register some sound from FM transmissions. Slope detection is inferior in distortion, noise rejection and adjacent signal rejection compared to the established FM detectors and seems to be kept in textbooks only for its educational value.Cuddlyable3 00:48, 18 February 2007 (UTC)

I have tidied up the references to FM detection, relegating the explanation of actual detector circuits to Detector (radio).84.210.139.189 18:26, 9 March 2007 (UTC)

[edit] Assignments

Could this article include the assigned frequencies for the different uses? Such as 118.00 – 136.975 for Aviation, 108.000-117.975 Navigation, 151.5125- 158.400 BRS. I think a side bar would be best served for this purpose. —The preceding unsigned comment was added by 70.41.64.103 (talk) 00:50, 15 February 2007 (UTC).

There should be a linking with Radio frequency and Frequency allocation pages to avoid duplication. Probably Wikipedia should only link to the most detailed frequency allocations with their national variations, found on the web. I would like it kept clear that FM is a modulation type, and not intrinsically any frequency range.Cuddlyable3 15:42, 15 February 2007 (UTC)

...although FM broadcasting is better allocated to higher frequencies than those historically allocated to AM broadcasting, because of issues of coverage and bandwidth.Cuddlyable3 21:29, 11 March 2007 (UTC)