Talk:Amplitude modulation

Formulas

I am not sure, but as far as I remmber the formulas should be $c(t) = C \cos(\omega_c t)\,$ and $m(t) = M \cos(\omega_m t + \phi)\,$ so $\ X_{AM}(t)=[C + M(t)]\cos(\omega_c t)=[C + M \cos(\omega_m t + \phi)]\cos(\omega_c t)$, i.e., $\ X_{AM}(t)= C\cos(\omega_ct)+{M \over 2} [\cos(\phi +(\omega_m+\omega_c)t)+\cos(\phi +(\omega_m-\omega_c)t)]$, thus, $\ X_{AM}(t)=C[1+m_a\cos(\omega_mt)]\cos(\omega_ct)$ while $m_a= {M \over C}$. The first formulas, here in this article, include sines but only the last one include cosines. I scammed the other Wikipedias and I found cosines in every forgin Wikipedia I checked. Am I mistaken? --Shimonnaim 13:25, 31 March 2007 (UTC)

Image

The FM article uses Image:Frequency-modulation.png as a static image showing the waveform. It seems like it would wise to include the equivalent image for AM on this page: Image:Amplitude-modulation.png. Thoughts? Tacvek 18:20, 29 June 2007 (UTC)

'Telephone' example

I don't think that the telephone is a good example. We can't consider a baseband medium 'modulated', even with DC bias, IMO. A better example is needed. Or am I missing a historical AM telephone? --Ktims 07:28, 31 August 2007 (UTC)

I am inclined to agree. But I will play devil's advocate for a moment. Rather than looking at it as "DC bias" or "transmission medium", the implication is that it is a 0 Hz "carrier", with a measureable power level of its own. Without that power source, the telephone apparently doesn't work.

In contrast, consider acoustic transmission of the same voice signal through air. The air provides no energy. It is truly just a medium. The only measureable power comes from the information source itself.

But I agree that the distinction is unnecessarily subtle.

--Bob K 14:17, 31 August 2007 (UTC)

amplitude modulation index

I don't think the amplitude modulation index is correctly explained.

See http://www.rfcafe.com/references/electrical/amplitude_modulation.htm —Preceding unsigned comment added by 192.91.172.36 (talk) 02:08, 2 October 2007 (UTC)

I agree the calculation for the modulation index is obviously wrong because earlier it the article it is explained that A=0 is used for carrier suppression. This equation therefore implies that m = M/0 = infinity when the carrier is suppressed.

Kris —Preceding unsigned comment added by 203.97.235.82 (talk) 07:49, 8 February 2008 (UTC)

Merge?

It seems to me this article is very similar or identical in purpose and scope to the Modulation article. Could/should they be merged? --166.70.188.26 (talk) 17:43, 30 June 2008 (UTC)

They are not in any way similar in purpose or scope! Modulation is an introduction to "all" modulation schemes. This article covers the maths, theory and application of one specific modulation scheme. Oli Filth^(talk) 19:34, 30 June 2008 (UTC)

Fig.1 is wrong

The depiction of the FM signal is wrong in Fig.1 image (it does not follow the modulating signal). —Preceding unsigned comment added by Gyll (talk • contribs) 16:54, 6 August 2008 (UTC)

Looks ok to me. Oli Filth^{(talk|contribs)} 17:06, 6 August 2008 (UTC)

Too many animations

See discussion at Talk:Frequency modulation#Too many animations

The idea of Heterodyning

This process is known as heterodyning.

This is a small stub gleaned from the original article which suggests that the AM carrier and its two sidebands are merged in a process known as heterodyning. This is a false statement. The sidebands occur as a modulation product just as they do in FM. FM has theoretically infinite sidebands but are of little use other than consuming spectrum space than AM. Actually the process of heterodyning takes place in the radio receiver circuitry. The local oscillator frequency is fed to a circuit called a mixer along with the received signal from the antenna after a preamplifier. This "mixing" of signals is known as heterodyning and produces sums and differences of the received signal and the signal from the local oscillator. Heterodyning has nothing to do with an AM carrier and its adjacent sidebands in a pure sense since the sidebands, carrier or both can be heterodyned with a local oscillator frequency then fed to an IF filter and once again mixed and then finally output to an amplifier for audio with everything except the modulating signal removed. The final ouput from an AM superheterodyne receiver has to be absolutely linear in that the received signal has to be exactly reproduced at the output. FM receivers are much more forgiving since only the frequency has to be preserved, so sloppy, non linear circuits can be used. --Skywalker45 (talk) 19:15, 11 November 2010 (UTC)

Heterodyne, modulate, and mix, are three words that mean exactly the same thing. We generally don't use heterodyne when we talk about modulators, but we are heterodyning. We take an audio frequency signal and a radio frequency signal, put them together, and end up with the original signals and the sum and difference of the two signals. The sidebands are the sum and the difference. When I read the article, the word heterodyne startled me, but it is technically correct. wa6bjh (talk) 20:57, 24 December 2010 (UTC)

The meanings overlap, but they're not exactly the same. Mixing is probably most general, as it means nonlinearly combining two signals (in audio it means linearly, but that's different). AM modulating is bilinear mixing a baseband signal with a carrier oscillator signal. Heterodyning is mixing an RF signal with a local oscillator signal of a frequency different from the RF carrier frequency. Homodyning is mixing an RF signal with a local oscillator signal of same frequency as the RF. Dicklyon (talk) 23:08, 24 December 2010 (UTC)

"When multiplication of two signals takes place, as opposed to their simple addition, mixing is involved. The result is multiple signals, including the sum and difference of the AF and RF frequencies. These two "products" will appear as sidebands alongside what was the original RF frequency. Mixing, modulation, detection, demodulation, and heterodyning all refer to this multiplication process and can all be analyzed by the same mathematical treatment." American Radio Relay League, Handbook for Radio Communications, 2008, p. 9.26. wa6bjh (talk) 23:54, 2 January 2011 (UTC)

Are you saying we should ignore subtle differences in meaning, just because this one sourced sentence does? Dicklyon (talk) 02:51, 3 January 2011 (UTC)

This discussion started because someone wrote that "heterodyning has nothing to do with an AM carrier and its adjacent sidebands..." I pointed out that all of those things are the same, and they are. It's important to understand that all of those things are the same, regardless of what you call them. I don't like heterodyning for the AM modulation process, and I wouldn't use the word, but I wouldn't change the article, because it's technically correct. In the 1950s, a single diode mixer was common in VHF receivers. I owned one in the 70s. The strong local oscillator (heterodyne oscillator) and the weaker VHF signal were both applied to the cathode of a diode. The filter after the diode selected the signal you wanted for the IF. We call this circuit a mixer. But when we take a diode in a crystal radio and we apply a strong carrier and weaker sidebands to the cathode, and select the resulting audio signal, we call this a detector or a demodulator. But it does the same thing as a the VHF mixer, except that we select an AF signal instead of an RF signal. The process is the same regardless of what we call it. In the 1920s people spoke of condensers and capacity, but now we say capacitor and capacitance. People may have once used homodyne, but the term now is direct conversion. They mean the same thing. There are some problems with this article and the modulation article, and we need to fix them so people understand the process.wa6bjh (talk) 01:36, 5 January 2011 (UTC)

What it said was "In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands. This process is known as heterodyning." I don't believe that this process of modulation has ever been called heterodyning. Do you know of a source where it is? Dicklyon (talk) 02:20, 5 January 2011 (UTC)

You're correct, modulation usually isn't called heterodyning. And I must admit that I've never seen that anywhere. But I have seen this: When describing what we would call heterodyning--mixing an RF signal with a local oscillator to produce an IF--the author called the products upper and lower sidebands instead of the usual sum and difference frequencies. I think it was in Experimental Methods of RF Design, but I'll look. I don't like heterodyne for modulation and I don't like upper and lower sidebands for sum and difference frequencies in a mixer. But they're really all the same. On a slightly different point, I prefer a modulation description that talks only about sidebands, because then you're well on your way to understanding single sideband. I don't like the time domain thing that's in figure 1, because that's not what's happening. wa6bjh (talk) 02:12, 7 January 2011 (UTC)

Why don't you like the time-domain view? It looks correct to me. And what's wrong with calling the products upper and lower sidebands, when they are not discrete frequencies? Or do you mean that at the mixer to IF, both sidebands get translated via sum and difference to two complete two-sideband images? Actually, there's nothing there about an mixer to IF, so I guess it's moot. Dicklyon (talk) 03:51, 7 January 2011 (UTC)

Spectrum section

While I'm no mathematician (which is putting it mildly), I know a little about radio—which I presume this article is trying to address, although it has far too many formulas IMO—and can't get my head around this section. The assertion that each sideband has the same carrier-plus-sidebands as the "fundamental" frequency confuses this concept with DSB-AM and its variants and since the whole negative-frequency thing is speculative to begin with, it just muddies the waters.--Miniapolis (talk) 20:22, 26 October 2011 (UTC)