Talk:Amplitude modulation

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[edit] 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)

[edit] 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)

[edit] '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)

[edit] 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)