Talk:Sideband
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[edit] Where sidebands come from?
From the modulation process, yes, but I would like to have some details. I've look through the modulation page, but found no concreete explanation of the generation of those sidebands. In particular concerning the AM process, if it's a modulation based on the amplitude of the carrier wave, why do we have sideband?? which are wave of different frequencies if I'm right. Anyhow, I missing something here, so maybe the article shall be reviewed in some way.JeDi 14:15, 22 July 2006 (UTC)
- The argument should run something like, say, for amplitude modulation, essentially the modulation process is multiplying the carrier by the signal -you could illustrate this by working out what sin (w1t) * sin(w2t) would expand to, if, unlike me, you could find your trig tables or remember the identity - you should get something with terms that look like sin ((w1+w2)t) and sin((w1-w2)t) in the resulting expressing. For FM you get into Fourier transforms and Bessel functions...my signals course was decades ago. --Wtshymanski 02:01, 25 July 2006 (UTC)
"While all forms of modulation have sidebands by definition, ..." This statement should be backed up by a mathematical theorem of some sort. I'm not sure what he means by "by definition," but I'm guessing there's some proof out there like was mentioned in the comment above. If you need a different proof for each kind of modulation, then it's not "by definition." 72.79.151.184 23:00, 16 March 2007 (UTC)
Asking again:
I have exactly the same question that JeDi had. Where do the sidebands come from? Let's start with AM. It seems to me that modulating a single high-frequency carrier with lower-frequency source tone, for instance, seems to me to only change the amplitude of the carrier but not introduce additional frequencies. Imagine a .0001Hz (very low) source tone modulating an audible carrier tone (a pure F# note) such that the amplitude (volume) of the carrier F# is increased to double at the source's peek (and halved? at the valley). This really just amounts to cranking the volume on your speaker playing the tone. Does this mean that there are sidebands (additional frequencies) introduced merely by increasing the volume?
- Yes, that is exactly what it means. In this case (if you manage to modulate the F# tone with a monotonous 0.001Hz sine wave) the original (carrier) frequency (the F# tone) won't even be present in the new signal anymore; instead, the new signal will contain two other frequencies, namely F#-0.001Hz and F#+0.001Hz. Multi io (talk) 17:30, 22 April 2008 (UTC)
And if this is so - if AM sidebands are distortion based on increased amplitude - what physics principle is at work here? Is my analogy to audible tones from eletromagnetic waves accurate? Cheers, Jason —Preceding unsigned comment added by 68.96.79.221 (talk) 22:21, 23 September 2007 (UTC)
- It's just mathematics at heart. If you have your carrier frequency fc and your modulated frequency fm (0.001Hz in your case above) that modulates the carrier, the resulting signal Sr(t) will be just the product of those two signals, i.e. something like Sr(t)=sin(2*Pi*fc*t)*cos(2*Pi*fm*t). This is equal to 1/2*sin(2*Pi*(fc-fm)*t) + 1/2*sin(2*Pi*(fc+fm)*t) (use trigonometric addition and subtraction theorems to prove), i.e. Sr(t) is just the sum of two waves whose frequencies are fc-fm and fc+fm, i.e. they're arranged symmetrically around fc. In the more general case, the signal you want to carry will be a whole (infinite) sum of different (continuous) fm's (e.g. a voice signal), which will result in Sr(t) being a corresponding (infinite) sum of fc+fm and fc-fm waves for all those fm's, which would end up looking like the diagram in the article. In all cases, the symmetric arrangement around fc will be retained. Multi io (talk) 17:30, 22 April 2008 (UTC)
[edit] LSSB and USSB
LSB is sometimes called LSSB, and USB is sometimes called USSB.Dima373 (talk) 19:40, 23 February 2008 (UTC)
