Modulation question

[~T]

I'm left confused by a number of different sources that suggest different things about modulation...

There are images such as this...
http://www.sfu.ca/sonic-studio/handbook/Graphics/Amplitude_Modulation2.gif
... which suggest that the modulated signal is ~0 when the modulating signal is in the middle (presumably ~0)

And there there are images such as this...
http://topperchoice.com/wp-content/uploads/2013/03/Introduction-To-Amplitude-Modulation-.jpg
... which suggest that the modulated signal is ~0 when the modulating signal has a trough, below zero.



I'm quite confident that the first image is correct and the second isn't. But I'm not entirely sure, and I think that my contention is based purely off what makes sense in my head. So firstly, which of the above is correct? And secondly, what actually happens with the modulating and carrier signals? Are they added together? Multiplied? Something else?

Thanks in advance,
Tim

[LazyZombie]

I would like to know which image is correct too.

As far as what actually happens with the modulating and carrier wavesyou're right i should use signal and wave interchangeable but i really didn't think that carefully before typing this answer :/, its addition of ordinates or sine waves or something (in methods/spesh? not sure) but its not in the study design and apparently complicated.
(I asked my teacher a similar question. After saying 'its complicated' she ranted about vce physics for a bit)

[Kiro Kompiro]

The first is correct.  But the terminology "carrier signal" and "modulating signal" can be confusing because of the word "signal".  I prefer carrier "wave" and signal "wave".  The signal "wave" is the "message" thats  being sent, the "carrier wave" carries that message.

There are 2 types of modulation:

1. Frequency modulation FM

http://www.m2m.ecs.soton.ac.uk/Wcc5e5a23ddd1e.htm

For FM modulation, the frequency of the carrier wave is varied by the signal wave

Here the amplitude of the carrier wave is NOT changed by the signal wave.  Draw the signal wave over the top of the carrier wave.  Basically the frequency of the carrier wave inside the "negative" part of the signal wave is increased, and decreased in the "positive" part of the signal wave.

2. Amplitude modulation AM

http://www.m2m.ecs.soton.ac.uk/Wc63e59ed037ae.htm

For AM: draw two modulating sine waves, one the with the negative amplitude of the other.  These two waves form the "envelope".  The carrier wave fits inside the envelope ie the regions bounded by the two signal sine waves, and keeps its frequency.  Therefore its maximums will be the maximums of the modulating waves, its minimum will be the intersections of the two modulating waves.

[~T]

Sweet, thank you!