Understanding Amplitude Modulation (AM)

[sphyics]

i have got some doudts regarding AM (amplitude modulation)
1) the amplitude of carrier wave is changed according to the signal --- defn of modulation
wats the amplitude of modulated wave is it A+B or other ... if "A" is the amplitude of carrier & "B" is the amplitude of signal
...



to be ctnd after the reply as my doubt is regarding modulating factor

 

[cepheid]

If the carrier wave is C(t) and the signal is A(t), then the amplitude modulated signal is given by

f(t) = A(t)C(t) (the carrier wave is simply multiplied by the signal).

Example:

C(t) = \sin(2\pi \nu t)

then

f(t) = A(t)\sin(2 \pi \nu t)

Another way to think of this is as follows: the standard form for a sinusoidal signal of amplitude A is (ignoring arbitrary initial phase factors for simplicity):

f(t) = A\sin(2 \pi \nu t)

Now imagine if, instead of that amplitude being fixed, it is time-varying (A(t)). Then we have:

f(t) = A(t)\sin(2 \pi \nu t)

An amplitude-modulated signal is just a sinusoid with a time-varying amplitude, where the variation in time is such that the modulating signal contains information.

 

[sphyics]

An amplitude-modulated signal is just a sinusoid with a time-varying amplitude, where the variation in time is such that the modulating signal contains information.

agreed,
i mean, what would be the maximum amplitude of the modulated signal.??
the modulating factor which is defined as ratio of maximum change of amplitude of a carrier wave to amplitude of unmodulated carrier wave.
if signal amplitude is equal to carrier amplitude then modulating factor is _______*
*my book suggested that it is 1, but i am not able to get into a conclusion (see how modulating factor is defined ) ------> the maximum change

 

[NoTime]

*my book suggested that it is 1

Yes, it is 1 or 100%.
If you modulate >1, then you can not recover the original signal.

 

[DefaultName]

Ideally, you want the abs( m(t) ) <= 1, otherwise you're overmodulating the signal. http://en.wikipedia.org/wiki/Overmodulation

 

[sphyics]

i was jus trying to work with in the limits of formulae for modulating factor.

agrred with u all that if m>1 (overmodulated), m=1(perfect modulation), m<1 (under modulated)

( if new modulated amplitude is "X"; "Y" amplitude of unmodulated carrier wave)
the change is X-Y then modulating factor is (X-Y)/Y)...
i jus want to to know what is the amplitude of modulated signal...then i can take the difference between and work out...

 

[zeitghost]

If 100% modulated, the minimum value of the carrier is 0% so the maximum is 200%.

Off topic, but to get a more punchy sound, rock stations overmodulate the positive peaks... you can't over modulate the negative peaks coz it cuts off the transmitter...

 

[NoTime]

Off topic, but to get a more punchy sound, rock stations overmodulate the positive peaks... you can't over modulate the negative peaks coz it cuts off the transmitter...

I think the FCC would come knocking on their door if they did that.
What they can do (especially for commercials) is to process the input so that the average modulation is higher.

 

[zeitghost]

Same thing.

 

[NoTime]

Same thing.

No it is not.
Simple clipping, either by directly over modulating or even just clipping audio signal peaks causes spatter interference (or out of band operation).

Here is some reading for you.
http://www.nrscstandards.org/SG%20ref%20docs/AM_Mod_Overmod_1986.pdf

 

[zeitghost]

This is what I was referring to...

This is where "Asymmetrical Peak Limiting" built into modern AM processing comes in handy! Again, the average male voice is asymmetric in nature. This means that when examined on an audio spectral analyzer, the amplitude peaks will be slightly higher on one side of the plot as opposed to the other. This is why proper "Phasing" of the audio is critical! We always want the positive peaks to be higher than the negative peaks in amplitude since the negative peaks cause distortion when modulated above 100% at the transmitter. There are devices out there called "Phase Rotators" and "Phase Scramblers" that keep the the voice phase positive at all times. However, these devices are not really needed if just one mic and one voice is used. Once the correct phase is established, it will rarely ever change with just one voice and microphone.

With modern AM processing where built in asymmetrical peak limiting is employed, we can manage the negative peaks at about 95% while letting the positive peaks travel well above 100% while still satisfying the transmitter with clean output. It's like having your cake and eating it too! Of course, like anything else, there are some purity issues and tradeoff when using real aggressive peak-limiting. There will always be a fine balance between low distortion and aggressive peak limiting. The trick is finding the balance that will satisfy both sides of the equation - Loudness vs. Cleanness. A little bit of peak limiting will go a long way, so don't get too carried away!

See the Inovonics Model 222 Data Sheet by Clicking Here

For those on a tight budget like me, I personally like the Inovonics Model 222 AM processor. This AM processor has a pre-emphasis network, a low-pass filter (either at 9.7kHz or 5kHz), a tight asymmetrical peak limiter, and filter overshoot compensation. Also, the "Positive Peaks" control can be adjusted for a maximum of 130% positive peak modulation relative to the negative peaks! All features are bypass switchable You can find the 222 at Broadcast Supply Worldwide (BSW) for around $550.00.

HTH.

 

[NoTime]

Hmmm, the net effect of signal processing like this would seem to be equivalent to increasing the voltage to your final (barring the inherent distortion induced into the original signal).
Calling this over modulation is at best suspect.
It does not quite conform to the definition of the term.

 

[zeitghost]

It seems weird to me too, but hey! it's LOUDER so it must be good... )

Or something...

 

[NoTime]

If you can't be good, be loud.