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Splatter

posted Jan 22, 2012, 7:41 PM by Charles Boling   [ updated Jan 22, 2012, 8:28 PM ]
(This is the 2nd, shorter version of this page; Google lost the draft when I was nearly complete the 1st time...)

Last week's post introduced amplitude modulation.  Let's see what the waveform looks like when we modulate to different degrees, e.g. by adjusting knobs (or otherwise changing a radio setting) or by speaking louder/softer or closer to / further from the microphone. 
 


Unmodulated Carrier







 Images from
http://www.cleanrf.com/applications.html#app_1
Visit that site for more information or to look at their test equipment.

 
50% Modulation
 
 
100% Modulation 
 
> 100% Modulation #1
 
 
> 100% Modulation #2
 
Note the two different styles of overmodulation; in one, it simply "hits bottom", but in the other, it actually "goes below the floor", resulting in a phase reversal.

As mentioned in the post on Fourier synthesis, overmodulation is a bad thing.  Not only does the resulting "splatter" sound bad, but it can cause you to transmit on other frequencies, and make others (including the FCC) very unhappy!

There are other areas of your transmitter that can introduce similar audio distortion without actually overmodulating the carrier; for example the audio processing circuit, or even the microphone itself (Just like you can only bounce a ball so high indoors, that little diaphragm in your mic can only move back & forth so far, no matter how loudly you yell at it!)  Here is a sample that was amplified 100x -- far more than the processing could handle:

 
 
 Original Signal
Listen
 Amplified 20dB
Listen


Ć
test1.wav
(176k)
Charles Boling,
Jan 22, 2012, 8:25 PM
Ć
test2.wav
(176k)
Charles Boling,
Jan 22, 2012, 8:25 PM
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