Talk:Amplitude modulation

Re lede sentence

Pat, re "In electronics and radio engineering, amplitude modulation (AM) is a method of injecting information onto an oscillating analog signal, such as a radio wave, by changing the height (amplitude) of the peaks of the periodic waveform.":

I'm trying to learn: Is the signal an 'analog' signal before the data is injected onto it, as the sentence seems to imply, or does it 'become' an 'analog' signal because the signal is carrying the data (in the way it does)?

I may not know what I'm talking about, as I'm way out of my subject area. I'm prepared to be embarrassed. Anthony.Sebastian 22:55, 15 May 2011 (CDT)

It is analog both before and after mixing the signals. When a carrier frequency C is mixed with an injected information frequency D, four new frequencies are produced: C, D, C+D, and C-D.

Admittedly, this gets more confusing when the periodic waveform is either a square wave, or bursts of square waves, sometimes called pulse amplitude modulation--a technique used in radar. Howard C. Berkowitz 23:02, 15 May 2011 (CDT)

Great, now I really need a bunch of diagrams.

What makes the carrier wave 'analog'.? Anthony.Sebastian 09:27, 16 May 2011 (CDT)

I'll try to draw some if I get time today, and can figure out a reasonable approximation of a sine wave in PowerPoint.

Think of analog as a signal where there is a theoretically infinite range of values. If, for example, you put a voltmeter or oscilloscope on a microphone, and talk into the microphone, the resulting voltage will vary continuously.

If we converted that to digital, we'd approximate the voltage level every so many milliseconds, and convert that to a (typically) 8-bit value. We can then send the speech through the Internet, which can't directly handle analog...it all comes down to one and zero bits. Howard C. Berkowitz 09:52, 16 May 2011 (CDT)

Followup question for diagrams--are you comfortable with standard (time domain) oscilloscope views? Ever used a frequency domain analyzer? Generally familiar with Fourier transforms? (sigh) of course this is all engineering stuff, for which I have, according to some, inadequate credentials. Howard C. Berkowitz 09:54, 16 May 2011 (CDT)

Natural sound is an oscillating wave. In analogue systems, electromagnetic waves (or varying voltages) are used to produce waves which are analogous to the waves they are trying to describe. Digital systems, on the other hand, produce a series of on/off values (bits) which bear little resemblance to the original continuous wave. The interpretation of those values is up to the receiver, and is often not obvious.

The main advantage of digital signals is that so long as the signal is intact (on and off can still be distinguished), full quality may be retained. The quality of an analogue signal will degrade as noise is introduced. Johan A. Förberg 11:01, 16 May 2011 (CDT)

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