Modulation - what is it?

There are two ways to explain amplitude modulation or AM.  The correct way and the one that is easy to understand and makes sense to the layman.

For starters, the carrier or continuous AC wave produced by an AM transmitter ideally is a pure sine wave.  Its strength depends on the power applied and the efficiency of the device.

Radio frequency power can travel large distances without wires.  Audio frequencies can not.  The radio frequency power is used to transport or carry the audio without wires connecting the origination point to the end point.

When you add your program audio to the radio frequency carrier, the loudness of the audio causes the amplitude of the carrier to change in step with the audio.  The audio is impressed on the carrier or modulates the carrier.

The audio is an AC signal.  The voltage of the audio alternately is positive and negative.  That causes the carrier to alternately increase and decrease at the audio rate.  The extra power is the audio.  If the carrier is 100 mW the audio needed for 100% modulation would be 50 mW.

The maximum the carrier can decrease is 100%.  If the audio is too loud the carrier can't go beyond 100% in the negative direction as the carrier would shut off at that point.  Trying to go beyond 100% in the negative direction creates RF interference over a wide range of frequencies and must be avoided.

Depending upon how the audio is impressed on the carrier, the carrier can increase more than 100% in the positive direction.  This does not have the same effect as more than 100% negative modulation because the carrier does not shut off it simply keeps getting larger.

When the carrier is modulated more than 100% in the positive direction this is called asymetrical modulation.  Technically, this is distortion but the end result is the received signal sounds louder.  Special modulators allow the positive going signal beyond 100% but limit the negative going signal to no more than 100%.

The other way to explain amplitude modulation is when the audio is mixed with the carrier, new frequencies are generated.  Among others you end up with the carrier frequency, carrier frequency plus audio frequency and carrier frequency minus audio frequency.  The new plus and minus frequencies are called sidebands.  When these three frequencies are mixed together the algebraic sum of the instantaneous voltages produces the modulated output signal.  A lot less words but a little more difficult for some to grasp.

The frequency difference between the plus and minus sidebands is the bandwidth of your signal.  Another words, the bandwidth of your signal is two times the highest audio frequency applied.  The AM broadcast band channel is 10 kHz so the maximum audio frequency should be 5 kHz but that seems to have been relaxed over the years.

OK, that's my stab at it.  Rip it up...

Here is a LINK TO A GREAT ARTICLE which explains the process.

Nice writeup, Bob, and it raises another question about 125% modulation. A calculation shows a 2 dB increase in peak power (which I assume is a measure of loudness in AM since the demodulated audio would also be up 2 dB) over that of 100%. It is usually agreed that a 1 dB increase is the minimum which a human perceives as "louder" and 10 dB is perceived as twice as loud.

From this link : http://www.sengpielaudio.com/calculator-levelchange.htm the closest tabulated dB value in the "loudness" chart is 3 dB which is perceived as 1.23 times as loud. This would suggest that a 2 dB increase would be less than this loudness increase.

So, the question is "Is asymmetrical modulation worth while?" Have you have ever done a side by side comparison and, if so, what did you think?

Neil

Years ago, AM broadcast stations began using audio processors that permitted at least 125% positive peak modulation (if the transmitter was capable of it).  That helped to extend their coverage range at the fringes of their useful daytime groundwave signals.

They also began to use other audio processor settings that greatly increased the density, or average value of the audio waveform prior to modulation of the transmitter, while keeping positive and negative peaks of the audio waveform under tight control.  Carried to an extreme, transmitter modulation rarely drops below about 125% positive, 99% negative (the present FCC limit on positive peaks for AM broadcast stations is 125%).

This greater density in the audio waveform actually has a much larger benefit than using +125% modulation toward improving the signal-to-noise ratio that a listener perceives in the audio output of an AM receiver.  But it adds distortion to the program waveform, and can be tiresome to listen to for long periods of time, for some people.

Also - most people adjust the volume control of an AM radio receiver to a setting producing the acoustic sound level they want in that listening environment (car, living room, etc).  This tends to reduce the effects of using highly processed audio at the transmitter in areas of high field strength from the AM station, except maybe to increase the "tune-out" factor.  More benefit could be possible in the coverage area fringe due to noticeably better signal-to-noise ratio there at a given setting of the receiver volume control.

I just installed a new Optimod 9300 at the commercial AM I work for.

They poopoo the concept of assymetrical modulation.  Go to section 1-15 of this manual for their theory.

http://www.orban.com/products/manuals/9300_1.0.5_Operating_Manual_rev_02.pdf

All I know is between the new transmitter and processor our signal, both daytime and night time is clearly heard further.  Although we're putting the same current into the antenna the ability to modulate to 125% with greater audio density has clearly increased our coverage. 

Kind of like my Part 15.  Dropping just one click on the audio feed clearly makes a difference as to how far I can be heard. 

Tim in Bovey

Whats this bit mean? "Dropping just one click on the audio feed clearly makes a difference as to how far I can be heard."

Tim,

Antenna current increases with amplitude modulation, actually.  For +/-100% modulation by a sine wave, antenna current increases to 122.5% of the unmodulated value.

I'd bet that your AM broadcast station will show more increase than 22.5% though, due to the higher density of the waveform produced by the Orban 9300.  So that system actually radiates more average power in the sidebands than otherwise -- which is the reason for the increased coverage.

On top of all the boosting techniques described by MRAM, Radio8Z, Rich and Tim, I would add that the AM talk personalites including Rush, Sean Hannity, Ann Caulter and more, all shout loudly so as to over-talk anyone or anything that's going on. You can hear them on the fringe of the fringe.

Ah, from the perspective as someone who isn't me "dropping one click" really doesn't mean much, does it!  OK, My audio goes right from the computer to the transmitter.  Withint the first couple days on the air I determined where to set the output volume on the computer for loudest audio on the air, with no discernable distortion. On an imac, when you tap the volume up and down keys a little level bar pops up on the screen showing a line of rectangles indicating your volume level ---- or ----- or ------- etc. If I bump it down one "click" or notch as it were I can notice a slight decrease in my coverage. Dropping 2 or three clicks can make quite a difference.

Clearly demonstrating maximum modulation = maximum range. 

Tim in Bovey

I've used an Inovonics 222 combined with a Rangemaster to achieve asymetrical modulation, and it did make a difference.  My signal was listenable (according to my definition) further than without it.  And I could hear the actual signal in the fringe areas much further (not that I would want to listen to it for long periods of time).  I could even hear the signal (barely) on a Sony ghetto blaster about 3/4 of a mile away).  Was it worth it?  Well, when you're dealing with a maximum listenable range of about a mile (and practically in urban areas, a lot less than that), even several hundred feet makes a difference.

The previous explanations of "modulation" are fine if you want technical. But allow me to demonstrate another way of saying this:

Stand up. Sing a sustained mid-range note by saying "AAAAAAAAAAAAAAHHHHH!!"

While singing the single tone of "AAAAHHH!!!" start thumping on your chest like a jungle gorilla. "thump, thump, thump, thump, thump."

Notice how the "AAAAAHHHH!" gets shifted and sounds like "AAAuuh AAAAeee AAAuuh AAAeee!" You are modulating yourself.

The sustained "aaahh" is the carrier. The thump thump is the modulation.

Try not to cause your heart to stop beating.

You can bypass the chest thumping by simply singing vibrato, a form of self-modulation without physical abuse.

I like the way you think Blare.

Artisan, I agree accomplishing an additional few hundered foot of range makes a difference, but I look at it like this; I'm assuming the 222 had cost you what at least $500-$700 for a used unit...??

I have always wanted one, but could never bring myself to spend that much, cause to me, it made more sense to buy another transmitter instead. Sure, then there's a couple hoops to jump through to get an audio feed to a remote transmitter, but the result is a doubling of range opposed to a few hundred feet, for about the same cost (if not less) of a 222.

I say perhaps less becuase it's not all that uncommon to pick up a used Rangemaster for $300-$400. The cheapest Inovonics 222 I've ever seen sell was for about $450, and thats rare.

That's the very reason I never bit the bullet to aquire a 222 unit.

By the way Bob.. No, Mram.. I'm going to continue calling you Mram unless you prefer not, I'm just more used to it.

By the way Mram, that is the best description of amplitude modulation I've ever read.. I comprehend it now much clearer than previously.

And Carl, yours was the second best.