# What does AM/FM stand for? **URL:** \ **Category:** Factual Questions\ **Created:** [May 13, 2001, 1:03am UTC](https://boards.straightdope.com/t/what-does-am-fm-stand-for/65272 "2001-05-13T01:03:17Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![imthjckaz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/imthjckaz/32/3223_2.png) [@imthjckaz](https://boards.straightdope.com/u/imthjckaz)\ **Post date:** [May 13, 2001, 1:03am UTC](https://boards.straightdope.com/t/what-does-am-fm-stand-for/65272/1 "2001-05-13T01:03:17Z")
Forgive me if this has been covered before, because I couldn’t find it in a search. I was driving with my 10 year old son the other day and he asked me what the letters AM and FM stood for on the radio. After some thought, I had to admit that I did not know. Does anybody out there know? If you do, could you please enlighten me, and help me restore “My dad knows everything” status to my son. ---
**Author:** ![Zweistein](https://avatars.discourse-cdn.com/v4/letter/z/54ee81/32.png) [@Zweistein](https://boards.straightdope.com/u/Zweistein)\ **Post date:** [May 13, 2001, 1:11am UTC](https://boards.straightdope.com/t/what-does-am-fm-stand-for/65272/2 "2001-05-13T01:11:06Z")
AM: is Amplitude Modulation; FM is Frequency Modulation and this indicates how the audio signal is encoded (modulated) on the radio wave. ---
**Author:** ![Dr.Lao](https://avatars.discourse-cdn.com/v4/letter/d/2bfe46/32.png) [@Dr.Lao](https://boards.straightdope.com/u/Dr.Lao)\ **Post date:** [May 13, 2001, 1:16am UTC](https://boards.straightdope.com/t/what-does-am-fm-stand-for/65272/3 "2001-05-13T01:16:22Z")
AM = Amplitude Modulation FM = Frequency Modulation In order for you to recieve information through radio wave there must be some sort of variation in the wave that the reciever can recognize as information (sound or pictures). AM transmiters vary the amplitude of the carrier wave, FM ones vary the frequency. This is likely an oversimplification and others will be around shortly to correct my oversights. ---
**Author:** ![imthjckaz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/imthjckaz/32/3223_2.png) [@imthjckaz](https://boards.straightdope.com/u/imthjckaz)\ **Post date:** [May 13, 2001, 1:27am UTC](https://boards.straightdope.com/t/what-does-am-fm-stand-for/65272/4 "2001-05-13T01:27:23Z")
Yeah, thats what I thought.😉 I knew I could count on the Teeming Millions to fight my ignorance on this subject. Now all I have to do is explain it on a 10 year old level(but first I have to understand it on a 39 year old level). ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [May 13, 2001, 4:45am UTC](https://boards.straightdope.com/t/what-does-am-fm-stand-for/65272/5 "2001-05-13T04:45:12Z")
This may not help, at all, but here is the explanation provided by the [Encyclopædia Britannica on modulation](http://www.britannica.com/eb/article?eu=54493&tocid=4919). To steal a few lines from David Macauley’s wonderful book, **The Way Things Work** (and if you have a 10-year-old and not that book, shame on you): > [@](#): > > In AM radio, the waves are modulated so that the amplitude (energy level) of the carrier wave varies at the same frequency as the changing voltage in the sound signal. In FM radio, the waves are modulated so that that frequency of the carrier wave varies with the voltage level of the sound signal. ---
**Author:** ![TheLoadedDog](https://avatars.discourse-cdn.com/v4/letter/t/35a633/32.png) [@TheLoadedDog](https://boards.straightdope.com/u/TheLoadedDog)\ **Post date:** [May 13, 2001, 5:45am UTC](https://boards.straightdope.com/t/what-does-am-fm-stand-for/65272/6 "2001-05-13T05:45:06Z")
IIRC, an FM station must broadcast **two** signals. One is a base signal which does not vary. The other is a carrier signal which varies in frequency, and it is the difference between the two which the radio reciever detects as information. This means that FM can broadcast peroids of silence (eg. spaces between spoken words), but AM can’t. As far as I understand, during a silent moment on an AM programme, the station isn’t broadcasting _anything_ momentarily, so the reciever will detect background hiss. The FM signal on the other hand, broadcasts a momentarily identical base signal and carrier signal, effectively suppressing background noise. ---
**Author:** ![TheLoadedDog](https://avatars.discourse-cdn.com/v4/letter/t/35a633/32.png) [@TheLoadedDog](https://boards.straightdope.com/u/TheLoadedDog)\ **Post date:** [May 13, 2001, 5:48am UTC](https://boards.straightdope.com/t/what-does-am-fm-stand-for/65272/7 "2001-05-13T05:48:09Z")
Dang. “i before e and except…” My Bad. ---
**Author:** ![TheLoadedDog](https://avatars.discourse-cdn.com/v4/letter/t/35a633/32.png) [@TheLoadedDog](https://boards.straightdope.com/u/TheLoadedDog)\ **Post date:** [May 13, 2001, 5:49am UTC](https://boards.straightdope.com/t/what-does-am-fm-stand-for/65272/8 "2001-05-13T05:49:33Z")
“And”? Think I’ll just hit myself on the head now and be done with it. ---
**Author:** ![Tripler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripler/32/21807_2.png) [@Tripler](https://boards.straightdope.com/u/Tripler)\ **Post date:** [May 13, 2001, 7:40am UTC](https://boards.straightdope.com/t/what-does-am-fm-stand-for/65272/9 "2001-05-13T07:40:04Z")
> [@](#): > > \*Originally posted by TheLoadedDog \* > \*\* > IIRC, an FM station must broadcast **two** signals. One is a base signal which does not vary. The other is a carrier signal which varies in frequency, and it is the difference between the two which the radio reciever detects as information. This means that FM can broadcast peroids of silence (eg. spaces between spoken words), but AM can’t. > \*\* Close: The FM station has it’s Carrier Wave and Information Wave (I call it that for easy reference). But the two are multiplied together in a “mixer”. Slight slight variations in the frequency are measured and separated from the carrier wave and put out as what you hear coming from the radio. Keep in mind that when you dial 105.3 that’s **Mega** hertz. A human voice or musical instrument is down around a couple hundred Hertz or so. It doesn’t take much to make variations. To dumb things down a shade, imagine an ocean wave: AM makes the wave “crests” get taller and bigger according to the Information Wave. FM squishes and stretches the crests in and out. That’s what I got from my Communications class. Tripler I could be wrong tho, he was a crappy professor. ---
**Author:** ![holdenmagroin](https://avatars.discourse-cdn.com/v4/letter/h/5daacb/32.png) [@holdenmagroin](https://boards.straightdope.com/u/holdenmagroin)\ **Post date:** [May 13, 2001, 8:56am UTC](https://boards.straightdope.com/t/what-does-am-fm-stand-for/65272/10 "2001-05-13T08:56:33Z")
I recently heard about “XM”, a new digital frequency that is broadcast nationwide, maybe even world wide. High-end vehicles are putting it on their radios.