# What is the basic difference between AM and FM radio?

I want to know the basic/fundamental difference between AM and FM radio. Why nowadays FM radio has replaced AM and has become more popular?

• Phase modulation is actually more popular then frequency modulation, but you are probably thinking of listening to music. I will bet money your phone uses a form of PSK. – Kortuk Sep 3 '12 at 22:27
• @Kortuk: As also does WLAN (802.11). – boardbite Sep 4 '12 at 9:26
• @boardbite - PM is often combined with AM as QAM (Quadrature Amplitude Modulation) to get 6 or even 8 bits per baud, which in pure PM would be nearly impossible. – stevenvh Sep 4 '12 at 9:40

Short answer: FM is far less susceptible to disturbance of the signal.

This is an AM modulated signal. The contours are the baseband signal which we recover by demodulation. Notice that there's a spike in the signal, which may be caused by a thunderstorm for instance.

This is the demodulated signal. The demodulator doesn't "know" that the spike isn't actual part of the signal, so it can't remove it, and the listener will hear a tick in the symphony she's listening to.

FM has a constant amplitude, and the demodulator won't be fooled by spikes in amplitude since it will detect variations in frequency.

This is an FM signal. The baseband signal determines the change in frequency of the carrier. Notice that the spike doesn't change the frequency, so it won't be audible after demodulation.

• @boardbite - Thanks! I don't know for sure, but I guess it's has to do with the simplicity of the AM modulator and demodulator (you can make a crystal radio receiver with 3 components). An insufficiently stable FM receiver sounds much worse than an equally unstable AM receiver. Maybe something like that. – stevenvh Sep 3 '12 at 19:51
• @stevenvh Indeed. Early AM radio hobbyists even made their own crystal diodes ('cat's whisker') - no need to buy an expensive valve (and no such thing as transistors in those days). – Bill Michell Sep 3 '12 at 22:08
• ooooooh, pictures.... – Kortuk Sep 3 '12 at 22:27
• I don't know about that, but in 1945 the FCC changed the FM band from 42-50 MHz to its current 88-108 MHz band, to make room for TV channel 1. This instantly rendered every existing FM radio obsolete. – dan04 Sep 4 '12 at 4:59
• @Gustavo - Yes, they will affect frequency/phase too, but far less than they affect amplitude. This graph is the same spike as the graph in my answer, compared with the undisturbed signal. While there's a 25 % error in amplitude the frequency error isn't noticeable. – stevenvh Sep 4 '12 at 7:42

AM radio is amplitude modulated, meaning that the amplitude of the carrier frequency is varying in the same manner as the audio signal you are transmitting.

FM radio is frequency modulated, meaning that the frequency of the carrier frequency is varying in the same manner as the audio signal you are transmitting.

Illustrative image:

• While your answer is correct it doesn't address OP's question (the sentence with the question mark). – Federico Russo Sep 4 '12 at 9:35
• +1 for cool animation, but a bit more substance in your answer would help. – Olin Lathrop Sep 4 '12 at 19:07
• Yes, it's an animated image, but I don't see the extra value of the animation. OP says he knows the fundamental differences between AM and FM, so this is not an answer. – radagast Dec 7 '13 at 12:13
• @radagast He says no such thing. he says he wants to know that. – Marquis of Lorne Apr 19 '15 at 1:05

A complementary question to:

Why nowadays FM radio has replaced AM and has become more popular?

Might be:

Why hasn't AM radio been replaced by FM yet?"

First, we'll infer from the question that we are talking about the broadcast range. I would like to simply add to the already excellent answers that broadcast AM radio (in the Americas) uses 540 to 1610 kHz whereas broadcast FM radio (in the same region) uses 88 to 108 MHz. These frequencies correspond inversely to wavelength; higher frequency has a smaller wavelength whereas lower frequency has a longer wavelength.

Where v = velocity, f = frequency, and λ = wavelength

One important property is that longer wavelengths have greater propagation. (Ignoring some other variables like atmospheric conditions, transmission power, antenna location and type, etc.) The lower frequency of the AM band gives it greater distance coverage than FM. One reason why broadcast AM radio continues to survive may be that the ability for listeners to tune in somewhat more distant stations gives it a unique quality property.

It's important to remember that amplitude modulation (AM) and frequency modulation (FM) are independent of frequency, so my answer is less about the modulation type and more about the frequency range of the broadcast bands you refer to.

• thanks for pointing out that low frequency travels a greater distance compared to high frequency – Silver Moon Aug 21 '14 at 13:04

FM Modulation is less sensitive to disturbances as stated in other answers, but this comes with the drawback of having a larger Bandwidth. To have an approximation of the FM bandwidth you should check the Carson Rule (the bandwith is theoretically infinite, but it can be constrained to a certain amount after which the values are no more significant).

AM radio the signal strength changes. on FM the frequency changes at the same rate as the audio rate. the advantages of FM less static do to to signal strength, so your ratio detector ignores the static allowing a static free reception on FM frequency's.

• Assuming you have a ratio detector. Most other FM detectors need limiters. – Marquis of Lorne Apr 19 '15 at 0:18
1. In FM signals, all the transmitted power can be used, but in AM wave the transmission carriers contain most of the power. So, complete use of power is not possible.

2. FM waves are waves having constant amplitude. These are independent of the modulation. So, due to this the power transmission of these waves is also constant. The power transmission of FM waves is better than that of the AM signals.

3. Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. frequency modulation (FM) conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant).

4. FM signals have a great advantage over AM signals. Both signals are susceptible to slight changes in amplitude. With an AM broadcast, these changes result in static. With an FM broadcast, slight changes in amplitude don't matter -- since the audio signal is conveyed through changes in frequency, the FM receiver can just ignore changes in amplitude. The result: no static at all.

1. AM broadcasting is simpler than FM but the difference in complexity and price are very marginal at present.
2. AM is more prone to signal distortion and degradation compared to FM.
3. FM doesn’t degrade linearly with distance.
4. AM usually broadcasts in mono which makes it sufficient for talk radio.
5. FM can transmit in stereo making it ideal for music.
6. AM has a longer range than FM.
• Item 6 is not technically correct. Modulation type does not determine range; frequency and wavelength do. – JYelton Jun 29 '17 at 16:07

Wavelength Difference AM waves work in the range of KHz as a result AM waves have a higher wavelength While FM waves work in the MHz so, these have a shorter wavelength. A higher wavelength increase in the range of AM signals while FM has a limited area of coverage.

• There's no reason why FM couldn't work at low frequencies, or AM at high frequencies. Your whole argument is false. – flup Dec 28 '13 at 9:46

actually in AM modulation the most of the power is wasted and s/n is less.but in fm almost the total power utiliezed to creat infinite no of side bands.and also sound quality less is in am because less bandwidth but due to more bandwidth sound quality is high in fm.