hai pl explain what is FM Radio and how it works.?

Answer 1

FM broadcasting is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation (FM) to provide high-fidelity sound over broadcast radio

Broadcast bands
The original FM broadcast band in the United States until 1946 was on 42 to 50 MHz with 0.2 MHz channel spacing. This band was abandoned after World War II and is now allocated to fixed, mobile, and land mobile radio services.

The term "FM band" can upset purists, because it conflates a modulation scheme with a range of frequencies. It is effectively shorthand for 'frequency band in which FM is used for broadcasting'. The exact range of frequencies used varies around the world, but always falls within the VHF part of the radio spectrum. The term "VHF" was previously in common use for "FM" within the EU. ("UKW," which stands for "Ultrakurzwellen" in German, means "ultra short wave" in that language, is still widely used in Germany.)


[edit] Broadcast bands around the world
Throughout the world, the broadcast band is 87.5 to 108.0 MHz, or some portion thereof. In the U.S. it is 87.8 to 108.0 MHz. Japan is the only exception, using the 76 to 90 MHz band with 0.1 MHz channel spacing.

In the former Soviet republics, and some Eastern Bloc nations, an additional older band from 65.9 to 74 MHz is also used. Assigned frequencies are at intervals of 30 kHz. This band, sometimes referred to as the OIRT band, is slowly being phased out in many countries.

The frequency of an FM broadcast station (more strictly its assigned nominal centre frequency) is usually an exact multiple of 100 kHz. In most of the Americas and the Caribbean, only odd multiples are used. In some parts of Europe, Greenland and Africa, only even multiples are used. In Italy, "half-channel" multiples of 50 kHz are used. There are other unusual and obsolete standards in some countries, including 0.001, 0.01, 0.03, 0.074, and 0.3 MHz.

For more information on FM frequency allocations, see FM broadcast band.


[edit] Technical characteristics

[edit] Pre-emphasis and de-emphasis
Random noise has a 'triangular' spectral distribution in an FM system, with the effect that noise occurs predominantly at the highest frequencies within the baseband. This can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver. Reducing the high frequencies in the receiver also reduces the high-frequency noise. These processes of boosting and then reducing certain frequencies are known as pre-emphasis and de-emphasis respectively.

The amount of pre-emphasis and de-emphasis used is defined by the time constant of a simple RC filter circuit. In most of the world a 50 µs time constant is used. In North America, 75 µs is used. This applies to both mono and stereo transmissions and to baseband audio (not the subcarriers).

The amount of pre-emphasis that can be applied is limited by the fact that many forms of contemporary music contain more high-frequency energy than the musical styles which prevailed at the birth of FM broadcasting. They cannot be pre-emphasized as much because it would cause excessive deviation of the FM carrier. (Systems more modern than FM broadcasting tend to use either program-dependent variable pre-emphasis; e.g. dbx in the BTSC TV sound system or none at all.)

The problems with pre-emphasis due to the high frequency energy in modern CD music can be greatly attenuated using psychoacoustics principles, as Oscar Bonello demonstrates at his March 2007 AES paper. A new device named IM cancelled high frequency clipper is able to give a heavy audio clipping at high audio frequencies with low ear annoying. [1] [2]


[edit] FM stereo
In the early 1960s, several systems to add stereo to FM radio were considered by the FCC, including one submitted by E. H. Armstrong, the inventor of FM, which avoided many of the problems that have haunted the Zenith-GE pilot tone multiplex system. The Armstrong system was rejected by the FCC because it did not allow sub-carrier services, and the Zenith system has gone on to become the standard method in most countries.

It is important that stereo broadcasts should be compatible with mono receivers. For this reason, the left (L) and right (R) channels are matrixed into sum (M) and difference (S) signals, i.e. M = (L+R)/2 and S = (L−R)/2. A mono receiver will use just the M signal. A stereo receiver will matrix the M and S signals to recover L and R: L = M+S and R = M−S.

The M signal is transmitted as baseband audio in the range 30 Hz to 15 kHz. The S signal is amplitude-modulated onto a 38 kHz suppressed carrier to produce a double-sideband suppressed carrier (DSBSC) signal in the range 23 to 53 kHz.

A 19 kHz pilot tone, at exactly half the 38 kHz subcarrier frequency and with a precisely defined phase relationship to it, is also generated. This is transmitted at 8-10% of overall modulation level and used by the receiver to regenerate the 38 kHz subcarrier with the correct phase.

The final multiplex signal from the stereo generator is the sum of the baseband mono audio (M), the pilot tone, and the DSBSC subcarrier. This multiplex, along with any other subcarriers, modulates the FM transmitter.

Converting the multiplex signal back to left and right is performed by a stereo decoder, which is built into stereo receivers.

In order to preserve stereo separation, it is normal practice to apply pre-emphasis to the left and right channels before matrixing, and to apply de-emphasis at the receiver after matrixing.

Stereo FM signals are far more susceptible to noise and multipath distortion than mono FM signals. This is due to several factors, including the following:

the addition of the two sidebands of the difference subcarrier to the baseband signal increases the noise bandwidth of the signal by a factor of three (9.5 dB) as compared with a mono signal.
as mentioned above, the pre-emphasis is applied to the audio signals before encoding. This results in the pre-emphasis acting in the wrong direction on the lower sideband of the difference subcarrier, i.e. decreasing the level as the frequency rises, which will have a further deleterious effect on the S/N of the difference signal.
For this reason many FM stereo receivers include a stereo/mono switch to allow listening in mono when reception conditions are less than ideal, and most car radios are arranged to reduce the separation as the S/N ratio worsens, eventually going to mono whilst still indicating by the pilot light that a stereo signal is being received.

In addition, the reception of vertically- and horizontally-polarised signals at different phase relationships from the same transmitter site will further corrupt stereo reception and invoke an earlier resolution within the receiver to mono presentation.

A short lived quadraphonic version of the Zenith-GE system used an additional subcarrier at 76 kHz.

Answer 2

The first answer you received has a URL that may shed some light on your question. I'll try and answer your question as non-technically as possible. In either an AM or FM transmitter, if there is no sound being transmitted, you have a signal from each which is transmitting on only one frequency. You couldn't tell which is AM or which is FM. If you were to play a single note into each transmitter (say A-440, a standard note in music), a big difference would appear. The AM signal will get stronger and weaker 440 times each second. How much stronger and weaker is dependant upon how loud the volume is. This is called modulation. AM stands for amplitude modulation and the amplitude (strength of the signal) varies along with the note (or music). With FM, that signal does not get stronger and weaker; instead, the darn *frequency* actually changes. Up and down with the note. How much the frequency changes is dependent upon how loud the volume is (and hence, *frequency* modulation). The two signals are decoded very differently, but the crux of the situation is that AM signals vary in *amplitude* and FM signals vary in frequency. AM is pretty much standardized. 100% modulation is defined as when the amplitude of the signal just reaches zero when maximum audio is applied. Apply more audio and you have severe distortion. With FM, it isn't quite as simple. More audio, more deviation. The problem here is that eventually the frequency change (deviation) of the signal is too much for the decoding process at the receiver and you get distortion. There are various FM services, each with different definitions as to what defines 100% modulation. This is set by the Federal Communications Commission in the United States and this allows proper communication within each service. Hope this helps ;) Best regards, Jim

Answer 3

I think your question deserves a better answer. A less technical answer. Oh, OK, technically, FM is a bandwidth. Stations transmit a signal on a frequency from around 88 to 108Khz and your device (a radio) picks it up. It's limited to "line-of-sight" meaning buildings, mountains etc. will get in the way and so its reach is limited. But the quality is great. That's FM. Frequency Modulation. Now that wasn't so hard - though still a bit technical.

But in the more Karmic sense: What FM really is - is the band that saved radio. Starting around 1970, just as today, people were becoming bored with radio, which was predominately on the AM band. (FM existed but was reserved mostly for non-commercial uses).

The music was stale and formatted. You would actually hear "Sugar, Sugar" by The Archies followed by Simon & Garfunkel and into The Partridge Family, The Carpenters, James Brown, Led Zeppelin... I know, hard to believe, but that was Top-40 back then; with a Glen Campbell and occasional Merle Haggard thrown in. The DJs were too talkative - and there were way too many commercials to deal with. Hard to imagine with today's very narrow niche-oriented stations. Well, except for the commercials part. Greedy buggers then - greedy buggers today.

But it had been the only game in town. The AM band has a more powerful signal, but the quality is junk - and there was no stereo available at that point. Then, stereo eight-tracks and cassettes and reel-to-reel recorders became inexpensively available and radio began to lose listeners, just as it is today with the advent of iPods, Internet and Satellite.

People lost interest in AM radio. Stations had fewer advertisers, things were going downhill fast.

Guys were coming home from Vietnam with incredible systems bought at ridiculously low prices while on R&R in Hong Kong or from their PX. Then the Japanese, who have always known a good thing when they heard it, started sending these systems, equipped with AM AND FM dials into the US. And the music was beyond Hi-Fi, it was STEREO. And not available on the AM band.

Suddenly the geniuses who owned the stations realized the public wanted better fidelity. These station owners thought to themselves, "Well we've got this FM signal we only use for elevator music (technically called The Schulke Format, invented by Jim Schulke), let's give it a try."

With all of this could come a re-invention of the medium. Marketing, branding, salvation!

Thus it started, originally with "progressive radio" (album cuts). And all of a sudden listeners could listen to the entire Allman Brothers "Live" Album, in stereo, on big ol' Pioneer CS-88 speakers from an Army PX. It was heaven in a box.

Listeners started buying tuners that would pick up FM signals. AM stations went south, fast - but eventually came back with talk and sports. Advertisers started buying commercials again.

Owners smiled, advertisers smiled, listeners smiled. Then came the Internet; a bunch of investors got together to launch a couple of satellites and beam music down to earth, Scottie; and Steve Jobs thought of a way to save his company.

But that's a story for another day.

-a guy named duh

Answer 4

http://en.wikipedia.org/wiki/FM_Radio