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INTRODUCTION OF FREQUENCY MODULATION (FM)

Edwin Howard Armstrong was an American electrical engineer who invented wideband frequency modulation (FM) radio. He patented the regenerative circuit in 1914, the super heterodyne receiver in 1918 and the super-regenerative circuit in 1922.He presented his paper which entitled ‘A Method of Reducing Disturbances in Radio Signalling by a System of Frequency Modulation’, which first described FM radio, before the New York section of the Institute of Radio Engineers on November 6, 1935. The paper was published in 1936.

1. Application by using FM – RADIO FM

FM radio uses frequency modulation, of course. The frequency band for FM radio is about 88 to 108 MHz. The information signal is music and voice which falls in the audio spectrum. The full audio spectrum ranges from 20 to 20 kHz, but FM radio limits the upper modulating frequency to 15 kHz. Although, some of the signal may be lost above 15 kHz, most people cannot hear it anyway, so there is little loss of fidelity. FM radio maybe appropriately referred to as ‘high-fidelity’.

An FM signal can also be used to carry a stereo signal. However, this is done by using multiplexing and demultiplexing before and after the FM process. A high-efficiency radio-frequency switching amplifier can be used to transmit FM signals. For a given signal strength which is measured at the receiver antenna, switching amplifiers use less battery power and typically cost less than a linear amplifier. This gives FM another advantage over other modulation schemes that require linear amplifiers, such as AM and QAM.

So, the information signal actually has a maximum modulating frequency of 53 kHz, requiring a reduction in the modulation index to about 1.0 to keep the total signal bandwidth about 200 kHz. (2080347)

2. Application by using FM – MAGNETIC TAPE STORAGE

Another application is magnetic tape storage.Magnetic tape storage is device for storing information, in which signals are recorded by lining up small bits of magnetic materials in the coating on the tape. Ordi nary tape recorders use magnetic tape.FM is also used at intermediate frequencies by all analog VCR systems and VHSto record both the luminance (black and white) and the chrominance portions of the video signal.

FM is the only method of recording video and retrieving video from Magnetic tape without extreme distortion, as video signals have a very large range of frequency components from a few hertz to several megahertz. FM also keeps the tape at saturation level, and therefore acts as a form of noise reduction, and a simple limiter can mask variations in the playback output, and the FM capture effect removes print-through.

These FM systems are unusual in that they have a ratio of carrier to maximum modulation frequency of less than two; contrast this with FM audio broadcasting where the ratio is around 10,000. Consider for example a 6 MHz carrier modulated at a 3.5 MHz rate; by Bessel analysis the first sidebands are on 9.5 and 2.5 MHz, while the second sidebands are on 13 MHz and −1 MHz. The result is a sideband of reversed phase on +1 MHz; on demodulation, this results in an unwanted output at 6−1 = 5 MHz. The system must be designed so that this is at an acceptable level. (2080348)

3. Application by using FM-SOUND

Sound
Another FM applications is sound.FM is also used at audio frequencies to synthesize sound. This technique, FM synthesis, was popularized by early digital synthesizers and became a standard feature for several generations of personal computer sound cards.

In audio and music frequency modulation synthesis (or FM synthesis) is a form of audio synthesis where the timbre of a simple waveform is changed by frequency modulating it with a modulating frequency that is also in the audio range, resulting in a more complex waveform and a different-sounding tone. For synthesizing harmonic sounds, the modulating signal must have a harmonic relationship to the original carrier signal. As the amount of frequency modulation increases, the sound grows progressively more complexA 'synthesizer' (often abbreviated "synth") is an electronic instrument capable of producing sounds by generating electrical signals of differentfrequencies. These electrical signals are played through a loudspeaker or set of headphones. Synthesizers can usually produce a wide range of sounds, which may either imitate other instruments ("imitative synthesis") or generate new timbres.

A sound card (also known as an audio card) is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs.(2080352)

4. Application by using FM –RADIOTELETYPE

Radioteletype (RTTY) is a telecommunications system consisting originally of two or more electromechanical teleprinters in different locations, later superseded by personal computers (PCs) running software to emulate teleprinters, connected by radio rather than a wired link. Radioteletype evolved from these earlier landline teleprinter operations. Commercial RTTY systems were in active service between San Francisco and Honolulu in as early as April 1932.

Electromechanical teleprinters were quite heavy, complex and noisy and they have been replaced with electronic units. The line output of a teleprinter can be at either digital logic levels (+5 V signifies a logical "1" or mark and 0 V signifies a logical "0" or space) or line levels (-80 V signifies a "1" and +80 V a "0"). When no traffic is passed, the line idles at the "mark" state.

A 5-bit character is generated when a key of the teleprinter keyboard is pressed. The teleprinter converts it to serial format and transmits a sequence of a start bit (a logical 0 or space), then one after the other the 5 data bits, finishing with a stop bit (a logical 1 or mark, lasting 1, 1.5 or 2 bits). When a sequence of start bit, 5 data bits and stop bit arrives at the input of the teleprinter, it is converted to a 5-bit word and passed to the printer or VDU. In electromechanical teleprinters these functions required complicated electromechanical devices, but they are easily implemented with standard digital electronics using shift registers. Special ICs have been developed for this function, for example the 6402 and 6403.[10] These are stand-aloneUART devices, similar to computer serial port peripherals. Teleprinters for languages using other alphabets also use an additional third shift state, in which they print letters in the alternative alphabet.