This type of modulation is commonly used for broadcasting music and speech, magnetic tape recording systems, two way radio systems and video transmission systems. When noise occurs naturally in radio systems, frequency modulation with sufficient bandwidth provides an advantage in cancelling the noise. In this type of modulation, the phase of the carrier signal varies in accordance with the message signal.
When the phase of the signal is changed, then it affects the frequency. So, for this reason, this modulation is also comes under the frequency modulation. Generally, phase modulation is used for transmitting waves. It is an essential part of many digital transmission coding schemes that underlie a wide range of technologies like GSM, WiFi, and satellite television. This type of modulation is used for signal generation in al synthesizers, such as the Yamaha DX7 to implement FM synthesis.
If noise enters into a system, it persists and gets carried up to the end receiver. So, this drawback can be overcome by the digital modulation technique. For a better quality and efficient communication, digital modulation technique is employed. The main advantages of the digital modulation over analog modulation include available bandwidth , high noise immunity and permissible power.
In digital modulation, a message signal is converted from analog to digital message, and then modulated by using a carrier wave. The carrier wave is switched on and off to create pulses such that the signal is modulated. Similar to the analog, in this system, the type of the digital modulation is decided by the variation of the carrier wave parameters like amplitude, phase and frequency.
In an Amplitude shift keying, the amplitude of the carrier wave changes based on the message signal or on the base-band signal, which is in digital format. It is sensitive to noise and used for low-band requirements.
In frequency shift keying, the frequency of the carrier wave is varied for each symbol in the digital data. It needs larger bandwidths as shown in the figure. Frequency Shift Keying FSK is the digital modulation technique in which the frequency of the carrier signal varies according to the discrete digital changes. FSK is a scheme of frequency modulation. The binary 1s and 0s are called Mark and Space frequencies. The phase of the output signal gets shifted depending upon the input.
The other one is DPSK which changes the phase according to the previous value. Phase Shift Keying PSK is the digital modulation technique in which the phase of the carrier signal is changed by varying the sine and cosine inputs at a particular time. If this kind of techniques are further extended, PSK can be done by eight or sixteen values also, depending upon the requirement.
The following figure represents the QPSK waveform for two bits input, which shows the modulated result for different instances of binary inputs. Instead of the conversion of digital bits into a series of digital stream, it converts them into bit-pairs. This decreases the data bit rate to half, which allows space for the other users. No reference signal is considered here. The signal phase follows the high or low state of the previous element.
It is seen from the above figure that, if the data bit is LOW i. If the data is HIGH i. PM changes the phase angle of the complex envelope in direct proportion to the message signal.
Suppose that the signal to be sent called the modulating or message signal is m t and the carrier onto which the signal is to be modulated is c t.
Binary symbol 1 is represented by transmitting carrier wave of fixed amplitude and fixed frequency for the bit duration T seconds. ASK refers to a type of amplitude modulation that assigns bit values to discrete amplitude levels.
The carrier signal is then modulated among the members of a set of discrete values to transmit information. Values are represented by different amplitudes of carrier wave. Usually, one amplitude is zero, i. Used over optical fiber. Ac cos 2Sfct where the symbols has usual meanings. Phase Shift Keying: Phase-shift keying PSK is a method of digital communication in which the phase of a transmitted signal is varied to convey information. There are several methods that can be used to accomplish PSK.
It uses two opposite signal phases 0 and degrees. The digital signal is broken up timewise into individual bits binary digits. The state of each bit is determined according to the state of the preceding bit. If the phase of the wave does not change, then the signal state stays the same 0 or 1. If the phase of the wave changes by degrees -- that is, if the phase reverses -- then the signal state changes from 0 to 1, or from 1 to 0. Because there are two possible wave phases, BPSK is sometimes called biphase modulation.
The two binary states, logic 0 low and 1 high , are each represented by an analog waveform. Logic 0 is represented by a wave at a specific frequency, and logic 1 is represented by a wave at a different frequency. A modem converts the binary data from a computer to FSK for transmission over telephone lines, cables, optical fibre, or wireless media. The modem also converts incoming FSK signals to digital low and high states, which the computer can "understand.
All of us have experienced the difference between the quality of AM radio and FM radio, generally the quality of FM waves is much better than AM waves because AM is more susceptible to noise because noise affects amplitude, which is where information is stored in an AM signal and as the information is represented in frequencies for FM waves it remains unaffected.
0コメント