Theory - 73,74 :- (1) pulse modulation (2) categories of pulse modulation (3) PAM (4) PWM (5) PCM (6) Demodulation (7) advantages of pulse modulation

The communication system which makes use of analog signals are called as analog communication systems. Such communication systems include telegraphy, telephony, radio broadcast, radar communication, television network, teleprinting, telex, etc. with the advancement of technology, analog communication systems are replaced by digital communication systems. The communication systems, which make use of digital signals, are called as digital communication systems. The communication systems include, fax, electronic mail, internet, mobile telephone network, telemetry.

ANALOG AND DIGITAL SIGNALS

ANALOG SIGNAL: in communication systems, a signal means a time varying electrical signal which contains all information or intelligence. In an analog signal, the current or voltage value varies continuously with time.

DIGITAL SIGNAL: A Digital signal is a discontinuous function of time. The digital signal has two voltage levels, i.e, either low or high. The zero of digital signal refers to open circuit and one to a closed circuit. Both 0 and 1 are called as bits.

PULSE MODULATION

We have seen that in analog modulation, some parameter of carrier wave is varied in accordance with the instantaneous value of modulating signal. In pulse modulation, the carrier is no longer a continues sinusoidal was but consists of pulse train. Moreover, the signal is sampled at regular intervals and information regarding the signal is transmitter only at the sampling times together with carrier. Hear, some parameter of carrier is varied in accordance with the sampled signal

The pulse modulation is divided as follows:

Pulse modulation may be broadly divided into two categories:

1. Analog   2.Digital

In pulse analog modulation the indication of sample, amplitude may be infinitely variable. Here some characteristic of pulse is varied continuously. There are three possible types viz.,

• Pulse amplitude modulation (PAM)
• Pulse position modulation (PPM)
• Pulse duration modulation (PWM)

In pulse digital modulation a code which indicates the sample amplitude to nearest predetermined level is sent.

Types are:

• Pulse code modulation (PCM)
• Delta modulation (DM)

Advantages of pulse Modulation:

1. In such system, amplitude limiting techniques may be employed to minimize noise effects.
2. Transmission can be made independent of fading.
3. Multiple signals may be transmitted on a signal carrier.
4. Since energy is radiated in short bursts, power consumption is low.

 

PULSE AMPLITUDE MODULATION (PAM)

In this process, the relative amplitudes of successive pulse serve as a measure of signal amplitudes at the corresponding sampling instant s. The width and position of the pulse remain fixed. The modulating signal is sampled at the basic rate; usually (1/2fm) Where FM is the maximum wave of frequency present in the modulating signal.

PULSE WIDTH MODULATION:

                                                                                         

In this system, we have fixed amplitude and starting time of each pulse but the width of each pulse is made proportional to the amplitude of the signal at the instant. In this context, a negative pulse width is not possible.  Generation. PWM may be generated by applying trigger pulses (at the sampling rate) to control The starting time of pulses from a monostable multi vibrator and feeding in the signal to be sampled to control the duration of the pulses.

PULSE POSITION MODULATION (PPM):

In PPM, the position of a pulse is changed as a function of sampled value. The amplitude and width of the pulses is kept content in this system, while the position of each pulse in relation to the position of a recurrent reference pulse, is varied by each instantaneous sampled value of the modulating wave.

Generation. PPM is easily obtained from PWM. In PWM the location of the leading edges are fixed, whereas those of the trailing edges are not. Their position depends on pulse width, which is determined by the signal amplitude at that instant.

PPM Advantages

1. In PWM the power requirement for transmitter was varying. In PPM it requires constant transmitter power output.
2. As most of the signal information is contained in the trailing edge of a pulse, only edges are transmitted rather than pulses. Also the power requirement of PPM is less than PWM.

PPM Disadvantage.

1. It requires synchronization between transmitter and receiver.

PULSE CODE MODULATION (PCM):

Pulse code modulation is completely digital modulation. To obtain pulse code modulated signal, the pulse amplitude modulated signal is used. The signal is first communication systems sampled, then sampled amplitude is selected in terms of nearest standard amplitude, called quantization and finally indicated by a code of pulse known as coding. The signal is no modulation at all and that the signal is not transmitted at all. The transmission is a code, inary code, until the final stages of detection.

The operations involved in PCM are:

• Sampling:- A process of generating pulses of zero width (duration) and of amplitude equal to the instantaneous amplitude of analog signal is defined as sampling.  In sampling process, the amplitude of analog signal is measured at an interval t during each cycle. This interval t is kept small so that a number of samples may be taken. 
• QUANTISATION:- The process of converting the amplitude of analog voltage into a nearest standard amplitude is called as quantization.
• Coding:- The process of digitizing the quantized pulse according to some code is called coding. The most popular code is the binary code and hence each quantized pulse is given a digital code which is a binary number.

PCM Advantages:-

1. It is cheap due to latest Technology.
2. PCM can be relayed without degradation when the signal to noise ratio exceeds about 21 DB
3. The ability to get relayed without distortion using poor-quality transmission paths is very significant incentive to used digital modulating system.
4. Interconnection is possible between radio links cable switching centers without decoding the digital signal
5. Optical wave guide and beams can be used only with PCM transmission.

PCM disadvantage

1. PCM require very complex encoding and quantizing circuitry
2. PCM requires a large BW compared to analog system.
3. Adjacent bits in the code of a signal quantized sample can interfere and cause inter – symbol interference
4. Normally the PCM system overload when the amplitude of the based band signal exceeds the range of the active device used to process the signal.

Communication systems

PCM applications: PCM is used for satellite communication, multichannel communication telephone links microwave links for long distance etc. PCM should always be used when there is multiplexing in the common channel, PCM technique is used in services like ISDN (integrated services digital network) internet LAN (local area network) etc.

PAM-SIGNAL (generation and demodulation )

Generation. It is very easy to generate and generate and demodulate PAM. In a PAM generator, the signal to be converted to PAM is fed to one input of an AND gate. Pulses at the sampling frequency are applied to the other input of the And gate to open it during the wanted time intervals. The output of the gate then consists of pulses at the sampling rate, equal in amplitude to the signal voltage at each instant. The pulses are then passed through a pulse[1]shaping network, which given them flat tops. FM is then employed so that the system becomes PAM-FM. Demodulation. Fig.19.5 (a) shows the block diagram for PAM demodulator. 

In the receiver the pulses are fist recovered with a standard FM demodulator. They are then fed to an ordinary diode detector followed by a LP (low pass ) filter. If the cut off frequency of this filter is high enough to pass the highest signal frequency, but low enough to remove the sampling frequency ripple an undistorted replica of the original signal is reproduced as shown in fig. 19.5(b)

Advantages:

1. It is easy to implement.
2. There is greater peak power compared to AM and FM.
3. Signal to noise ratio is improved.
4. In PAM, pulses are of uniform duration and equally spaced. Therefore, receiver synchronization is not a problem.

Disadvantages:-

1. Compared to constant amplitude systems Pm has greater noise interference.
2. As the amplitude of PAM signal varies, the peak power required by the transmitter also varies with the modulating signal.
3. The bandwidth required for PAM transmission is large in comparison to maximum frequency present in the modulating signal.