Sampling 1

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DIGITAL MODULATION

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Why digital modulation???

The RF spectrum must be shared, yet every day there are more users for that spectrum as demand for communications services increases.

Digital modulation schemes have greater capacity to convey large amounts of information than analog modulation schemes

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IntroductionIntroduction

Digital ModulationDigital Modulation Advantages :

Immunity to noise (due to its finite process)Easy storage and processing: RegenerationEasy to measureEnables encryptionData from several sources can be integrated and

transmitted using the same digital communication system

Error correction detection can be utilized

Disadvantages :Requires a bigger bandwidthAnalog signal need to be changed to digital firstNot compatible to analog systemNeed synchronization

MP, DSP, RAM, ROM, Computer

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TRANSMISSION METHOD FOR ANALOG & DIGITAL SIGNALS

Analog input

Analog channel Baseband

Analog output

Analog input Modulator De

modulatorAnalog output

Analog channel

Digital input encoder decoderDigital

channelDigital output

Digital input Modem ModemAnalog

channelDigital output

Analog input

ADC & encoder

Decoder & DAC

Analog output

Digital channel

Analog input

Analog output

Analog channel

ADC & encoder

ModemADC & decoder Modem

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Block diagram for digital transmission system

Analog ADC Line coding Digitaltransmission

SamplingQuantizationCoding

RZ, NRZ, AMI

ASK, FSK, PSK

Block diagram for digital transmission system

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OutlineSampling

Analog Pulse Modulation:• PAM• PDM/PWM• PPM

Digital Pulse Modulation:• PCM• Delta Modulation• DPCM

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7

X

Digital signals(t)

ms(t)m(t)

m(t)

t

ms(t)

t

s(t)

tTs

A process of periodically sampling the continually changing analog input voltage and convert it to a series of

constant amplitude pulses

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Sampling TheoremSampling Theorem states that an analogue signal iscompletely described by its samples, taken at equal timeIntervals, the sampling frequency/sampling rate is greater than, or equal to, twice the maximum frequency component of the analogue signal

ms ff 2

Nyquist theorem states that:

fs= 2 x (bandwidth of analogue signal) = 2B Hz

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The choice of sampling frequency, fs must follow the sampling theorem to overcome the problem of aliasing and loss of information

(a) Sampling frequency=> fs1 < 2fm (max)

f2fs1 3fs1fs1fm

Aliasingms(f)

(b) Sampling frequency=> fs2 > 2fm (max)

f2fs2 3fs2fs2fm

ms(f)

Shannon sampling theorem=> fs 2fm

Nyquist frequency fs = 2fm= fN

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Two types of sampling

1. Ideal Sampling It is the instantaneous sampling and is not possible practically.

2. Natural Sampling The tops of the sample pulses retain their natural shape, making it difficult for ADC to convert to PCM codes.

3. Flat-top Sampling The input voltage is sampled with narrow pulses and then held relatively constant until next sampling.

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Sr. No.

Parameter ofcomparison

Ideal or Instantaneous sampling

Natural sampling

Flat-Top sampling

1. Sampling principle

It uses multiplication.

It uses chopping principle.

It uses sample and hold circuit.

2. Generation circuit

3. Waveform involved

4. Feasibility This is not a practically possible method.

This method is used practically.

This method is also used practically.

5. Sampling rate

Sampling rate tends to infinity.

Sampling rate satisfies Nyquist criteria.

Sampling rate satisfies Nyquist criteria.

6. Noise interference

Noise interference is maximum.

Noise interference is minimum.

Noise interference is maximum.April 8, 202313

Pulse Modulation

Modulation of a carrier wave may be accomplished by short pulses. Conventional telegraphy is the simplest example of this mode of modulation.Pulse systems are based on sampling of the information signal at periodic intervals, usually twice the maximum frequency present (2B). They transmit a very short pulse of radio-frequency carrier for each sample, with pulse characteristics varied in some manner proportional to the amplitude at the sampling instant. A general name given to these modes of modulation is the pulse modulation.

The common pulse systems employed in pulse modulation of analog signals are: (i) Pulse – amplitude modulation (PAM) (ii) Pulse – position modulation (PPM)(iii) Pulse – duration/width modulation (PDM/PWM)(iv) Pulse – code modulation (PCM

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Pulse-Amplitude Modulation (PAM)

PAM: Sample and hold Sample the message signal every Ts seconds.Vary the amplitude of regularly spaced pulses in proportion to the corresponding sample values. discrete in time, continuous in amplitude, still

analog. The pulses can be rectangular or other

appropriate shapes.The shape and duration of the pulse determine

the bandwidth.

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Pulse Duration/Width ModulationPDM/PWMIt is modulation tech. in which the time

duration or the width of the pulse is varied according to the instantaneous value of the modulating wave.

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PPMThe position of the pulses are varied

according to the instantaneous value of the modulating wave.

To generate PPM ,PDM is first differentiated and then clipped.

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Pulse Amplitude Modulation(PAM)

Pulse Width/Duration Modulation(PDM/PWM)

Pulse Position Modulation(PPM)

Waveform Waveform Waveform

Amplitude of the pulse is proportional to the amplitude of the modulating signal.

Width of the pulse is proportional to the amplitude of the modulating signal.

The relative position of the pulse is proportional to the amplitude of the modulating signal.

The bandwidth of the transmission channel depends on the width of the pulse.

The bandwidth of the transmission channel depends on the rising time of the pulse.

The bandwidth of the transmission channel depends on the rising time of the pulse.

The instantaneous power of the transmitter varies.

The instantaneous power of the transmitter varies.

The instantaneous power of the transmitter remains constant.

Noise interference is high.

Noise interference is minimum.

Noise interference is minimum.

System is complex. Simple to implement. Simple to implement.

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PULSE CODE MODULATION (PCM)

DEFINITION: Pulse code modulation (PCM) is essentially analog-to-digital conversion of a special type where the information contained in the instantaneous samples of an analog signal is represented by digital words in a serial bit stream.

The advantages of PCM are: •Relatively inexpensive digital circuitry may be used extensively. •PCM signals derived from all types of analog sources may be merged with data signals and transmitted over a common high-speed digital communication system. •In long-distance digital telephone systems requiring repeaters, a clean PCM waveform can be regenerated at the output of each repeater, where the input consists of a noisy PCM waveform. •The noise performance of a digital system can be superior to that of an analog system.•The probability of error for the system output can be reduced even further by the use of appropriate coding techniques.

Components of PCM encoder

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Sampling, Quantizing, and Encoding

The PCM signal is generated by carrying out three basic operations:

1. Sampling 2. Quantizing3. Encoding

1. Sampling operation generates a flat-top PAM signal.2. Quantizing operation approximates the analog values

by using a finite number of levels. This operation is considered in 3 steps

a) Uniform Quantizer b) Quantization Error c) Quantized PAM signal output

3. PCM signal is obtained from the quantized PAM signal by encoding each quantized sample value into a digital word.

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QuantizationThe output of a sampler is still continuous in

amplitude.Each sample can take on any value e.g.

3.752, 0.001, etc.The number of possible values is infinite.

To transmit as a digital signal we must restrict the number of possible values.

Quantization is the process of “rounding off” a sample according to some rule.E.g. suppose we must round to the nearest

tenth, then: 3.752 --> 3.8 0.001 --> 0

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Illustration of the Quantization Error

EncodingThe output of the quantizer is one of M possible

signal levels.If we want to use a binary transmission system,

then we need to map each quantized sample into an n bit binary word.

Encoding is the process of representing each quantized sample by an bit code word.The mapping is one-to-one so there is no

distortion introduced by encoding.Some mappings are better than others.

A Gray code gives the best end-to-end performance.

The weakness of Gray codes is poor performance when the sign bit (MSB) is received in error.

22 , log ( )nM n M

Quantization and encoding of a

sampled signal

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ApplicationsUsed in many applications:

Telephone systemsDigital audio recordingCD laser disksvoice maildigital video etc.

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Components of a PCM decoder

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DELTA MODULATIONDelta modulation transmits only one bit per sample.The present sample value is compared with the

previous sample value and this result whether the amplitude is increased or decreased is transmitted.

The step size is kept fixed.The difference between the input signal x(t) and the

staircase approximate signal is confined to two levels,i.e. +∆ and -∆.

Now if the difference is positive, then approximated signal is increased by one step,i.e., +∆.

If the difference is nagetive ,then approximated signal is reduced by -∆.

When the step is reduced,”o” is transmitted and if the step is increased,”1” is transmitted.

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The process of delta modulation

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Delta modulation components

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Delta demodulation components

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DIFFERENTIAL PULSE CODE MODULATION (DPCM)Some time the information carrying signal does not

change much.The codes for the information are the same,i.e.

redundant information.In the DPCM these redundant information is not

send,instead only one sample is send.By reducing the redundancy the overall bit rate will

decrease and the number of bits required to transmit one sample will also be reduced.

DPCM works on the principle of prediction.The value of the present sample is predicted from the past samples.

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COMPARISION OF DIGITAL PULSE MODULATION METHODSSr.No

Parameter s of comparison

PCM DM DPCM

1. Numbers of bits.

It can use 4,8 or 16 bits per samples.

It uses only one bit for one sample.

Bits can be more than one but are less than PCM.

2. Levels and step size.

The number of levels depend on number of bits. Level size is kept fixed.

Step size is kept fixed and cannot be varied.

Here,fixed number of levels are used.

3. Quantization error and distortion.

Quantization error depends on number of levels used.

Slope overload distortion and granular noise are present.

Slope overload distortion and quantization noise is present.

4. Transmission bandwidth.

Highest bandwidth is required since number of bits are high.

Lowest bandwidth is required.

Bandwidth required is lower than PCM.

5. Feedback. There is no feedback in transmitter or receiver.

Feedback exists in transmitter.

Here,feedback exist.

6. Complexity of implementation.

System complex. Simple. Simple.

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