PULSE MODULATION

Pulse Modulation

Advantages of pulse modulation: Noise immunity, because pulses are

evaluated based on precise time interval and amplitude compared with reference level

Multiplexing is possible Signal regeneration is used instead of

amplification Simpler to measure and evaluate

Pulse Modulation

Disadvantages of pulse modulation: Use more bandwidth – generation of pulses

require more bandwidth Need additional encoding and decoding

circuitry Require precise time synchronization in

receiver and transmitter Incompatible with older analog

transmission systems

Basic types of pulse modulation: Phase amplitude modulation (PAM) Pulse width modulation (PWM) Pulse position modulation (PPM) Pulse code modulation (PCM)

Pulse Amplitude Modulation (PAM) PAM waveform characteristics:

Pulse width is constant Position of pulse is constant Amplitude of pulse is varied according to the

amplitude of the sample of the analog signal Application: PAM is used as an intermediate

form of modulation with PSK (phase-shift keying), QAM (quadrature amplitude modulation) and PCM

Pulse Width Modulation (PWM) Width of pulse is varied proportional to

the amplitude of the analog signal at the time the signal is sampled

The resulting PWM waveform has constant amplitude

Application: Special-purpose communication systems, mainly for military, rarely for commercial digital transmission systems

Pulse Position Modulation (PPM) Position of the pulse is varied according

to the amplitude of the analog signal The resulting PPM waveform has

constant amplitude and constant width Application: Special-purpose

communication systems, mainly for military, rarely for commercial digital transmission systems

Comparing PAM, PWM and PPM

Resulting Pulse

Pulse Amplitude Modulation (PAM)

Pulse Width Modulation (PWM)

Pulse Position Modulation (PPM)

Pulse width (duration)

Constant Variable Constant

Pulse position

Constant Constant Variable

Pulse height (amplitude)

Variable Constant Constant

Pulse Code Modulation (PCM) A form of digitally encoding analog

signal Pulses are of fixed length and amplitude Is a binary system i.e. represented by

logic 1 or 0 PCM is the most widely used pulse

modulation technique

Pulse Code Modulation (PCM)

Bandpass filter

Sample and hold

Analog to digital

converter

Parallel to serial

converter

Regenerative repeater

Regenerative repeater

Serial to parallel

converter

Digital to analog

converterHold

Low pass filter

Analog input signal

Output signal

PCM Transmitter

PCM Receiver

PCM

At transmitter: Bandpass filter limits frequency of analog signal Sample-and-hold periodically samples analog

signal to obtain PAM samples ADC change PAM samples to parallel PCM codes Parallel-to-serial converter change parallel PCM

codes to serial digital codes Repeaters regenerate digital pulses in the

transmission line Integrated circuit which performs encoding

and decoding is called codec (coder/decoder) At receiver, the operation is reverse to that of

the transmitter. The “hold” circuit converts PAM signals to original analog form

In summary, 3 steps in PCM: Sampling Quantization Encoding

PCM sampling

PCM sampling will periodically sample continually changing analog input voltage

Sampling rate must fulfill Nyquist sampling theorem i.e. sampling rate must be at least twice the highest frequency of input signal

2 types of PCM sampling: Natural sampling Flat-top sampling

Natural sampling

Top of the pulse takes shape of input waveform for the sample interval

Difficult for ADC to convert to PCM code because amplitude is not constant

Flat-top sampling

Top remains at the sampled value for the duration of the sample

Works with sample-and-hold circuit Most commonly used

Quantization

Quantization convert sampled amplitudes to discrete amplitudes taken from a set of possible amplitudes

Quantization level = the individual voltage levels

Quantization interval (or quantum) = difference between adjacent voltage levels

No. of quantization levels, L

Where n = no. of bits used in the code

nL 2

Maximum quantization level is given by2(n-1) – 1, where n is the no. of bits

Encoding

Encoding assigning each quantization level to a set of codes

The codes are sign-magnitude codes, where the most significant bit (MSB) is the sign bit, and the others represent magnitude

Dynamic Range (DR)

Dynamic Range (DR):

Where Vmax = maximum voltage magnitude

Vmin = quantum value (interval size)

min

max

V

VDR

Example 1

Given a PCM system with these parameters:

maximum analog input frequency = 4 kHz

no. of bits used in PCM code = 8 Find

Minimum sampling rate Number of quantization levels, and thus the

number of codes Maximum quantization level

Delta Modulation

Only a single bit is transmitted, which indicates whether that sample is larger or smaller than the previous sample

If current sample is smaller than previous sample, a ‘0’ is transmitted

If current sample is larger than previous sample, a ‘1’ is transmitted

Delta Modulation Transmitter

Sample and hold

Analog input

Digital to

analog converte

r

Up/down counter

Delta PCM

+

-

clock

Sampling pulse

1 = up0 = down

Delta Modulation Transmitter

Analog input is sampled and converted to PAM signal

PAM signal is compared with output of Digital-to-Analog Converter (DAC)

Output of DAC is a voltage equal to the regenerated magnitude of the previous sample which was stored in the up-down counter as a binary number

Up-down counter is incremented/decremented depending on whether the previous sample is larger/smaller than current sample

Up-down counter is clocked at a rate equal to the sample rate

Delta Modulation Receiver

Low pass filter

Digital to

analog converte

r

Up/down counter

Delta PCM clock

Recovered analog signal

Delta Modulation Receiver

As logic ‘1’ or ‘0’ are received, the up-down counter is incremented/decremented accordingly

Output of DAC in the decoder is identical to the output of DAC in the transmitter

Problems associated with Delta Modulation 1) Slope overload

Slope of analog signal will be greater than the delta modulator can maintain

Happens when analog input signal changes at a faster rate than the DAC can maintain

How to avoid: Increase clock frequency Increase magnitude of minimum step size

Problems associated with Delta Modulation 2) Granular noise

The reconstructed signal has variations that were not present in the original signal

Happens when the original analog input signal has a relatively constant amplitude

How to avoid: Decrease step size

Problems associated with Delta Modulation Small resolution is needed to reduce

granular noise, but large resolution is needed to reduce slope overload. Hence, a compromise is needed

Granular noise – more prevalent in analog signals that have gradual slopes

Slope overload – more prevalent in analog signals that have steep slopes or rapid variations in amplitude

Differential PCM (DPCM)

Often in PCM, there are successive samples that are almost of equal amplitudes. This cause several identical PCM codes to be transmitted, which is redundant

DPCM can solve this problem by transmitting the amplitude difference of the two successive samples instead of the actual samples

Hence, fewer bits are required for DPCM

DPCM Transmitter Low pass filter limits the input signal to half the sample rate Differentiator subtractor compares the bandlimited input

signal with the preceding accumulated signal level in the differentiator

The difference between the two signals is PCM encoded and transmitted

Low pass filter

Sample and hold

Analog to digital converte

r

Parallel to serial

converterAnalog input

Differentiator subtractor

Digital to

analog converte

r

Integrator

Binary adder

Serial DPCM

DPCM Receiver

Each received sample is converted to analog, stored and summed with the next sample received

Serial to parallel

converter

Digital to analog

converter

HoldLow pass filter

Sum signal out

Adder + Integrator

Serial DPCM in

Analog out