Post on 22-Jun-2015
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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