1. Communication Systems
Chapter 6
Pulse Modulation

2. Communication Systems, chapter 6
Pulse Modulation
Some parameter of pulse train is varied in accordance with the message signal
Analog pulse modulation
A periodic pulse train is used as the carrier wave
Some characteristic feature of each pulse is varied (e.g. amplitude, duration, position)
Information transmitted basically in analog form
Digital pulse modulation
Information transmitted in digital form as a sequence of coded pulses
Communication Systems, chapter 6

3. Communication Systems, chapter 6
The Sampling Process
An analog signal is converted into a corresponding sequence of samples that are usually spaced uniformly in time
It is necessary that we choose sampling rate properly
Sequence of samples uniquely defines the original analog signal
Communication Systems, chapter 6

4. Communication Systems, chapter 6
The Sampling Process
Communication Systems, chapter 6

5. Pulse-Amplitude Modulation
The amplitudes of regularly spaced pulses are varied in proportion to the corresponding sample values of a continuous message signal
Pulses can be of a rectangular form or some other appropriate shape
PAM is somewhat similar to natural sampling
Communication Systems, chapter 6

6. Pulse-Amplitude Modulation
Two operations involved in the generation of the PAM signal :
Instantaneous sampling of the message signal m(t) every Ts seconds, where the sampling rate fs = 1/Ts is chosen in accordance with the sampling theorem
Lengthening the duration of each sample so obtained to some constant value T
Communication Systems, chapter 6

7. Time-Division Multiplexing
Enables the joint utilization of a common communication channel by a plurality of independent message sources without mutual interference among them
Each input message if first restricted in bandwidth by a low-pass filter to remove the frequencies that are nonessential to an adequate signal representation
Outputs are then applied to the commutator
To take a narrow sample of each of the N input message at a rate fs
To sequentially interleave these N samples inside the sampling interval Ts
Communication Systems, chapter 6

8. Time-Division Multiplexing
Pulse modulator (following the commutator)
To transform the multiplexed signal into a form suitable for transmission over the common channel
At the receiving end of the system
Signal is applied to a pulse demodulator
reverse operation of the pulse modulator
Outputs are distributed to the appropriate low-pass reconstruction filters by means of a decommutator
Operates in synchronism with the commutator
Communication Systems, chapter 6

9. Time-Division Multiplexing
Communication Systems, chapter 6

10. Pulse-Position Modulation
The samples of the message signal are used to vary the duration of the individual pulses
Known also as : pulse-width or pulse-length modulation
May vary the time of occurrence of leading edge, trailing edge, or the both edges of the pulse
Communication Systems, chapter 6

11. Pulse-Position Modulation
Communication Systems, chapter 6

12. Pulse-Position Modulation
Communication Systems, chapter 6

13. Pulse-Position Modulation
Communication Systems, chapter 6

14. Bandwidth-Noise Trade-Off
PPM and FM exhibit similar noise performance :
Both have a figure of merit proportional to the square of the transmission bandwidth normalized with respect to the message bandwidth
Both exhibit a threshold effect as the signal-to-noise ratio is reduced
Communication Systems, chapter 6

15. The Quantization Process
A continuous signal has a continuous range of amplitudes
Infinite number of amplitude levels
The original continuous signal may be approximated by a signal constructed of discrete amplitudes selected on a minimum error basis from available set
Communication Systems, chapter 6

16. Communication Systems, chapter 6
Quantization noise
Difference between the input signal and output signal is called quantization noise
Communication Systems, chapter 6

17. Pulse-Code Modulation
A message signal is represented by a sequence of coded pulses, which is accomplished by representing the signal in discrete form in both time and amplitude
Basic operations in transmitter are :
sampling, quantizing and encoding
Basic operations in receiver are :
regeneration, decoding and reconstruction
Communication Systems, chapter 6

18. Pulse-Code Modulation
Communication Systems, chapter 6

19. Pulse-Code Modulation
Sampling :
The incoming signal is sampled with a train of narrow rectangular pulses
Low-pass filter is used before the sampler in order to exclude frequencies greater than W before sampling
W = the highest frequency component
Communication Systems, chapter 6

20. Pulse-Code Modulation
Quantization :
To provide a new representation of the signal that is discrete in both time and amplitude
Works as presented in earlier slides
It is preferable to use a variable separation between the representation levels
Communication Systems, chapter 6

21. Pulse-Code Modulation
Encoding :
To translate the discrete set of sample values to a more appropriate form of signal
Code word/character -> code element/symbol -> code
There are several line codes that can be used for the electronical representation of binary symbols 1 and 0
On-Off signalling
Nonreturn-to-zero (NRZ) signalling
Return-to-zero (RZ) signalling
Bipolar return-to-zero (BRZ) signalling
Split-phase (Manchester code)
Differential encoding
Communication Systems, chapter 6

22. Pulse-Code Modulation (Encoding)
Communication Systems, chapter 6

23. Pulse-Code Modulation
Regeneration :
Reconstruct the PCM signal with the regenerative repeaters located along transmission route to avoid the distortion and noise produced transmission through a channel
The regenerated signal departs from the original
The unavoidable presence of channel noise and interference causes the repeater to make wrong decisions occasionally
If the spacing between received pulses deviates from its assigned value, a jitter is introduced into the regenerated pulse position, thereby causing distortion
Communication Systems, chapter 6

24. Pulse-Code Modulation (Regeneration)
Communication Systems, chapter 6

25. Pulse-Code Modulation
Decoding :
The first operation in the receiver is to regenerate the received pulses one last time
These pulses are then regrouped into code words and decoded into a quantized PAM signal
Communication Systems, chapter 6

26. Pulse-Code Modulation
Filtering :
The final operation in the receiver is to recover the message signal wave by passing the decoder output through a low-pass reconstruction filter
Communication Systems, chapter 6

27. Pulse-Code Modulation
Multiplexing :
In PCM-applications it is natural to multiplex different message sources by time division
Communication Systems, chapter 6

28. Pulse-Code Modulation
Synchronization :
To synchronize transmitter and receiver
Local clocks to keep the same time at the transmitter and the receiver
Communication Systems, chapter 6

29. Noise Considerations in PCM Systems
Channel noise
Anywhere between the transmitter output and the receiver input
Always present
Quantization noise
In the transmitter and is carried all the way to the receiver output
Signal dependent
Communication Systems, chapter 6

30. Communication Systems, chapter 6
Delta Modulation
An incoming sample is over sampled to purposely increase the correlation between adjacent samples of the signal
Provides a staircase approximation to the over sampled version of the message signal
Difference between the input and approximation is quantized into only two levels, namely ±Δ
Communication Systems, chapter 6

31. Communication Systems, chapter 6
Delta Modulation
Communication Systems, chapter 6

32. Communication Systems, chapter 6
Differential PCM
Remove redundancy (not absolutely essential to the transmission of information) before encoding to obtain a more efficient coded signal
As PCM suffers from quantization noise
Communication Systems, chapter 6

33. Communication Systems, chapter 6
Differential PCM
Communication Systems, chapter 6

34. Coding Speech at Low Bit Rates
To remove redundancies from the speech signal as far as possible
To assign the available bits to code the nonredundant parts of the speech signal in a perceptually efficient manner
Adaptive DPCM
adaptive quantization and prediction
Adaptive subband coding
The number bits used to encode each subband is varied dynamically
Communication Systems, chapter 6