1. Lecture 10 Basic Modulation Techniques (VI)
Fall 2008
NCTU EE
Tzu-Hsien Sang 1 1

2. Outlines Linear Modulation Angle Modulation Interference
Feedback Demodulators
Analog Pulse Modulation
Delta Modulation and PCM
Multiplexing 2 2

3. One unfinished business: the Costas loop
One unfinished business: the Costas loop. It is very useful for carrier recovery in both analog and digital communication systems.

4. Analog Pulse Modulation
We now study the progression from techniques dealing with pure waveforms ones for “discrete” samples.
Historically, these methods are the early attempts to achieve modern communications. They are in the twilight zone between analog and digital modulations.
Today, their spirits can be still found in components such as ADC.

5. Analog pulse modulation: A pulse train is used as the carrier wave
Analog pulse modulation: A pulse train is used as the carrier wave. Some characteristic feature of each pulse (e.g., amplitude, duration, or position) is used to represent message samples.
PAM – pulse amplitude
PDM – pulse duration
PPM – pulse position
Digital Pulse Modulation: Messages are discrete-amplitude (finite levels) samples.
DM – delta modulation
PCM – pulse-code modulation

7. PAM

8. The “conceptual” demodulation scheme:
The idea of equalizer: Anything that goes through a known distortion can be recovered.

9. PWM Spectrum: complicated (Fourier-Bessel spectra)
Demodulation: area of “pulse.” Low-pass filtering (integration)

10. PPM

11. Delta Modulation

12. Slope overload: The message signal m(t) has a slope greater than can be followed by the stair-step approximation ms(t). Assume the step-size = 0  slope (max) = 0/Ts.

13. Solution (may not be perfect): adaptive delta modulation -- adjust the step-size 0 based on xc(t). Idea: If m(t)  constant, xc(t) alternates in sign  get 0 . If m(t)  ( or  ) rapidly, xc(t) has the same polarity  get 0.

14. Question: How does the receiver knows the time-varying step size?
Answer: Regenerate the step-size rule. Many so-called “adaptive” schemes, for example, in adaptive video coding, rely on this type of regeneration mechanism.

15. PCM
m(t)  samples (analog amplitude)  quantized samples  binary representation  binary modulated waveform (ASK (AM), PSK (PM), FSK (FM) )
Main advantages of digital communication
– more reliable communication
Main disadvantages of digital communication
– wide BW (reduced by “compression”)
complicated circuits (cost reduced by VLSI)