Pulse modulation
- 1. Pulse Modulation This lecture: Pulse‐Amplitude Modulation Pulse‐Time Modulation Pulse‐Duration and Pulse‐Position Modulation Pulse Code Modulation Delta Modulation Adaptive Delta Modulation Differential Pulse Code Modulation
- 2. Pulse Amplitude Modulation (PAM) If a message waveform is adequately described by periodic sample values, it can be transmitted using analogue pulse modulation wherein the sample values modulate the amplitude of pulse train. Therefore, the amplitudes of regularly spaced pulses are varied in proportion to the corresponding sample values of a continuous message signal x ( t ) . This technique is termed Pulse Amplitude Modulation
- 3. Generation of the PAM signal There are two operations involved in the generation of the PAM signal: Instantaneous sampling of the message signal x ( t ) every Ts seconds, where the sampling rate f s = 1/ Ts is chosen in accordance with the sampling theorem Lengthening the duration of each sample so obtained to some constant value τ (sample‐and‐hold)
- 7. Block diagram of PAM generation System for recovering message signal m(t) from PAM signal s(t).
- 14. Pulse Code Modulation PCM is the most commonly used technique in digital communications Used in many applications: Telephone systems Digital audio recording CD laser disks voice mail digital video etc. They are a primary building block for advanced communication systems
- 15. Pulse Code Modulation Based on the sampling theorem Each analog sample is assigned a binary code Analog samples are referred to as pulse amplitude modulation (PAM) samples The digital signal consists of block of n bits, where each n -bit number is the amplitude of a PCM pulse
- 16. Pulse Code Modulation (PCM) As in the case of other pulse modulation techniques, the rate at which samples are taken and encoded must conform to the Nyquist sampling rate. The sampling rate must be greater than, twice the highest frequency in the analog signal, f s > 2 f A (max) Telegraph time-division multiplex (TDM) was conveyed as early as 1853, by the American inventor M.B. Farmer. The electrical engineer W.M. Miner, in 1903. PCM was invented by the British engineer Alec Reeves in 1937 in France. It was not until about the middle of 1943 that the Bell Labs people became aware of the use of PCM binary coding as already proposed by Alec Reeves.
- 17. Figure The basic elements of a PCM system. Pulse Code Modulation
- 18. Encoding
- 19. Robustness to noise and interference Efficient regeneration Efficient SNR and bandwidth trade-off Uniform format Ease add and drop Secure Advantages of PCM
- 20. Noise in PCM Systems
- 21. Quantizing The process of converting analog signals to PCM is called quantizing Since the original signal can have an infinite number of signal levels, the quantizing process will produce errors called quantizing errors or quantizing noise The dynamic range of a system is the ratio of the strongest possible signal that can be transmitted and the weakest discernible signal In a linear PCM system, the maximum dynamic range is found by: DR = (1.76 + 6.02 m ) dB
- 23. Two types of quantization: ( a ) midtread and ( b ) midrise.
- 24. Illustration of the quantization process. (Adapted from Bennett, 1948, with permission of AT&T.)
- 25. Companding Companding is used to improve dynamic range Compression is used on the transmitting end and expanding is used on the receiving end, hence companding
- 26. Intersymbol Interference If the system impulse response h(t) extends over more than 1 symbol period, symbols become smeared into adjacent symbol periods Known as intersymbol interference (ISI)
- 27. Intersymbol Interference Example Response h ( t ) is Resistor-Capacitor (R-C) first order arrangement- Bit duration is T Time (bit periods) amplitude Time (bit periods) amplitude For this example we will assume that a binary ‘0’ is sent as 0V. Modulator input Slicer input Binary ‘1’ Binary ‘1’
- 28. Intersymbol Interference The received pulse at the slicer now extends over 4 bit periods giving rise to ISI. The actual received signal is the superposition of the individual pulses time (bit periods) amplitude ‘ 1’ ‘ 1’ ‘ 0’ ‘ 0’ ‘ 1’ ‘ 0’ ‘ 0’ ‘ 1’
- 29. Intersymbol Interference For the assumed data the signal at the slicer input is, Clearly the ease in making decisions is data dependant time (bit periods) amplitude Note non-zero values at ideal sample instants corresponding with the transmission of binary ‘0’s ‘ 1’ ‘ 1’ ‘ 0’ ‘ 0’ ‘ 1’ ‘ 0’ ‘ 0’ ‘ 1’ Decision threshold
- 30. Delta Modulation In Delta Modulation, only one bit is transmitted per sample That bit is a one if the current sample is more positive than the previous sample, and a zero if it is more negative Since so little information is transmitted, delta modulation requires higher sampling rates than PCM for equal quality of reproduction
- 32. DM System: Transmitter and Receiver.
- 33. The modulator consists of a comparator, a quantizer, and an accumulator. The output of the accumulator is Slope overload distortion and granular noise
- 35. Adaptive delta modulation system: ( a ) Transmitter. ( b ) Receiver.
- 36. Waveforms resulting from the computer experiment on delta modulation: ( a ) Linear delta modulation. ( b ) Adaptive delta modulation.
- 38. Figure 3.28 DPCM system. ( a ) Transmitter. ( b ) Receiver.