Generation of PPM

1. Generation of PPM

2. Detection Of PPM

3. Pulse Code Modulation (PCM)

4. PCM 8 8 Volts 5 -1 time -2 -7 -9 10001 00101 10111 01000 01000 10010 11001 time

5. Merits of Digital Communication: • Digital signals are very easy to receive. The receiver has to just detect whether the pulse is low or high. • AM & FM signals become corrupted over much short distances as compared to digital signals. In digital signals, the original signal can be reproduced accurately. • The signals lose power as they travel, which is called attenuation. When AM and FM signals are amplified, the noise also get amplified. But the digital signals can be cleaned up to restore the quality and amplified by the regenerators. • The noise may change the shape of the pulses but not the pattern of the pulses. • AM and FM signals can be received by any one by suitable receiver. But digital signals can be coded so that only the person, who is intended for, can receive them. • The digital signals can be stored, or used to produce a display on a computer monitor or converted back into analog signal to drive a loud speaker.

6. Error correction Easy processing and storage Disadvantages: More bandwidth Need of ADC and DAC Need of clock recovery circuit Incompatible with older analog system

7. Pulse Code Modulation (PCM): • Analog signal is converted into digital signal by using a digital code. Analog to digital converter employs two techniques: • 1. Sampling: The process of generating pulses of zero width and of amplitude equal to the instantaneous amplitude of the analog signal. The no. of pulses per second is called “sampling rate”. • 2. Quantization: The process of dividing the maximum value of the analog signal into a fixed no. of levels in order to convert the PAM into a Binary Code. • The levels obtained are called “quanization levels”. • * A digital signal is described by its ‘bit rate’ whereas analog signal is described by its ‘frequency range’. • * Bit rate = sampling rate x no. of bits / sample

8. Voltage Time 111 110 101 100 011 010 001 000 76543210 Binary Levels Codes Voltage Time 0 1 0 1 0 1 1 1 0 1 1 1 1 1 0 1 0 1 0 1 0 Time Sampling,QuantizationandCoding

9. Quantization • Is the process of converting the sampled signal to a binary value • Each voltage level will correspond to a different binary number • The magnitude of the minimum step size is called the resolution. • Resolution with respect to digitizing system refers to the accuracy of the digitizing system in representing the sampled signal. • The error resulting from quantizing is called the quantization noise. Its value is 1/2 the resolution

10. 111 110 101 100 011 010 001 000 t0 t1 t2 t8 t3 t7 t4 t5 t6 • Sampling ( Time axis) • Quantization (Amplitude axis) • encoding Encode 011 101 110 101 011 011 000 001 011

11. Quantization example Quant. levels boundaries x(nTs): sampled values xq(nTs): quantized values amplitude x(t)‏ Step size 111 3.1867 110 2.2762 101 1.3657 100 0.4552 011 -0.4552 010 -1.3657 001 -2.2762 000 -3.1867 Ts: sampling time t PCM codeword 110 110 111 110 100 010 011 100 100 011 PCM sequence

15. Quantization Noise 20 levels Red = magnitude Black = timing interval

16. 4 levels Red = magnitude Black = timing interval

17. Process of quantizing noise Qauntizer Model of quantizing noise AGC + Quantization error • Quantizing error: The difference between the input and output of a quantizer • Maximum quantization error is • Signal to Quantization noise ratio

18. Dynamic Range This is the ratio of the largest to smallest analogue signal that can be transmitted. But Vmin is the resolution and can be written as It follows that

19. Types of Quantization • Uniform quantizer: if the step size remains constant throughout the input range. There are two types of quantizer: • Symmetric quantizer of the mid tread type • Symmetric quantizer of the mid riser type. 2. Non uniform quantizer: if the quantizer characteristics is non-linear and the step size is not constant instead it varies depending on the amplitude of input.

20. Mid riser Mid trader

21. Uniform and nonuniform quantization

22. compression+expansion companding Non-uniform quantization • It is achieved by uniformly quantizing the “compressed” signal. • At the receiver, an inverse compression characteristic, called “expansion” is employed to avoid signal distortion. Compress Qauntize Expand Channel Transmitter Receiver

23. Companding characteristics

25. Block Diagram of Companding Process

26. Laws of companding • The µ-law algorithm is a companding algorithm, primarily used in the digital telecommunication systems of North America and Japan. • Companding algorithms reduce the dynamic range of an audio signal. • In analog systems, this can increase the signal-to-noise ratio (SNR) achieved during transmission, and in the digital domain, it can reduce the quantization error (hence increasing signal to quantization noise ratio).

27. Laws of companding • An A-law algorithm is a standard companding algorithm, used in European digital communications systems to optimize, i.e., modify, the dynamic range of an analog signal for digitizing.

29. PCM Receiver

30. DIFFERENTIAL PCM • Only transmit the difference between the predicated value and the actual value • Voice changes relatively slowly • It is possible to predict the value of the simplest form

31. DPCM TRANSMITTER

32. DPCM Receiver

33. Delta Modulation • PCM is powerful, but quite complex coders and decoders are required. • An increase in resolution also requires a higher number of bits per sample. • An alternative, which overcomes some limitations of PCM is to use past information in the encoding process. One way of doing this is to perform source coding using delta modulation: • Transmits information only to indicate whether the sample of analog signal is larger or smaller then the previous sample.

34. Principle of DM

35. Delta modulation components

37. Types of error • Slope overload : happens when analog signal changes at a faster rate than the DAC can maintain. • increasing the clock frequency reduces the probability of slope overload occurring. • Another way to prevent is to increase the magnitude of step size

38. Types of error • Granular noise: when original analog signal has relatively constant amplitude, the reconstructed signal has variations that were not present in the original signal. this is called as granular noise. • Can be reduced by decreasing the step size

39. To reduce the granular noise, a small resolution(small step size) is needed and to reduce slope overload occurring , a large resolution is required .

40. DM RECEIVER

41. Adaptive Delta Modulation • Is a delta modulation system where the step size of the DAC is varied , depending upon the amplitude characteristics of the analog input signal. • A common algorithm for adaptive delta modulator is when three consecutive 1s or 0s occur, the step size of the DAC is increased or decreased by a factor of 1.5.