CE_PCM.ppt
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- 7. Definition….. The amplitude and of the width of the pulses are kept constant, while the position of each pulse, with the reference to the position of reference pulse is changed according to the instantaneous sampled value of the modulating signal. 7
- 8. First generate PWM To generate PPM , PWM signal as a Trigger input to one monostable multivibrator. The monostable output remains zero untill it is triggered. The monostable is triggered ON the falling edge of PWM. The output of monostable is high for the fixed period then goes low. 8
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- 10. It consists of Differentiator & Monostable Multivibrator 10 Differentiator Positive and Negative Spikes Monostable Multivibrator
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- 14. AM , FM – Analog modulation (continuous form of modulation) PAM, PWM,PPM – also Analog pulse modulation, ie., Signal was sampled and sent in pulse form PCM – PCM is also sampling technique but it differ from the others, in that it is a digital process 14
- 15. . i.e., instead of sending a pulse train capable of continuously varying one of the parameter. The PCM generator produce a series of numbers or digit (digital process). Each one of these digit, almost always in binary code, represent the approximate amplitude of the signal samples. 15
- 16. Any pulse can be approximated to one of the values of quantization levels. The Presence of pulse may represented by 1 16 Quantization level Binary code 0 0 0 0 1 0 0 1 2 0 1 0 3 0 1 1 4 1 0 0 5 1 0 1 6 1 1 0 7 1 1 1
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- 18. If noise occur , it makes no difference. The probability of occurrence of such error is extremely small. Practically error-free transmission is possible. Quantization error can be minimized by reducing the step size. The only error that is possible is due to quantization. 18
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- 20. 20 Quantization is a non linear transformation which maps elements from a continuous set to a finite set. It is also the second step required by A/D conversion. Sample Quantize Analog Signal - Continuous time - Continuous value Digital Signal - Discrete time - Discrete value - Discrete time - Continuous value
- 22. Quantization is defined as the process of transforming the sample amplitude of a message signal into discrete amplitudes. Quantization Noise: Difference between input and quantization output signal is called quantization error or quantization noise. ◦ Step size of quantization is δ 22
- 23. Quantization Uniform Quantization/linear Non- Uniform Mid tread mid riser biased µ - law A- law 23
- 24. The samples of signals are highly correlated with each other. Its value from present sample to next sample does not differ by large amount. The adjacent samples of the signal carry the same information with little difference. When these samples are encoded by PCM, the resulting encoded signals contains redundant information 24
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- 26. Definition of DPCM…. If this redundancy is reduced, then overall bit rate will decrease and no of bits required to transmit one sample will also be reduced. 26
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- 29. In PCM - it transmits all the bits which are used to code the sample. Hence signaling rate and transmission channel BW are large. To overcome this problem Delta Modulation is used. 29
- 30. Delta Modulation is a technique which provides a staircase approximation to an over-sampled version of the message signal (analog input). sampling is at a rate higher than the Nyquist rate – aims at increasing the correlation between adjacent samples; simplifies quantizing of the encoded signal 30
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- 32. It transmits only one bit per sample i.e, the present sample value is compared with the previous sample(whether the amplitude is increased or decreased). Input signal m(t) is approximated to step signal, this step size is fixed. The difference between the x(t) and approximated signal produces 2 levels ±δ. 32
- 33. If the difference is +ve approximated signal is increased by one step (i.e., δ) 1 is transmitted If the difference is –ve approximated signal is reduced by one step size. 0 is transmitted 33
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- 35. Slope overload distortion Granular Noise /45
- 36. Staircase cannot approximate it , the step size δ is too small (because of m(t) is too large). Thus there is large error between the staircase approximated and original signal m(t). This error is called Slope overload distortion. To reduce this error , the step size should be increased when m(t) is increased. 36
- 37. Granular noise occur when δ is too large for small variation in m(t). The staircase signal keep on oscillating ±δ around the signal. To solve this problem to make step size small 37
- 38. 1. Large step-size is necessary to accommodate a wide dynamic range 2. Small step-size is required for accuracy with low-level signals compromise between slope overhead and granular noise adaptive delta modulation, where the step size is made to vary with the input signal 38/ 45
- 39. Slope overload distortion – δ is too small Granular noise - δ is too large ( since DM has fixed δ size) Need variable step size….. ADAPTIVE Delta Modulation (ADM) 39
- 40. When the input is varying slowly, the step size is reduced. Then this method is called ADM. It takes continuous changes in step size. δ is automatically varied according to variation in input of ADM The receiver must be able to adapt step sizes in exactly the same manner as the transmitter 40
- 41. Using logic for step size control. The δ is increased or decreased according to certain rule depending on one bit quantizer output. ◦ If one bit quantizer output is high (1) then δ may doubled for next samples. ◦ If one bit quantizer output is low (0) then δ may decreased by one step. 41
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