1
play

1 LOGO LAB 4&5 PCM Modulator & Demodulator Block - PowerPoint PPT Presentation

Dec 20, 2022 •452 likes •1.03k views

1 LOGO LAB 4&5 PCM Modulator & Demodulator Block diagram of PCM modulation LPF is used to remove the noise in the audio signal 3 PCM modulation is a kind of source coding. is commonly used in audio and telephone transmission Source

  1. 1

  2. LOGO LAB 4&5 PCM Modulator & Demodulator

  3. Block diagram of PCM modulation LPF is used to remove the noise in the audio signal 3

  4. PCM modulation is a kind of source coding. is commonly used in audio and telephone transmission Source coding means the conversion from analog signal to digital signal. 4

  5. PCM in Wired Telephony Voice circuit bandwidth is 3400 Hz. Sampling rate is 8 KHz (T s =125  s). Each sample is quantized to one of 256 levels. Each quantized sample is coded into a 8-bit word. The 8-bit words are transmitted serially (one bit at a time) over a digital transmission channel. The bit rate is: bit sample bit   8 *8,000 64,000 sample sec sec

  6. The bits are regenerated at digital repeaters. The received words are decoded back to quantized samples, and filtered to reconstruct the analog signal. 6

  7. 7

  8. Parallel transmission

  9. Serial transmission

  10. Example of PCM: Voice & Audio Telephone voice CD Audio  F = 4 kHz →  F = 22 kHz → 8000 samples/sec 44000 samples/sec  8 bits/sample  16 bits/sample  R s =8 x 8000 =  R s =16 x 44000= 64 kbps 704 kbps per audio channel  High quality than telephone communication

  11. Quantization Non- Uniform Uniform 11

  12. Uniform Quantization  A system (with uniform quantization) would be wasteful for speech signals  Many of the quantizing steps would rarely be used.  The SNR is worse for low level signals than for high level signals. 12

  13. 13

  14. Nonuniform quantization  Example: Voice analog signal  Peak value(1V) is less appears while weak value(0.1V, 20dB down) around 0 is more appears (nonuniform amplitude distribution)  Thus nonuniform quantization is used  Implementation of nonuniform quantization PCM with Compression Analog PCM Uniform (Nonlinear) Input output filter Quantization

  15. ? 15

  16. Compression(Nonlinear) filter 16

  17. 17

  18. 18

  19. Nonuniform quantization • can provide fine quantization for the weak signals. • For telephone users with loud voices & quiet voices, quantisation noise will have same power(same q). • If SQNR made acceptable for quiet voices it may be better than necessary for loud voices. • Can be used to make the SNR a constant for all signals within the input range. 19

  20. Quantization Uniform Nonuniform The more steps (levels) the less quantization noise. Nonuniform quantization (e.g.  -law) allows a larger dynamic range (important for speech).

  21. Nonuniform quantization 21

  22. 22

  23. In uniform we assign as many reconstruction levels for larger amplitudes as for smaller amplitudes, which are more probable to occur. 23

  24. the histogram of the same speech signal after mu-law companding. 24

  25. Uniform Non-uniform 25

  26. Companding Companding = compression + expansion ADC with Expander Com- Uniform uniform pressor DAC quantiser Transmit or store Compressor: Compression filter in transmitter Expander: Inverse Compression filter in receiver 26

  27. Compression characteristic  A-law  µ -law

  28. DCS5-1 on ETEK DCS-6000-03 28

  29. Circuit diagram of PCM modulation 29

  30.  CW6694 is used as PCM modulator and PCM demodulator.  The sampler, quantizer and encoder are built in the IC  FS0(pin5) and FS1(pin7) are the data format selection of PCM encoder. pin5 pin7 30

  31. 31

  32. Buffer U1 is used to transfer a voltage from a first circuit, having a high output impedance level, to a second circuit with a low input impedance level. 32

  33. 33

  34. • Lab#5 PCM Modulator 34

  35.  Pulse wave modulation can be classified as :  pulse amplitude modulation (PAM)  pulse width modulation (PWM)  pulse position modulation (PPM)  pulse code modulation (PCM). - PAM, PWM, and PPM modulations belong to analog modulation and the PCM modulation belongs to the digital modulation - PAM, PWM, and PPM modulations are similar to AM, FM, and PM modulations, respectively . 35

  36. 36

  37. Block diagram of PCM demodulation 37

  38. 38

  39. LOGO 39

  40. 40

  41. 1  f  c 2 RC 41

  42.  f 1600 Hz c 42

  43. 1st-order Low Pass Filter Frequency Response of a 1st-order Low Pass Filter 43

  44. 1 st order LPF 1 1    f 1600 Hz    c 2 R C 2 (100)(1 ) 1 1 V R 10 k       o 2 Gain 1 1 2 V R 10 k 44 i 1

  45. 45

  46. 2.0V 1.0V 0V 1.0Hz 100Hz 10KHz 1.0MHz V(U1:OUT) V(U1:-) Frequency 46

  47. Second-order Low Pass Filter 47

  48. Frequency Response of a 2nd-order Low Pass Filter 48

  49. 1  f c  V R 2 R C R C    o 4 Gain 1 1 1 2 2 V R 1     i 3 (if R R & C C ) f  1 2 1 2 c 2 RC 49

  50. The High Pass Filter Circuit 50

  51. Frequency Response of a 1st Order High Pass Filter. 51

  52. Second-order High Pass Filter 52

  53. 53

  54. Inverting Integrator Configuration 54

  55. The RC Differentiator Circuit 55

  56. Inverting Differentiator Configuration 56

Recommend

More recommend