communication system lec4
- 2. 2 review Signals Complex Waves and Harmonics Sine Wave Square Wave Saw tooth Wave Conversion/Signal Formatting Character Coding
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- 4. 4 Formatting • Character coding • Sampling • Quantization • Pulse Code Modulation(PCM)
- 5. Formatting and Transmission of baseband signal 5
- 6. 6 Input Signal 1. Textual 2. Digital 3. Analog
- 8. 1. Digital to Digital conversion • done
- 9. 2. Analog to Digital Conversion (Codec) First Step: Pulse Amplitude Modulation (PAM) • According to Nyquist theorem, the sampling rate must be at least two times the frequency to ensure the accurate reproduction of the original signal. K. Salah
- 10. Second step: Quantized PAM Signal Quantizing Using Sign and Magnitude K. Salah
- 11. Third Step: Pulse Code Modulation (PCM) Remember this Out of each sample, Telephone companies only uses the upper 7 bits. The lower bit (bit0) is always assumed to be 0. For example, samplingvalues: +024 +024, +038 +038, +025 +024, +039 +038 This is not affecting sampling values much. In transmission, the most significant bit is used for control purposes, as we will see later. K. Salah
- 12. From Analog to PCM digital signal K. Salah
- 13. Sample Theorem Need for sampling: In analog signal, distortion is produced, and it is difficult to recover the signal, However it is easy to do so in digital signal. 13
- 16. 16 Impulse Sampling Use of Impulse: At a particular point, we get a sample. When any function is multiplied by impulse function at t=xsec, we get a sample of that function at x sec. If we want to recover complete signal we have to multiply by impulse train.
- 18. Impulse sampling : Using convolution property 18
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- 24. Sampling Theorem using frequency shifting property of Fourier Transform 24
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- 26. Under and over sampling 26
- 27. Over and under sampling 27
- 28. Aliasing Aliasing refers to an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled. It also refers to the distortion that results when the signal reconstructed from samples is different from the original continuous signal. 28
- 29. Aliasing 29
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- 33. Converting bit into samples • Quantizing • Similar concept to pixelization. • Breaks wave into pieces, assigns a value in a particular range • 8-bit range allows for 256 possible sample levels • More bits means greater detail, fewer bits means less detail
- 34. Quantization/Digitizing/PCM Uniform Quantization Non uniform Quantization 34
- 35. Analog to Digital Conversion (Sampling) 35
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- 39. PCM
- 40. PCM
- 41. PCM
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- 47. Companding 47
- 49. PCM PCM uses a sampling rate of 8000 samples per second. Each sample is an 8 bit sample resulting in a digital rate of 64,000 bps (8 x 8000).
- 50. 50 • n=number of pulses(bit) • L=M=number of levels=2^n • Step size= 2A/M • s0=m^2(t)
- 51. Quantization Error • Analog voice data must be translated into a series of binary digits before they can be transmitted. • With Pulse Code Modulation (PCM), the amplitude of the sound wave is sampled at regular intervals and translated into a binary number. • The difference between the original analog signal and the translated digital signal is called quantizing error.
- 52. 3. Digital to Analog Modulation (Modem) K. Salah
- 53. 4. Analog to analog Conversion
- 56. Structure of Telephone System • End office, known also as local central office • Local loops (twisted pairs, analog signaling) • Trunks (fiber optics or microwave, mostly digital) • Intermediate switches K. Salah
- 57. Need for A/D and D/A conversions • Modem converts D/A signals • Codec converts A/D signals K. Salah
- 58. 58 Question of Lecture • What is the difference between time and frequency domain? • Any complex waveform is composed of? • A square wave is composed of? • A saw tooth wave is composed of? • Prove that sine wave is composed of main frequency and all harmonics using Fourier series? • Prove that square wave is composed of main frequency and all odd harmonics using Fourier series? • Read what is ASCII coding? • What is M-ary pulse modulation
- 59. Asssignment#1 Due Date: 21Feb, 2014 1. Prove through Fourier series that sine waveform is a fundamental waveform? 2. What is the harmonic composition of a square wave? 3. Draw and explain the basic communication system taking PSTN as an exemplary network? 4. What is m-ary pulse modulation? 59