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- 1. Muhammad Uzair Rasheed ROLL# 2009-CPE-03 UCE&T BZU MULTAN
- 2. PAM(Pulse Amplitude Modulation)PCM(Pulse Code Modulation)
- 3. Pulse ModulationAnalogue modulated systems are quite widely used, becauseof their simplicity.An alternative to analogue modulated systems is Pulsedsystems.This system is based on digital signals or pulses.The basis of such a system is the use of a digital carriersignal, which is modulated by an analogue signal.There are various ways in which this can be achieved, givingrise different systems.
- 4. Definition PCM is essentially analog to digital conversion of a signal type where the information contained in the instantaneous samples of an analog signal is represented by digital words in a serial bit stream Analog signal is first sampled at a rate higher than Nyquist rate, and then samples are quantized Uniform PCM : Equal quantization interval Nonuniform PCM : Unequal quantization interval
- 5. 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
- 6. Pulse Code ModulationPCM is the most commonly used technique in digitalcommunicationsUsed in many applications: Telephone systems Digital audio recording CD laser disks voice mail digital video etc.They are a primary building block for advancedcommunication systems
- 7. PCM requires much wider bandwidth But, Inexpensive digital circuitry PCM signal from analog sources(audio, video, etc.) may be merged with data signals(from digital computer) and transmitted over a common high-speed digital communication system (This is TDM)
- 8. Uniform quantizer Error signalsWaveform of signals PCM signal PCM word
- 9. 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.
- 10. Analog LPF Analog signal Samplersignal BW=B & Hold Quantizer Encoder PCM Quantized No. of levels=M signal PAM signal Channel, Telephone lines with regenerative repeater Reconstruction Decoder PCM LPF Quantized signal PAM signal Analog Signal output
- 11. Uniform distribution Let M=2n is large enough Xmax ∆=2Xmax/M x ) xi Distortion ∆2 Di = 12 M M ∆2 D = ∑ Di = i =1 12 -Xmax x -∆/2 ∆/2 x ) xi
- 12. Distortion 2 xmax 2 ( ) ∆ 2 2 xmax 2 xmax 2 xmax D= = M = = = 12 12 3M 2 3(2n ) 2 3(4n ) SQNR Let normalized input : E[ X 2 ] X = xmax E[ X 2 ] 3M 2 E[ X 2 ] 3(4 n ) E[ X 2 ] SQNR = = = = 3(4 n ) X 2 D xmax xmax SQNR dB = 10 log10 SQNR ≈ 4.77 + 6.02n + 10 log10 X 2 SQNR dB _ pk ≈ 4.77 + 6.02n
- 13. Hard to analyze because PCM is nonlinear Bandwidth of PCM If sinc function is used to generate PCM R = , nf s 1 1 where R is bit rate BPCM ≥ 2 2 If rectangular pulse is used , first BPCM = R = nf s null bandwidth If fs=2B (Nyquist sampling rate)
- 14. Quantizer n bits Bandwidth SQNR| dB_PKLevel, M M=2 n >nB 4.8+6n2 1 2B 10.84 2 4B 16.88 3 6B 22.816 4 8B 28.932 5 10B 34.964 6 12B 40.9128 7 14B 46.9256 8 16B 52.9512 9 18B 59.01024 10 20B 65.02048 11 22B 71.04096 12 24B 77.08192 13 26B 83.016384 14 28B 89.132768 15 30B 95.165536 16 32B 101.1
- 15. Telephone communication Voice frequency : 300 ~ 3400Hz Minimum sampling frequency = 2 x 3.4KHz = 6.8KHz In US, fs = 8KHz is standard Encoding with 7 information bits + 1 parity bit Bit rate of PCM : R = fs x n = 8K x 8 = 64 Kbits/s Required Bandwidth of PCM If sinc function is used: B > R/2 = 32KHz If rectangular is used: B = R = 64KHz SQNR|dB_PK = 46.9 dB (M = 27) Parity does not affect quantizing noise but decrease errors caused by channels
- 16. THANKS

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