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

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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 coding means the conversion from analog signal to digital signal. 4

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

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

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Parallel transmission

Serial transmission

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

Quantization Non- Uniform Uniform 11

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

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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

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Compression(Nonlinear) filter 16

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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

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

Nonuniform quantization 21

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

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

Uniform Non-uniform 25

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

Compression characteristic  A-law  µ -law

DCS5-1 on ETEK DCS-6000-03 28

Circuit diagram of PCM modulation 29

 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

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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

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• Lab#5 PCM Modulator 34

 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

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Block diagram of PCM demodulation 37

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LOGO 39

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1  f  c 2 RC 41

 f 1600 Hz c 42

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

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

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

Second-order Low Pass Filter 47

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

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

The High Pass Filter Circuit 50

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

Second-order High Pass Filter 52

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Inverting Integrator Configuration 54

The RC Differentiator Circuit 55

Inverting Differentiator Configuration 56