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

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

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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

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LOGO

LAB 4&5 PCM Modulator & Demodulator

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Block diagram of PCM modulation

LPF is used to remove the noise in the audio signal

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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.

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PCM in Wired Telephony

Voice circuit bandwidth is 3400 Hz. Sampling rate is 8 KHz (Ts=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:

8 *8,000 64,000 sec sec bit sample bit sample  

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

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Parallel transmission

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Serial transmission

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Example of PCM: Voice & Audio

Telephone voice  F = 4 kHz → 8000 samples/sec  8 bits/sample  Rs=8 x 8000 = 64 kbps CD Audio  F= 22 kHz → 44000 samples/sec  16 bits/sample  Rs=16 x 44000= 704 kbps per audio channel

 High quality than telephone communication

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Quantization

Uniform Non- Uniform

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 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.

Uniform Quantization

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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

Compression (Nonlinear) filter

PCM with Uniform Quantization

Analog Input PCM

  • utput
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?

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Compression(Nonlinear) filter

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  • 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.

Nonuniform quantization

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Quantization

Uniform Nonuniform

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

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Nonuniform quantization

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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.

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

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Non-uniform Uniform

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Com- pressor ADC with uniform quantiser Expander Transmit

  • r store

Uniform DAC Compressor: Compression filter in transmitter Expander: Inverse Compression filter in receiver

Companding= compression + expansion

Companding

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A-law µ-law

Compression characteristic

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DCS5-1 on ETEK DCS-6000-03

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Circuit diagram of PCM modulation

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  • 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

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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.

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  • Lab#5 PCM Modulator
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  • 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.

Pulse wave modulation can be classified as :

  • pulse amplitude modulation (PAM)
  • pulse width modulation (PWM)
  • pulse position modulation (PPM)
  • pulse code modulation (PCM).
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Block diagram of PCM demodulation

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LOGO

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1 2

c

f RC  

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1600 Hz

c

f 

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Frequency Response of a 1st-order Low Pass Filter

1st-order Low Pass Filter

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1 1 1600 2 2 (100)(1 )

c

f Hz R C      

2 1

10 1 1 2 10

  • i

V R k Gain V R k      

1st order LPF

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

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Second-order Low Pass Filter

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Frequency Response of a 2nd-order Low Pass Filter

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1 2 1 (if & ) 2

c c

f R C R C R R C C f RC       

4 3

1

  • i

V R Gain V R   

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The High Pass Filter Circuit

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Frequency Response of a 1st Order High Pass Filter.

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Second-order High Pass Filter

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Inverting Integrator Configuration

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The RC Differentiator Circuit

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Inverting Differentiator Configuration