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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.

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 64,000sec 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

Serial transmission

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

UniformNon-

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 nonuniformquantization

Compression

(Nonlinear)

filter

PCM with

Uniform

Quantization

Analog

InputPCM

output

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?

16Compression(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

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

or store

Uniform

DAC

Compressor: Compression filter in transmitterExpander: Inverse Compression filter in receiver

Companding= compression + expansion

Companding

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

2cf

RC

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1600 Hzcf

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Frequency Response of a 1st-order Low Pass Filter

1st-order Low Pass Filter

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1 1

1 11600

2 2 (100)(1 )cf Hz

R C

2

1

101 1 2

10

o

i

V R kGain

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 1 2 2

1 2 1 2

1

2

1(if & )

2

c

c

fR C R C

R R C C fRC

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3

1o

i

V RGain

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