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Pulse Code Modulation (PCM) : Modulating (Analog) Signal

Pulse code modulation (PCM) works by sampling an analog signal, quantizing the samples, and encoding the values as binary numbers. The document describes a PCM board that uses an internal 33 kHz clock to generate a pulse train carrier signal. An analog modulating signal is converted to digital by an A/D converter then represented as parallel bits shown on LEDs. These are converted to a serial bitstream which is reconverted to analog by a DAC. Testing showed the output approximates the input signal, with more noise at higher frequencies, which the low-pass filter and demodulation control can reduce.

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142 views5 pages

Pulse Code Modulation (PCM) : Modulating (Analog) Signal

Pulse code modulation (PCM) works by sampling an analog signal, quantizing the samples, and encoding the values as binary numbers. The document describes a PCM board that uses an internal 33 kHz clock to generate a pulse train carrier signal. An analog modulating signal is converted to digital by an A/D converter then represented as parallel bits shown on LEDs. These are converted to a serial bitstream which is reconverted to analog by a DAC. Testing showed the output approximates the input signal, with more noise at higher frequencies, which the low-pass filter and demodulation control can reduce.

Uploaded by

Mohamd barca
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Pulse Code Modulation (PCM)

Like any modulation circuit, the PCM has:

Modulating (Analog) signal: it is the signal to be modulated, and we


can generate it form "MODULATING SIGNAL GENRATOR" block. In our
board, it has a fixed amplitude and variable frequency (15-300 Hz) which
we can control.

Carrier Signal (pulse train): In the PCM board, the pulse train carrier
signal will be generated internally with both fixed amplitude and
frequency; form the "TIMING & CONTROL" block that in turn takes this
signal form "33 KHz CLOCK GENERATOR" block, so the frequency of
pulse train is 33 KHz.

Modulating signal (PCM): After both the pulse train and analog
signal enter the modulator, the PCM signal will be generated. The PCM
signal is a serial of bits, each 8-bit represent one sample.
 Each sample bits is represented in the LED's of the modulator.

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PCM Modulator: the modulator in our board consist of the following
parts:
1. A/D: to convert the analog signal to digital:
Analog input LPF  S/H  Quantizer  Encoding  Parallel digital

2. LED's: To represent the parallel bits (the bits of each sample of the
analog input).
3. Parallel to serial converter

 Because the analog sampled signal is very fast we cannot notice


the representation of each sample in the LED's, so for that we are
going to use a variable DC voltage to represent different samples
in state of the actual analog signal.

PCM Demodulator: the demodulator consist of the ( serial to parallel


convertor, Buffer, LED's, D/A and LPF) to regenerate the analog input.

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The Procedure:
1. Connect the clock generator O/P to I/P of the Timing and control
block.
2. Connect the variable DC output to the analog input of the modulator
( we connected the DC in state of the analog input to see the
representation of each bit)
 Try connecting the analog input and you will see the LED's
changing very fast.
3. Vary the DC voltage form (-3 to 3) and record the LED's state and
draw the digital output for each step.
It should look like this:

4. Now connect remove the DC connection and connect the modulating


signal with f=200 Hz to the analog input of the modulator. And the
output of the modulator to the input of the demodulator.
5. Draw the signal before the and after the LPF and compare each with
the original modulating input.
 The signal before the LPF (the output of the D/A circuit) is a
staircase representation of modulating signal. When decreasing
the frequency of the modulating signal this signal approach the
original modulating signal (but it stay in the stare case shape).

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 The signal after the LPF is the regeneration of the original
modulating signal but with noise. When decreasing the frequency
of the modulating signal the noise will decrease and the signal
approach the original one.

Before LPF After LPF

 The demodulation fine control (in the demodulated circuit) can


also reduce the noise of the demodulated signal.

Notes:
 In our experiment, we used 8-bits to represent each sample (we
can see that form the number of LED's in the modulator) which
mean there is 28 number of different sample level but we only
took seven (-3 to 3).
 In this experiment, to reduce the quantization error we need to
represent each sample with more bits, but that will increase the
bandwidth of our signal.

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