PCM Encoder’s output code PCM Encoder’s input voltage
11111111
00000000
EE 4-05: PCM modulation and demodulationPage 11 / 23
Question 11
Based on the information in Table 1, what is the maximum allowable amplitude (peak-to-peak)
for an AC signal on the PCM Encoder module’s INPUT?
Part C – Quantisation
This next part of the experiment lets you investigate quantisation.
32. Return the Variable DCV module’s Variable DC control to about the middle of its
travel.
33. See if you can vary the Variable DC control left and right without causing the output
code to change.
The sampled voltage can be changed without causing the output code to change because it is
compared to a set of quantisation levels but there are a finite number of them. This means that,
in practice, there’s a range of sample voltages for each quantisation level.
Question 12
What’s the name for difference between a sampled voltage and its closest quantisation level?
Tip: If you’re not sure, see the preliminary discussion.
It’s possible to work how far apart a PCM encoder’s quantisation levels are using the information
you’ve gathered so far. To do so, answer the following question.
Question 13
Calculate the difference between the quantisation levels in the PCM Encoder module by
subtracting the values in Table 1 and dividing the number by 256 (the number of codes).
Question 14
To reduce quantisation error it’s better to have
Fewer quantisation levels between ±2V.
More quantisation levels between ±2V.
EE 4-05: PCM modulation and demodulationPage 12 / 23
Part D – PCM encoding of continuously changing voltages
Now let’s see what happens when the PCM encoder is used to convert continuously changing
signals like a sinewave.
34. Return the scope’s Trigger Source control to the CH1 (or INT) position.
35. Return the scope’s Trigger Source Coupling control to the AC position.
36. Set the scope’s Channel 1 and Channel 2 Vertical Attenuation controls to the 2V/div
position.
37. Locate the VCO module and set its Range control to the HI position.
38. Turn the VCO module’s Frequency Adjust control fully anti-clockwise.
Note: The VCO module will be used to provide the PCM Encoder module with a 50kHz (approx)
clock.
39. Disassemble the current set-up.
40. Connect the set-up as shown in Figure 9 below.
Figure 9
41. Set the scope’s Timebase control to the 50 m s/div position.
42. Watch the PCM Encoder module’s PCM DATA output on the scope’s display.
Question 15
Why does the PCM DATA change continuously?
EE 4-05: PCM modulation and demodulationPage 13 / 23
43. Return the scope’s Variable Sweep control to the detent (locked) position.
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PCM decoding
Preliminary discussion
The previous experiment introduced you to the basics of pulse code modulation (PCM) which
you’ll recall is a system for converting message signals to a continuous serial stream of binary
numbers (encoding). Recovering the message from the serial stream of binary numbers is
called decoding.
At its simplest, decoding involves:
• Identifying each new frame in the data stream.
• Extracting the binary numbers from each frame.
• Generating a voltage that is proportional to the binary number.
• Holding the voltage on the output until the next frame has been decoded (forming a
pulse amplitude modulation (PAM) version of the original message signal).
• Reconstructing the message by passing the PAM signal through a low-pass filter.
The PCM decoder’s clock frequency is crucial to the correct operation of simple decoding
systems. If it’s not the same frequency as the encoder’s clock, some of the transmitted bits are
read twice while others are completely missed. This results in some of the transmitted numbers
being incorrectly interpreted, which in turn causes the PCM decoder to output an incorrect
voltage. The error is audible if it occurs often enough. Some decoders manage this issue by
being able to “self-clock”.
There is another issue crucial to PCM decoding. The decoder must be able to detect
the beginning of each frame. If this isn’t done correctly, every number is incorrectly, every
number is incorrectly interpreted. The synchronising of the frames can be managed in one of
two ways. The PCM encoder can generate a special frame synchronisation signal that can be
used the decoder though this has the disadvantage of needing an additional signal to be sent.
EE 4-05: PCM modulation and demodulationPage 14 / 23
Alternatively, a frame synchronisation code can be embedded in the serial data stream that is
used by the decoder to work out when the frame starts.