Part B – Decoding the PCM data
14. Return the scope’s Slope control to the “+” position.
15. Set the scope’s Mode control to CH1 position.
16. Modify the set-up as shown in Figure 14 below.
Figure 14
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This entire set-up can be represented by the block diagram in Figure 15 below. Notice that the
decoder’s clock and frame synchronisation information are “stolen” from the encoder.
Figure 15
17. Adjust the scope’s Timebase control to view two or so cycles of the message.
18. Set the scope’s Mode control to the DUAL position to view the PCM Decoder
module’s output as well as the message signal.
Question 16
What does the PCM Decoder’s “stepped” output tell you about the type of signal that it is? Tip:
If you’re not sure, see the preliminary discussion for this experiment or for Experiment 4-04.
The PCM Decoder module’s output signal looks very similar to the message. However, they’re
not the same. Remember that a sampled message contains many sinewaves in addition to the
message. This can be better appreciated if you compare the message and the PCM Decoder
module’s output by listening to them.
19. Add the Buffer module to the set-up as shown in Figure 16 below leaving the
scope’s connections as they are.
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Figure 16
20. Turn the Buffer module’s Gain control fully anti-clockwise.
21. Without wearing the headphones, plug them into the Buffer module’s headphone
socket.
22. Put the headphones on.
23. Turn the Buffer module’s Gain control clockwise until you can comfortably hear the
PCM Decoder module’s output.
24. Disconnect the Buffer module’s lead where it plugs to the PCM Decoder module’s
output.
25. Modify the set-up as shown in Figure 17 below, again leaving the scope’s
connections as they are.
Figure 17
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26. Compare the sound of the two signals. You should notice that they’re similar but
clearly different.
Question 17
What must be done to the PCM Decoder module’s output to reconstruct the message properly?
Part C – Encoding and decoding speech
So far, this experiment has encoded and decoded a sinewave for the message. The next part of
the experiment lets you do the same with speech.
27. Completely remove the Buffer module from the set-up while leaving the rest of the
leads in place.
28. Disconnect the plugs to the VCO module’s SINE output.
29. Modify the set-up as shown in Figure 18 below.
Figure 18
30. Talk, sing or hum while watching the scope’s display.
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Part D – Recovering the message
As mentioned earlier, the message can be reconstructed from the PCM Decoder module’s
output signal using a low-pass filter. This part of the experiment lets you do this.
31. Locate the Tuneable Low-pass Filter module and set its Gain control to about the
middle of its travel.
32. Turn the Tuneable Low-pass Filter module’s Cut-off Frequency Adjust control fully
anti-clockwise.
33. Disconnect the plugs to the Speech module’s output.
34. Modify the set-up shown in Figure 19 below.
Figure 19
The entire set-up can be represented by the block diagram in Figure 20. The Tuneable Lowpass
Filter module is used to reconstruct the original message from the PCM Decoder module’s
PAM output.
35. Slowly turn the Tuneable Low-pass Filter module’s Cut-off Frequency control
clockwise and stop the moment the message signal has been reconstructed
(ignoring phase shift).
The two signals are clearly the same so let’s see what your hearing tells you.
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Figure 20
36. Add the Buffer module to the set-up as shown in Figure 21 below leaving the
scope’s connections as they are.
Figure 21
37. Turn the Buffer module’s Gain control fully anti-clockwise.
38. Put the headphones on.
39. Turn the Buffer module’s Gain control clockwise until you can comfortably hear the
Tuneable Low-pass Filter module’s output.
40. Disconnect the Buffer module’s lead where it plugs to the PCM Decoder module’s
output and connect it to the VCO module’s output (like you did when wiring Figure 17).
41. Compare the sound of the two signals. You should find that they’re very similar.
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Question 18
Even though the two signals look and sound the same, why isn’t the reconstructed message a
perfect copy of the original message? Tip: If you’re not sure, see the preliminary discussion for
PCM modulation part.
Virasit, Sa-Nguan and Jirun, (2009).