EE 4-05: PCM modulation and demodulation Page 1 / 23
KHON KAEN UNIVERSITY
DEPARTMENT OF ELECTRICAL ENGINEERING
Experiment EE4 - 05: Pulse Code Modulation and Demodulation
PCM Modulation
Preliminary discussion
As you know, digital transmission systems are steadily replacing analog systems in commercial
communications applications. This is especially true in telecommunications. That being the
case, an understanding of digital transmission systems is crucial for technical people in the
communications and telecommunications industries. This experiment will let you explore pulse
code modulation (PCM).
PCM is a system for converting analog message signals to a serial stream of 0s and 1s.
The conversion process is called encoding. At its simplest, encoding involves:
• Sampling the analog signal’s voltage at regular interval using a sample-and-hold
scheme (demonstrated in Experiment 4-04).
• Comparing each sample to a set of reference voltages called quantisation levels.
• Deciding which quantisation level the sampled voltage is closest to.
• Generating the binary number for that quantisation level.
• Outputting the binary number one bit at a time (that is, in serial form).
• Taking the next sample and repeating the process.
An issue that is crucial to the performance of the PCM system is the encoder’s clock frequency.
The clock tells the PCM encoder when to sample and, as the previous experiment shows, this
must be at least twice the message frequency to avoid aliasing (or, if the message contains
more than one sinewave, at least twice its highest frequency).
Another important PCM performance issue relates to the difference between the sample
voltage and the quantisation levels that it is compared to. To explain, most sampled voltages
will not be the same as any of the quantisation levels. As mentioned above, the PCM Encoder
assigns to the sample the quantisation level that is closest to it. However, in the process, the
original sample’s value is lost and the difference is known as quantisation error. Importantly, the
EE 4-05: PCM modulation and demodulation Page 2 / 23
error is reproduced when the PCM data is decoded by the receiver because there is no way for
the receiver to know what the original sample voltage was. The size of the error is affected by
the number of quantisation levels. The more quantisation levels there are (for a given range of
sample voltages) the closer they are together and the smaller the difference between them and
the samples.