Pulse Code Modulation:

Pulse Code Modulation (PCM) is different from the other forms of pulse modulations.

• Although PCM also uses sampling techniques, it differs from the others and that it is a digital process.
• Instead of sending a pulse train capable of continuously varying one of the parameters, the PCM generator produces a series of numbers or digits (hence the name digital process).
• Each one of these digits, almost always in a binary code, represents the approximate amplitude of the signal sample at that instant. The approximation can be made as close as desired.
• Thus Pulse-code modulation (PCM) is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a digital (usually binary) code.

Principle of operation:

• In PCM, the total amplitude range which the signal may occupy is divided into a number of standard levels, as shown in the figure.
• Since these levels are transmitted in binary code, the actual number of levels is a power of 2. 16 levels are shown here for simplicity, but practical systems use as many as 128.

• By a process called quantizing, the level actually sent by any sampling time is the nearest standard (as Quantum) level.

   

Modulation:

In the diagram, a sine wave (red curve) is sampled and quantized for PCM. The sine wave is sampled at regular intervals, shown as ticks on the x-axis. For each sample, one of the available values (ticks on the y-axis) is chosen by some algorithm.

This produces a fully discrete representation of the input signal that can be easily encoded as digital data for storage or manipulation.

For the sine wave example above, we can verify that the quantized values at the sampling moments are 9, 11, 13, 14, 15, 15, 14, 13, etc. Encoding these values as binary numbers would result in the following set of nibbles: 1001, 1011, 1101, 1110, 1111, 1111, 1110, 1101, etc. These digital values could then be further processed or analyzed by a purpose-specific digital signal processor or general purpose CPU.

Demodulation:

To produce output from the sampled data, the procedure of modulation is applied in reverse.

• After each sampling period has passed, the next value is read and the output of the system is shifted instantaneously (in an idealized system) to the new value.
• As a result of these instantaneous transitions, the discrete signal will have a significant amount of inherent high frequency energy, mostly harmonics of the sampling frequency.
• To smooth out the signal and remove these undesirable harmonics, the signal would be passed through analog filters that suppress artifacts outside the expected frequency.
• Some systems use digital filtering to remove the lowest and largest harmonics. In some systems, no explicit filtering is done at all; as it's impossible for any system to reproduce a signal with infinite bandwidth, inherent losses in the system compensate for the artifacts — or the system simply does not require much precision.
• The sampling theorem suggests that practical PCM devices, provided a sampling frequency that is sufficiently greater than that of the input signal, can operate without introducing significant distortions within their designed frequency bands.

• The electronics involved in producing an accurate analog signal from the discrete data are similar to those used for generating the digital signal. These devices are DACs (digital-to-analog converters), and operate similarly to ADCs. They produce on their output a voltage or current (depending on type) that represents the value presented on their inputs. This output would then generally be filtered and amplified for use.

   

Advantages of PCM technique:

1. It can easily carry a mixture of traffic, such as telephony, telegraphy, data and encoded video information, provided the medium has sufficient capacity.
2. It can increase the capacity of single telephone channels over cable pairs by multiplexing.
3. It can lend itself to such novel facilities as cryptography, storage and other forms of digital processing.
4. It is more suitable for the newer types of transmission media, such as light beams in optical fibers and multiple access satellites.
5. Its signal characteristics allow easy access to electronic switching in which groups of digits are selected to be switched in turn onto various highways.

Disadvantages of PCM technique:

1. Quantization noise
2. Aliasing error