Radio 101: Advantages and applications of Pulse Code Modulation

The principal advantage of Pulse Code Modulation (PCM) is the noise immunity. But it is not used exclusively. Other pulse modulation systems are still used in spite of highly superior performance of PCM. The reasons are that firstly those systems came earlier secondly PCM needs very complex encoding and quantizing circuitry and thirdly PCM requires larger bandwidth as compared to analog systems.

In spite of these three implementations PCM is fast gaining popularity and is being used increasingly. The reasons are very simple. PCM no doubt requires much more complex modulating procedures than analog systems. But multiplexing equipment is much cheaper. Further distance between repeaters is large because PCM tolerates much worse signal to noise ratios. Finally advent of very large scale integration (VLSI) has reduced the cost of complex circuits needed in PCM. Regarding the increased bandwidth requirements by PCM the problems is no longer a serious one because of the advent of large band width fiber optic systems. PCM also finds use in space communications. Way back in 1965 PCM was used by mariners to transmit back pictures of Mars. Of course each picture took several minutes for transmission.

PCM was obviously the first digital system. However, today several others have come up and are being used occasionally. Few of them are differential PCM and delta modulation. Differential modulation is a PCM with the modification that each word in this system indicates the difference in amplitude, positive or negative, between this sample and the previous one. Thus this system indicates the relative rather than the absolute value of each sample. In this, therefore, the speech is redundant, since amplitude is related to the previous one and large variations from one sample to the next are unlikely. As a result fewer bits are needed to indicate the size of the magnitude change relative to the case of absolute magnitude. Smaller bandwidth is therefore needed for transmission. Differential PCM is not popularly used because increased complexity of encoding and decoding processes outweighs advantages and gained through its use.

Delta modulation is a digital modulation system which in its simplest form may be equated with the basic form of differential PCM. In a simple delta modulation system just one bit is sent per sample to indicate whether the signal is larger or smaller than the previous sample. This system has the merits of having extremely simple coding and decoding procedures. Further the quantizing process is also very simple. But it has the drawback that it can not easily handle rapid changes in magnitude and as a result quantizing noise tends to be quite high. Even on using compounding and modified and complex versions of delta modulation, the transmission race must be close to hundred kilo bits per second to give the same performance for a telephone channel as PCM provides with only sixty four kilo bits.

Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect XM Satellite Radio system for their needs.