The signals from various sensors are sampled sequentially and the samples are converted to subtable formats, in the time-division multiplexing scheme. The process of scanning produces a train of pulses, with the samples corresponding to a particular sensor repeating at regular intervals. Each sensor output thus receives a time slot in the time pulse sequence. The individual amplitudes corresponding to the sample values can be sent as such or after further conversion. In pulse amplitude modulation (PAM) the sample values of the sensors are directly sent through the transmission medium to the receiver without any further change. Due to the susceptibility of the signal to get affected by the noise in the medium, this method is seldom employed in practice.
The amplitude information in PAM can be converted to equivalent pulse durations. This is referred to as pulse duration modulation (PDM). Here all the pulses start at the name relative point, but the duration of the pulse is proportional to the sampled value of the signal. PDM since most of the disturbances affects the pulse height rather than its duration Nevertheless, any distortion of the pulse shape will be misrepresented in the reconstructed display. The problem becomes severe at high sampling rates since the inductance and capacitance of the transmission line can introduce severe distortion of the pulse duration of the signal. In pulse position modulation (PPM), two pulses corresponding to the beginning and end of the duration are transmitted This scheme is superior to PAM and PDM since it is not susceptible to amplitude or shape distortion.
However, the spikes in noisy lines can seriously affect the reconstruction scheme. In addition, the bandwidth required for transmission of the short duration pulses is also higher For higher immunity against interferences, the pulse duration can be coded and the codes corresponding to each sample can then be far less susceptible to noise than PAM be sent over to the receiver This scheme is called pulse code modulation (PCM). The PCM represents the most efficient and the most practical coding technique today. PCM however requires a wide bandwidth as required for PPM. Nevertheless, the position of the pulses is not so critical and hence PCM is one of the most popular schemes used for the digital transmission of analog data.