In Pulse Time Modulation the signal is sampled in the same way as in PAM, but the pulses indicating instantaneous sample amplitudes have a constant amplitude themselves. However, one of their timing characteristics is varied, being directly proportional to the sampled signal amplitude at that instant.
The variable characteristics may be width, position or frequency of the pulses, so that three different types of PTM are possible.
Pulse frequency modulation has no significant practical applications, and is hence omitted.
The other two forms are discussed below.
All forms of Pulse Time Modulation have an advantage over PAMs in that the pulse amplitude remains constant, so that amplitude limiters can be used to provide a good degree of noise immunity.
The sampling theorem states the following.
If the sampling rate in any pulse modulation system exceeds twice the maximum signal frequency, the original can be reconstructed in the receiver with minimal distortion.
This theorem is used in practice to determine the minimum sampling speeds, e.g. pulse modulation used for speech modulation. Transmission is generally over standard telephone channels, so that the audio frequency range is 300 to 3400 Hz. For this application, a sampling rate of 8000 samples/s is almost a worldwide standard. This pulse rate is, as can be seen, more than twice the highest audio frequency. The sampling theorem is satisfied, and the resulting system is free from sampling error.
Pulse position or pulse width measurements are usually based on the leading or trailing edge of the pulse. Usually the uncertainty of the pulse position is stated in terms of the rise and fall time of the pulse. An analysis of typical low pass filter shows that the rise time tr is inversely proportional to the bandwidth. As a result, the band widths required for PDM and PPM are much greater than for PCM, where only the presence or absence of a pulse is of interest.