Nyquist criterion

In addition to the noise already mentioned in previous sections, another effect that appears in the digital channel is distortion. Given that the channel has a bandwidth, that is, it lets a limited number of frequency components of a signal pass through it unmodified, any pulse that passes through it will be broadened. This broadening is not noticable if the bandwidth of the channel is much greater than the bandwidth of the pulse spectrum. If both bandwidths are comparable, however, the pulse at the output of the channel will be noticably broadened. This means that when looking at a series of consecutive pulses, each of them will overlap with the ones next to it, and this effect provokes errors (a “0” may be interpreted as a “1”). In short, this will increase the probability of errors in the bit and lower the transmission quality.