Practical audio interfacing
Unlike analog audio, digital audio has a discrete-time structure, because it is a sampled signal in which the samples may be further grouped into frames and blocks having a certain time duration. If digital audio devices are to communicate with each other, or if digital signals are to be combined in any way, then they need to be synchronized to a common reference in order that the sampling frequencies of the devices are identical and do not drift with relation to each other. It is not enough for two devices to be running at nominally the same sampling frequency (say both at 44.1 kHz). Between the sampling clocks of professional audio equipment it is possible for differences in frequency of up to 10 parts per million (ppm) to exist and even a very slow drift means that two devices are not truly synchronous. Consumer devices can exhibit an even greater range of sampling frequencies that are nominally the same.