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Certain special features must be taken into account in the design of a servo system which is to operate satisfactorily with digital data. In particular, the data are quantized in amplitude, thereby introducing non-linear effects; they constitute a series of samples instead of a continuous function, and there may be significant delays due to low-speed transmission circuits or digital equipment or both. All of these contribute in various ways towards reducing the stability of the system. The method of synthesis takes full account of the sampling and delay features and enables a degree of prediction to be incorporated which ensures that the regenerated data do not lag on the original under steady-state conditions. Effects especially attributable to amplitude quantization are then studied qualitatively. Since stability diminishes and the sensitivity to noise and instrumental errors increases as the time of prediction is increased, it is essential that the delay involved in the transmission of each sample should be minimized. Furthermore, the sampling servo system is less efficient than a continuous one as a smoothing device, and as much smoothing as possible should be applied at the sending end before sampling.