(a) Suggest and explain the action of an ideal interpolation function for reconstruction of a sampled signal, assuming the minimum theoretical sampling rate has been employed. Explain how a practical interpolation function can minimise inter-symbol interference (ISI) in a binary signalling system, indicating how this function can be realised. Comment on how the definition of bandwidth in the associated channel response differs from that normally employed. (b) Sketch the general spectral diagram for the output signal from a frequency-shift-keyed (FSK) modem for a 200 baud, binary input. Indicate an acceptable transmission bandwidth on the spectral diagram and, with analogy to a sinusoidally modulated FM system, deduce an expression for the modulation index of the modem. Briefly explain why in data transmission over the public switched telephone network (PSTN): (i) phase-shift keying (PSK) is preferred to FSK at higher bit rates, a differential form of PSK is used, (iii) data rates are so restricted