Abstract: A digital filter 23 receives a control signal which specifies what filter coefficient values should be used. The control signal can be changed repeatedly so as to dynamically select the coefficients in response to the value of a control parameter. The filter 23 can be configured as a data rate converting filter in which the control signal represents the current phase relationship between the input data stream and the output data stream. The phase control signal can be provided as the phase output of a numerically controlled oscillator 67 having an oscillation frequency which is a multiple of one of the data rates and which is clocked at a multiple of the frequency of the other data rate. The data rate converter can be used as part of a data modulator or demodulator, thereby allowing an analog-to-digital converter 19 or a digital-to-analog converter 143 of the modulator or demodulator to run at a fixed arbitrary frequency substantially regardless of the data rate of modulated symbols.

Abstract: A filter, in which digital representations of a signal are stored as samples in a store (8) preferably in the order of their occurrence. Multipliers (9) multiply the samples by coefficients (C1 to C36) representing a function with which the samples are to be convolved. To compensate for the variable time delays in the signal, sets of coefficients representing the function with different time delays are stored. The set to be convolved with the signal is chosen in dependence upon the time delay of the signal. A signal indicative of the time delay is derived from an input to the shift register and fed-forward. The filter is preferably used in a digital demodulator of phase and/or amplitude modulated signals. The function is then an impulse function defining (at least in part) a desired frequency response of the filter, and the filter corrects the timing of symbols represented by the signal.

Abstract: A Viterbi decoder is provided with synchronization error detector which detects synchronization errors by monitoring the growth of the path metric associated with a state having itself as a predecessor and to indicate synchronization errors independently of the behavior of metrics associated with other states. The difference between the integral of the selected metric over a certain period of time and the sum of the absolute values of the input encoded data signal is determined and compared to a threshold to indicate whether or not a synchronization error exists.