A signal is transmitted to a reciever and is digitalised in signal points (S1). These points are viterbi analysed in an analyser with a path memory (PM) having a desired number of states (00, 01, 10, 11) with memory cells (MC). Bit sequences, corresponding to the signal points (S1), are generated and final metric values (m0, m1 + g1, m1, m3) for the bit sequences are calculated. In an indicated bit position the best bit sequence, with the smallest final metric value (m2), has a "1", which is the decied bit value. An alternative bit sequence is generated, which is the best bit sequence with the smallest final metric value (m0 + g0) under the condition that in the indicated bit position there is a bit with the opposite bit value, a "0". The difference between both the final metric values (m0 + g0 - m2) is a quality factor for the decided bit value "1". The calculation is made in three steps. At a time point metric values (m0, m1, m2, m3) have been calculated for the different states (00, 01, 10, 11). For a signal point in the indicated bit position, additive metric values ( (0-0), (1-0)) are calculated in the first step. From the additive metric values are calculated new metric values and part values of the quality factor. In the second step the part values are modified for the calculation steps through the path memory (PM), so that final part values (g0, g1, g2, g3) of the quality factor are obtained. In the third step bit sequences are selected as above and the quality factor (m0+g0-m2) is calculated.
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