a large value indicates a good reliability large probability of a correct bit decision whereas a small value indicates a poor reliability small probability of a correct bit decision a soft bit mentioned above is composed of a hard bit i e 1 or 0 and the minimum metric information reliability thus the minimum metric information can be extracted from a soft bit by taking the absolute value of it the classifier cl_ 1 of the current speech frame can be the mean value i e the mean signal strength of the smallest 1 a soft bits of course also only the value i e signal strength of the lowermost class 1 a bit can be taken as the classifier however a mean value gives a more reliable result in step s 2 classifiers cl_ 1 to cl_n of n previous speech frames are acquired from a memory then in step s 3 the mean value m of the previous classifiers cl_ 1 to cl_n is calculated in step s 4 the standard deviation u03c3 of the previous classifiers cl_ 1 to cl_n is calculated in step s 5 from the calculated mean value m and standard deviation u03c3 of the last n classifiers a rejection threshold th is calculated according to the following formula the values a and b are constants and can be chosen in accordance with a particular system thus the threshold th calculated in this manner depends on the current speech frame and also on previous speech frames hence the history of the speech frame transmission on this particular radio channel is considered in step s 6 the thus calculated threshold th is used to check whether the current speech frame is satisfactory or not i e whether.