is in a decreasing trend if yes step 604 is performed otherwise step 605 is performed in the g 729 each frame is 10 ms long and can be divided into two 5 ms long sub frames it can be known whether the pitch period of the last good frame before the lost frame is in a decreasing trend by comparing lengths of pitch periods of two sub frames of the last good frame before the lost frame if the pitch periods of the two sub frames of the last good frame before the lost frame are identical the pitch period of the last good frame before the lost frame is considered in a decreasing trend in step 604 a value obtained by subtracting n u2212 1 sampling point durations from the pitch period t 0 of the last good sub frame before the lost frame serves as a pitch period tn of the current lost frame and then step 606 is performed in this step n is a frame number of the current lost frame in continual lost frames further an integer td 20 u2266 td u2266 143 is preset and it is determined whether n gt td if yes the pitch period tn of the current lost frame equals the pitch period t 0 of the last good frame minus td sampling point durations otherwise tn equals the pitch period t 0 of the last good sub frame before the lost frame minus n u2212 1 sampling point durations in step 605 a value obtained.