is aperiodic v 0 the contribution of the adaptive codebook is set to zero this is accomplished by switch 45 which switches states in the direction of the arrow from its normal biased operating position coupling amplifier 55 to summer 85 to a position which decouples the adaptive codebook contribution from the excitation signal u n this switching of state is accomplished in accordance with the control signal developed by and gate 75 which tests for the condition that the frame is erased e 1 and it was an aperiodic frame not v 1 on the other hand the contribution of the fixed codebook is maintained in its normal operating position by switch 42 since e 1 but v 0 the fixed codebook index i and codebook vector sign are not available do to the erasure in order to synthesize a fixed codebook index and sign index from which a codebook vector c n could be determined a random number generator 45 is used the output of the random number generator 45 is coupled to the fixed codebook 10 through switch 40 switch 40 is normally is a state which couples index i and sign information to the fixed codebook however gate 47 applies a control signal to the switch which causes the switch to change state when an erasure occurs of an aperiodic frame e 1 and not v 1 to generate the fixed codebook index and sign the initial seed value for the generator 45 is equal to 21845 for a given coder subframe the codebook index is the 13 least significant bits of the random number the random sign is the 4 least significant bits of the next random number thus the random number.