The definitions of terms used in the present document can be found in GSM 06.20 [3], GSM 06.41 [4], GSM 05.03 [2].

For the purposes of the present document, the following abbreviations apply: BFI BSS DTX GSM LPC MS RSS RX SID UFI For abbreviations not given in this clause, see GSM 01.04 [1].

A cleared BFI flag (BFI= "0") and a set UFI flag (UFI="1") indicate a probably erroneous speech or SID frame. To improve the subjective quality, the probability of decoding erroneous frames shall be decreased by additional error detection which is based on both the exploitation of the frame parameters' properties and the decoder output signal's properties.

A set BFI flag (BFI="1") indicates a lost speech or SID frame. Normal decoding of these frames would result in a degradation of the subjective quality of the speech. To improve the subjective quality of the speech, the frame parameters shall be appropriately modified prior to the execution of the speech decoder functions.

With the parameter R0, the average signal energy is transmitted. Except at the beginning or the end of a talk spurt, this parameter shows a smooth behaviour from frame to frame. For error detection purposes, the difference of the R0 value between the actual and the last good frame is computed. If this difference exceeds a level dependent threshold and the frame is marked as unreliable, it is declared unusable by setting the BFI flag equal to "1".

In the speech decoder, the output signal is created by processing an excitation sequence through an LPC synthesis filter. The output signal energy of each subframe is calculated and compared to the output signal energy of the previous subframes. If the difference exceeds a level dependent threshold, the excitation sequence is attenuated such that the output signal energy corresponds to the output signal energy of the previous subframes.

For the half rate speech decoder, a possible substitution and muting strategy is based on an 8 state machine. The state, with the exception of states 6 and 7, indicates how many consecutive frames were lost. For example, state 5 indicates that 5 consecutive frames (including the current frame) were lost. State 7 is the initial state of the system, so that if the first decoded frame is lost, the frame is muted. Usually, the system will operate in state 0. The state machine remains in this state unless a frame is lost. On each successive lost speech frame, the state machine moves to the next higher numbered state. As soon as a frame is not declared lost, the machine returns to state 0 (unless it is in state 6). The machine remains in state 6 in the case of additional lost speech frames. If the machine is in state 6, a single frame without detected errors moves the machine to state 7 (i.e., two successive frames with no detected errors are needed to return to state 0 from state 6). This provides additional protection during prolonged intervals of very poor channel conditions which might cause false indications of valid speech data. If the machine is in state 7 and a lost speech frame is received, the machine returns to state 6. In each state, the following occurs: State 0: No error is detected. The received decoded speech data is output. The current frame parameters are stored. State 1: State 2: State 3: State 4: State 5: State 6: State 7:

The first and second lost SID frame is replaced by the last good SID frame. For subsequent lost SID frames the last good SID frame is repeated, but the frame energy R0 (coded on the interval from 0 to 31) is decreased with a constant value of 2 in each frame down to R0 = 0. This value is maintained if additional lost SID frames occur.