5 Requirements
3GPP46.021Half rate speechRelease 17Substitution and muting of lost frames for half rate speech traffic channelsTS
5.1 Error detection and concealment in case of unreliable speech or SID frames
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.
5.1.1 Error detection
By investigating the frame parameter properties, it shall be decided whether the frame is to be considered as usable or unusable. In the latter case, the BFI flag is set and substitution and muting is performed (clause 5.2). Clause 6.1 gives an example solution for error detection in case of unreliable frames.
5.1.2 Output signal concealment
If the frame is considered as usable, properties of the decoder output signal shall be compared to the corresponding signal properties of the previous valid frames. In case of large differences, the output signal shall be modified such that these differences are limited. Clause 6.2 gives an example solution for output signal concealment in case of unreliable frames.
5.2 Frame substitution and muting in case of lost speech or SID frames
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.
5.2.1 First and second lost speech frame
The first and second lost speech frame shall be partly or completely substituted with the last valid speech frame. For the first and second lost speech frame, the output shall not be muted directly.
5.2.2 Subsequent lost speech frames
For subsequent lost speech frames, a muting technique shall be used that will gradually decrease the output level, resulting in silencing of the output after a maximum of 80 ms. For subsequent lost speech frames, the muting of the output shall be maintained. Clause 6.3 gives an example of such a method.
5.2.3 First and second lost SID frame
The first and second lost SID frame shall be substituted with the last good SID frame. For the first and second lost SID frame, the output shall not be muted directly.
5.2.4 Subsequent lost SID frames
For subsequent lost SID frames, a muting technique shall be used that will gradually decrease the output level, resulting in silencing of the output after a maximum of 320 ms. For subsequent lost SID frames, the muting of the output shall be maintained. Clause 6.4 gives an example of such a method.