Content for TS 26.091 Word version: 18.0.0
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1 Scope p. 5
The present document defines an error concealment procedure, also termed frame substitution and muting procedure, which shall be used by the AMR speech codec receiving end when one or more lost speech or lost Silence Descriptor (SID) frames are received.
The requirements of the present document are mandatory for implementation in all networks and User Equipment (UE)s capable of supporting the AMR speech codec. It is not mandatory to follow the bit exact implementation outlined in the present document and the corresponding C source code.
2 References p. 5
The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
3 Definitions and abbreviations p. 5
3.1 Definitions p. 5
For the purposes of the present document, the following terms and definitions apply:
N-point median operation:
consists of sorting the N elements belonging to the set for which the median operation is to be performed in an ascending order according to their values, and selecting the (int (N/2) + 1) -th largest value of the sorted set as the median value
Further definitions of terms used in the present document can be found in the references.
3.2 Abbreviations p. 5
For the purposes of the present document, the following abbreviations apply:
AN
Access Network
BFH
Bad Frame Handling
BFI
Bad Frame Indication from AN
BSI_netw
Bad Sub-block Indication obtained from AN interface CRC checks
CRC
Cyclic Redundancy Check
ECU
Error Concealment Unit
medianN
N-point median operation
PDFI
Potentially Degraded Frame Indication
prevBFI
Bad Frame Indication of previous frame
RX
Receive
SCR
Source Controlled Rate (operation)
SID
Silence Descriptor frame (Background descriptor)
4 General p. 6
The purpose of the error concealment procedure is to conceal the effect of lost AMR speech frames. The purpose of muting the output in the case of several lost frames is to indicate the breakdown of the channel to the user and to avoid generating possible annoying sounds as a result from the error concealment procedure.
The network shall indicate lost speech or lost SID frames by setting the RX_TYPE values [3] to SPEECH_BAD or SID_BAD. If these flags are set, the speech decoder shall perform parameter substitution to conceal errors.
The network should also indicate potentially degraded frames using the flag RX_TYPE value SPEECH_PROBABLY_DEGRADED. This flag may be derived from channel quality indicators. It may be used by the speech decoder selectively depending on the estimated signal type.
The example solutions provided in paragraphs 6 and 7 apply only to bad frame handling on a complete speech frame basis. Sub-frame based error concealment may be derived using similar methods.
5 Requirements p. 6
5.1 Error detection p. 6
If the most sensitive bits of the AMR speech data (class A in [4]) are received in error, the network shall indicate RX_TYPE = SPEECH_BAD in which case the BFI flag is set. If a SID frame is received in error, the network shall indicate RX_TYPE = SID_BAD in which case the BFI flag is also set. The RX_TYPE = SPEECH_PROBABLY_DEGRADED flag should be set appropriately using quality information from the channel decoder, in which case the PDFI flag is set.
5.2 Lost speech frames p. 6
Normal decoding of lost speech frames would result in very unpleasant noise effects. In order to improve the subjective quality, lost speech frames shall be substituted with either a repetition or an extrapolation of the previous good speech frame(s). This substitution is done so that it gradually will decrease the output level, resulting in silence at the output. Clauses 6, and 7 provide example solutions.
5.3 First lost SID frame p. 6
A lost SID frame shall be substituted by using the SID information from earlier received valid SID frames and the procedure for valid SID frames be applied as described in [3].
5.4 Subsequent lost SID frames p. 6
For many subsequent lost SID frames, a muting technique shall be applied to the comfort noise that will gradually decrease the output level. For subsequent lost SID frames, the muting of the output shall be maintained. Clauses 6 and 7 provide example solutions.
