ETSI TS 126 091 V8.0.0 (2009-01) Technical Specification Digital cellular telecommunications system (Phase 2+); Universal Mobile Telecommunications System (UMTS); LTE; AMR speech Codec; Error concealment of lost frames (3GPP TS 26.091 version 8.0.0 Release 8) 3GPP TS 26.091 version 8.0.0 Release 8 1 ETSI TS 126 091 V8.0.0 (2009-01) Reference RTS/TSGS-0426091v800 Keywords GSM, LTE, UMTS ETSI 650 Route des Lucioles F-06921 Sophia Antipolis Cedex - FRANCE Tel.: +33 4 92 94 42 00 Fax: +33 4 93 65 47 16 Siret N° 348 623 562 00017 - NAF 742 C Association à but non lucratif enregistrée à la Sous-Préfecture de Grasse (06) N° 7803/88 Important notice Individual copies of the present document can be downloaded from: http://www.etsi.org The present document may be made available in more than one electronic version or in print. In any case of existing or perceived difference in contents between such versions, the reference version is the Portable Document Format (PDF). 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ETSI 3GPP TS 26.091 version 8.0.0 Release 8 3 ETSI TS 126 091 V8.0.0 (2009-01) Contents Intellectual Property Rights................................................................................................................................2 Foreword.............................................................................................................................................................2 Foreword.............................................................................................................................................................4 1 Scope........................................................................................................................................................5 2 References................................................................................................................................................5 3 Definitions and abbreviations...................................................................................................................5 3.1 Definitions..........................................................................................................................................................5 3.2 Abbreviations.....................................................................................................................................................5 4 General.....................................................................................................................................................6 5 Requirements............................................................................................................................................6 5.1 Error detection....................................................................................................................................................6 5.2 Lost speech frames.............................................................................................................................................6 5.3 First lost SID frame............................................................................................................................................6 5.4 Subsequent lost SID frames................................................................................................................................6 6 Example ECU/BFH Solution 1................................................................................................................6 6.1 State Machine.....................................................................................................................................................7 6.2 Assumed Active Speech Frame Error Concealment Unit Actions.....................................................................8 6.2.1 BFI = 0, prevBFI = 0, State = 0....................................................................................................................8 6.2.2 BFI = 0, prevBFI = 1, State = 0 or 5.............................................................................................................8 6.2.3 BFI = 1, prevBFI = 0 or 1, State = 1...6........................................................................................................9 6.2.3.1 LTP-lag update........................................................................................................................................9 6.2.3.2 Innovation sequence................................................................................................................................9 6.3 Assumed Non-Active Speech Signal Error Concealment Unit Actions...........................................................10 6.3.1 General........................................................................................................................................................10 6.3.2 Detectors.....................................................................................................................................................10 6.3.2.1 Background detector.............................................................................................................................10 6.3.2.2 Voicing detector....................................................................................................................................10 6.3.3 Background ECU Actions..........................................................................................................................10 6.4 Substitution and muting of lost SID frames.....................................................................................................10 7 Example ECU/BFH Solution 2..............................................................................................................11 7.1 State Machine...................................................................................................................................................11 7.2 Substitution and muting of lost speech frames.................................................................................................11 7.2.1 BFI = 0, prevBFI = 0, State = 0..................................................................................................................11 7.2.2 BFI = 0, prevBFI = 1, State = 0 or 5...........................................................................................................11 7.2.3 BFI = 1, prevBFI = 0 or 1, State = 1...6......................................................................................................12 7.2.3.1 LTP-lag update......................................................................................................................................12 7.2.4 Innovation sequence...................................................................................................................................12 7.3 Substitution and muting of lost SID frames.....................................................................................................12 Annex A (informative): Change history...............................................................................................13 History..............................................................................................................................................................14 ETSI 3GPP TS 26.091 version 8.0.0 Release 8 4 ETSI TS 126 091 V8.0.0 (2009-01) Foreword This Technical Specification has been produced by the 3rd Generation Partnership Project (3GPP). The contents of the present document are subject to continuing work within the TSG and may change following formal TSG approval. Should the TSG modify the contents of the present document, it will be re-released by the TSG with an identifying change of release date and an increase in version number as follows: Version x.y.z where: x the first digit: 1 presented to TSG for information; 2 presented to TSG for approval; 3 or greater indicates TSG approved document under change control. y the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections, updates, etc. z the third digit is incremented when editorial only changes have been incorporated in the document. ETSI 3GPP TS 26.091 version 8.0.0 Release 8 5 ETSI TS 126 091 V8.0.0 (2009-01) 1 Scope 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 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. [1] 3GPP TS 26.102: "AMR Speech Codec; Interface to Iu snd Uu". [2] 3GPP TS 26.090: "Transcoding functions". [3] 3GPP TS 26.093: "Source Controlled Rate operation". [4] 3GPP TS 26.101: "Frame Structure". 3 Definitions and abbreviations 3.1 Definitions 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 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 ETSI 3GPP TS 26.091 version 8.0.0 Release 8 6 ETSI TS 126 091 V8.0.0 (2009-01) prevBFI Bad Frame Indication of previous frame RX Receive SCR Source Controlled Rate (operation) SID Silence Descriptor frame (Background descriptor) 4 General 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 5.1 Error detection 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 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 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 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. 6 Example ECU/BFH Solution 1 The C code of the following example is embedded in the bit exact software of the codec. In the code the ECU is designed to allow subframe-by-subframe synthesis, thereby reducing the speech synthesis delay to a minimum. ETSI 3GPP TS 26.091 version 8.0.0 Release 8 7 ETSI TS 126 091 V8.0.0 (2009-01) 6.1 State Machine This example solution for substitution and muting is based on a state machine with seven states (Figure 1). The system starts in state 0. Each time a bad frame is detected, the state counter is incremented by one and is saturated when it reaches 6. Each time a good speech frame is detected, the state counter is reset to zero, except when we are in state 6, where we set the state counter to 5. The state indicates the quality of the channel: the larger the value of the state counter, the worse the channel quality is. The control flow of the state machine can be described by the following C code (BFI = bad frame indicator, State = state variable): if(BFI != 0 ) State = State + 1; else if(State == 6) State = 5; else State = 0; if(State > 6 ) State = 6; In addition to this state machine, the Bad Frame Flag from the previous frame is checked (prevBFI). The processing depends on the value of the State-variable. In states 0 and 5, the processing depends also on the two flags BFI and prevBFI. ETSI 3GPP TS 26.091 version 8.0.0 Release 8 8 ETSI TS 126 091 V8.0.0 (2009-01) The procedure can be described as follows: STATE = 0 BFI = 0 PrevBFI = 0 or 1 STATE = 1 BFI = 1 PrevBFI = 0 STATE = 2 BFI = 1 PrevBFI = 1 STATE = 3 BFI = 1 PrevBFI = 1 STATE = 4 BFI = 1 PrevBFI = 1 STATE = 5 BFI = 0 or 1 PrevBFI = 1 STATE = 6 Bad frame (BFI=1) BFI = 1 PrevBFI = 0 or 1 Good frame (BFI=0) Figure 1: State machine for controlling the bad frame substitution 6.2 Assumed Active Speech Frame Error Concealment Unit Actions 6.2.1 BFI = 0, prevBFI = 0, State = 0 No error is detected in the received or in the previous received speech frame. The received speech parameters are used in the normal way in the speech synthesis. The current frame of speech parameters is saved. 6.2.2 BFI = 0, prevBFI = 1, State = 0 or 5 No error is detected in the received speech frame, but the previous received speech frame was bad. The LTP gain and fixed codebook gain are limited below the values used for the last received good ⎧gp, gp ≤ gp(−1) subframe:gp = ⎨ (1) ⎩gp(−1), gp > gp(−1) ETSI 3GPP TS 26.091 version 8.0.0 Release 8 9 ETSI TS 126 091 V8.0.0 (2009-01) where gp= current decoded LTP gain,gp(−1)= LTP gain used for the last good subframe (BFI = 0), and ⎧ gc, gc ≤ gc(−1) gc = ⎨ ⎩gc(−1), gc > gc(−1) (2) where gc= current decoded fixed codebook gain and gc(−1)= fixed codebook gain used for the last good subframe (BFI = 0). The rest of the received speech parameters are used normally in the speech synthesis. The current frame of speech parameters is saved. 6.2.3 BFI = 1, prevBFI = 0 or 1, State = 1...6 An error is detected in the received speech frame and the substitution and muting procedure is started. The LTP gain and fixed codebook gain are replaced by attenuated values from the previous subframes: ⎧⎪P(state) gp(−1), gp(−1)≤median5(gp(−1),...,gp(−5)) gp = ⎨ ⎩⎪P(state) median5(gp(−1),...,gp(−5)), gp(−1)>median5(gp(−1),...,gp(−5)) (3) where gp= current decoded LTP gain, g p (−1),...,g p (−n)= LTP gains used for the last n subframes, median5() = 5-point median operation, P(state) = attenuation factor (P(1) = 0.98, P(2) = 0.98, P(3) = 0.8, P(4) = 0.3, P(5) = 0.2, P(6) = 0.2), state = state number, and ⎧⎪C(state) gc(−1), gc(−1)≤ median5(gc(−1),...,gc(−5)) gc = ⎨ ⎩⎪C(state) median5(gc(−1),...,gc(−5)), gc(−1) >median5(gc(−1),...,gc(−5)) (4) where gc= current decoded fixed codebook gain, gc(−1),...,gc(−n)= fixed codebook gains used for the last n subframes, median5() = 5-point median operation, C(state) = attenuation factor (C(1) = 0.98, C(2) = 0.98, C(3) = 0.98, C(4) = 0.98, C(5) = 0.98, C(6) = 0.7), and state = state number. The higher the state value is, the more the gains are attenuated. Also the memory of the predictive fixed codebook gain is updated by using the average value of the past four values in the memory: ener(0) = 1 ∑4 ener(−i) 4 i=1 (5) The past LSFs are shifted towards their mean: lsf_q1(i) = lsf_q2(i) =αpast_lsf_q(i)+(1−α)mean_lsf(i), i = 0...9 (6) where α = 0.95, lsf_q1 and lsf_q2 are two sets of LSF-vectors for current frame, past_lsf_q is lsf_q2 from the previous frame, and mean_lsf is the average LSF-vector. Note that two sets of LSFs are available only in the 12.2 mode. 6.2.3.1 LTP-lag update The LTP-lag values are replaced by the past value from the 4th subframe of the previous frame (12.2 mode) or slightly modified values based on the last correctly received value (all other modes). 6.2.3.2 Innovation sequence The received fixed codebook innovation pulses from the erroneous frame are used in the state in which they were received when corrupted data are received . In the case when no data were received random fixed codebook indices should be employed. ETSI