ETSI EN V8.0.1 ( )

Transcription

1 EN V8.0.1 ( ) European Standard (Teleommuniations series) Digital ellular teleommuniations system (Phase 2+); Comfort noise aspets for Enhaned Full Rate (EFR) speeh traffi hannels (GSM version Release 1999) GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS R

2 2 EN V8.0.1 ( ) Referene REN/SMG Q8 Keywords Digital ellular teleommuniations system, Global System for Mobile ommuniations (GSM) 650 Route des Luioles F Sophia Antipolis Cedex - FRANCE Tel.: Fax: Siret N NAF 742 C Assoiation à but non luratif enregistrée à la Sous-Préfeture de Grasse (06) N 7803/88 Important notie Individual opies of the present doument an be downloaded from: The present doument may be made available in more than one eletroni version or in print. In any ase of existing or pereived differene in ontents between suh versions, the erene version is the Portable Doument Format (PDF). In ase of dispute, the erene shall be the printing on printers of the PDF version kept on a speifi network drive within Seretariat. Users of the present doument should be aware that the doument may be subjet to revision or hange of status. Information on the urrent status of this and other douments is available at If you find errors in the present doument, send your omment to: editor@etsi.fr Copyright Notifiation No part may be reprodued exept as authorized by written permission. The opyright and the foregoing restrition extend to reprodution in all media. European Teleommuniations Standards Institute All rights reserved.

3 3 EN V8.0.1 ( ) Contents Intelletual Property Rights... 4 Foreword Sope Referenes Definitions, symbols and abbreviations Definitions Symbols Abbreviations General Funtions on the transmit (TX) side Bakground aousti noise evaluation Modifiation of the speeh enoding algorithm during SID frame generation SID-frame enoding Funtions on the reeive (RX) side Averaging and deoding of the LP and fixed odebook gain parameters Comfort noise generation and updating Computational details Annex A (informative): Change Request History History... 17

4 4 EN V8.0.1 ( ) Intelletual Property Rights IPRs essential or potentially essential to the present doument may have been delared to. The information pertaining to these essential IPRs, if any, is publily available for members and non-members, and an be found in SR : "Intelletual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to in respet of standards", whih is available from the Seretariat. Latest updates are available on the Web server ( Pursuant to the IPR Poliy, no investigation, inluding IPR searhes, has been arried out by. No guarantee an be given as to the existene of other IPRs not erened in SR (or the updates on the Web server) whih are, or may be, or may beome, essential to the present doument. Foreword This European Standard (Teleommuniations series) has been produed by Tehnial Committee Speial Mobile Group (SMG). The present doument defines operation of the bakground aousti noise evaluation, noise parameter enoding/deoding and omfort noise generation in Mobile Stations (MSs) and Base Station Systems (BSSs) during Disontinuous Transmission (DTX) on Enhaned Full Rate speeh traffi hannels within the digital ellular teleommuniations system. The ontents of the present doument is subjet to ontinuing work within SMG and may hange following formal SMG approval. Should SMG modify the ontents of the present doument it will be re-released with an identifying hange of release date and an inrease in version number as follows: Version 8.x.y 8 indiates Release 1999 of GSM Phase 2+. x the seond digit is inremented for all hanges of substane, i.e. tehnial enhanements, orretions, updates, et. y the third digit is inremented when editorial only hanges have been inorporated in the speifiation. National transposition dates Date of adoption of this EN: 3 November 2000 Date of latest announement of this EN (doa): 28 February 2001 Date of latest publiation of new National Standard or endorsement of this EN (dop/e): 31 August 2001 Date of withdrawal of any onfliting National Standard (dow): 31 August 2001

5 5 EN V8.0.1 ( ) 1 Sope The present doument gives the detailed requirements for the orret operation of the bakground aousti noise evaluation, noise parameter enoding/deoding and omfort noise generation in Mobile Stations (MSs) and Base Station Systems (BSSs) during Disontinuous Transmission (DTX) on Enhaned Full Rate speeh traffi hannels. The requirements desribed in the present doument are mandatory for implementation in all GSM MSs apable of supporting the Enhaned Full Rate speeh traffi hannel. The reeiver requirements are mandatory for implementation in all GSM BSSs apable of supporting the Enhaned Full Rate speeh traffi hannel, the transmitter requirements only for those where downlink DTX will be used. In ase of disrepany between the requirements desribed in the present doument and the fixed point omputational desription of these requirements ontained in GSM [2], the desription in GSM [2] will prevail. 2 Referenes The following douments ontain provisions whih, through erene in this text, onstitute provisions of the present doument. Referenes are either speifi (identified by date of publiation, edition number, version number, et.) or non-speifi. For a speifi erene, subsequent revisions do not apply. For a non-speifi erene, the latest version applies. A non-speifi erene to an ETS shall also be taken to er to later versions published as an EN with the same number. For this Release 1999 doument, erenes to GSM douments are for Release 1999 versions (version 8.x.y). [1] GSM 01.04: "Digital ellular teleommuniations system (Phase 2+); Abbreviations and aronyms". [2] GSM 06.53: "Digital ellular teleommuniations system (Phase 2+); ANSI-C ode for the GSM Enhaned Full Rate (EFR) speeh ode". [3] GSM 06.60: "Digital ellular teleommuniations system (Phase 2+); Enhaned Full Rate (EFR) speeh transoding". [4] GSM 06.61: "Digital ellular teleommuniations system (Phase 2+); Substitution and muting of lost frame for Enhaned Full Rate (EFR) speeh traffi hannels". [5] GSM 06.81: "Digital ellular teleommuniations system (Phase 2+); Disontinuous transmission (DTX) for Enhaned Full Rate (EFR) speeh traffi hannels". 3 Definitions, symbols and abbreviations 3.1 Definitions For the purposes of the present doument, the following terms and definitions apply: frame: time interval of 20 ms orresponding to the time segmentation of the Enhaned Full Rate speeh transoder, also used as a short term traffi frame. SID frame: frame haraterized by the SID (SIlene Desriptor) odeword. It onveys information on the aousti bakground noise.

6 6 EN V8.0.1 ( ) SID odeword: fixed bit pattern for labelling a traffi frame as a SID frame. SID field: bit positions of the SID odeword within a SID frame. speeh frame: traffi frame that annot be lassified as a SID frame. VAD flag: Voie Ativity Detetion flag. SP flag: SPeeh flag. Other definitions of terms used in the present doument an be found in GSM [3] and GSM [5]. The overall operation of DTX is desribed in GSM [5]. 3.2 Symbols For the purposes of the present doument, the following symbols apply. Boldfae symbols are used for vetor variables. f T f f... f Unquantized LSF vetor f$ T f$ f$... f$ Quantized LSF vetor f ( m) $f ( m) mth unquantized LSF vetor of the frame mth quantized LSF vetor of the frame $f Referene LSF parameter vetor f mean g Averaged LSF parameter vetor Unquantized fixed odebook gain $g Quantized fixed odebook gain $g Referene fixed odebook gain g mean e LP e Averaged fixed odebook gain Linear predition residual signal Computed LSF parameter predition residual $e Quantized LSF parameter predition residual $ b na x( n) Computed fixed odebook gain orretion fator Quantized fixed odebook gain orretion fator 1 K 1 x a x a x b x b

7 7 EN V8.0.1 ( ) 3.3 Abbreviations For the purposes of the present doument, the following abbreviations apply: BSS DTX LP LSF LSP MS RX SID TX VAD Base Station Subsystem Disontinuous Transmission Linear Predition Line Spetral Frequeny Line Spetral Pair Mobile Station Reeive SIlene Desriptor Transmit Voie Ativity Detetor For abbreviations not given in this lause, see GSM [1]. 4 General A basi problem when using DTX is that the bakground aousti noise, whih is transmitted together with the speeh, would disappear when the radio transmission is ut, resulting in disontinuities of the bakground noise. Sine the DTX swithing an take plae rapidly, it has been found that this effet an be very annoying for the listener - espeially in a ar environment with high bakground noise levels. In bad ases, the speeh may be hardly intelligible. The present doument speifies the way to overome this problem by generating on the reeive (RX) side syntheti noise similar to the transmit (TX) side bakground noise. The omfort noise parameters are estimated on the TX side and transmitted to the RX side before the radio transmission is swithed off and at a regular low rate afterwards. This allows the omfort noise to adapt to the hanges of the noise on the TX side. 5 Funtions on the transmit (TX) side The omfort noise evaluation algorithm uses the following parameters of the GSM Enhaned Full Rate speeh enoder, defined in GSM [3]: - the unquantized and quantized Linear Predition (LP) parameters, using the Line Spetral Pair (LSP) f T f f... f, representation, where the unquantized Line Spetral Frequeny (LSF) vetor is given by the quantized LSF vetor is given by f$ $ $... $ T f f f , and the two sets of unquantized and quantized LSF vetors (one for eah half of a frame) are given by f ( 1 ), f ( 2 ), f $ ( 1) and f $ ( 2 ), respetively; - the quantized fixed-odebook gain $g. The algorithm also omputes the following parameters to assist in omfort noise generation: - the erene LSF parameter vetor $ f (average of the quantized LSF parameters of the hangover period); - the averaged LSF parameter vetor f mean (average of the LSF parameters of the eight most reent frames); - the erene fixed odebook gain $g period); (average of the quantized fixed odebook gain values of the hangover - the averaged fixed odebook gain g mean (average of the fixed odebook gain values of the eight most reent frames); - the unquantized fixed odebook gain g.

8 8 EN V8.0.1 ( ) These parameters give information on the level ( g, $g, $g, g mean ) and the spetrum (f ( 1 ), f ( 2 ), f $ ( 1 ), f $ ( 2 ), $f, f mean ) of the bakground noise. Two of the evaluated omfort noise parameters (f mean and g mean ) are enoded into a speial frame, alled a Silene Desriptor (SID) frame, for transmission to the RX side. Sine the erene LSF parameter vetor f $ and the erene fixed odebook gain $g an be evaluated in the same way in the enoder and deoder, as given in lause 5.1, no transmission of these parameters is neessary. The averaged LSF parameter and fixed odebook gain values, f mean and g mean, are omputed in the enoder using both quantized and unquantized parameter values if the period of the eight most reent frames (the SID averaging period) is overlapping with the hangover period (the parameters from the frames overlapping with the hangover period have quantized values, while the parameters of the more reent frames of the SID averaging period have unquantized values). If the period of the eight most reent frames is non-overlapping with the hangover period, the averaged LSF parameter and fixed odebook gain values are omputed using only unquantized parameter values. The SID frame also serves to initiate the omfort noise generation on the reeive side, as a SID frame is always sent at the end of a speeh burst, i.e., before the radio transmission is terminated. The sheduling of SID or speeh frames on the radio path is desribed in GSM [5]. 5.1 Bakground aousti noise evaluation The omfort noise parameters to be enoded into a SID frame are alulated over N = 8 onseutive frames marked with VAD = 0, as follows: The averaged LSF parameter vetor f mean i of the frame i shall be omputed aording to the equation: f mean ( m) i f i n n m1 (1) f ( m) i is the mth (unquantized) LSF parameter vetor of the urrent frame i (n = 0); f ( m) i n is the mth (quantized or unquantized) LSF parameter vetor of one of the last frames (n = 1,..., 7); n is the averaging period index (n = 0, 1,..., 7); m is the LSF parameter vetor index within a frame (1 or 2); i is the frame index. NOTE: When the averaging is performed at the end of the hangover period (first SID update), all of the LSF f ( m) i n of the 7 previous frames (the hangover period) have quantized values, parameter vetors while the LSF parameter vetors f ( m) i of the urrent frame i have unquantized values. In the subsequent SID updates, the LSF parameter vetors of the SID averaging period in the frames overlapping with the hangover period have quantized values, while the parameter vetors of the more reent frames of the SID averaging period have unquantized values. The averaged LSF parameter vetor f mean i of the frame i is enoded using the same enoding tables that are also used by the GSM Enhaned Full Rate speeh ode for the enoding of the non-averaged LSF parameter vetors in ordinary speeh enoding mode, but the quantization algorithm is modified in order to support the quantization of omfort noise. The LSF parameter predition residual to be quantized is obtained aording to the following equation: i i mean e f f $ (2)

9 9 EN V8.0.1 ( ) f mean i is the averaged LSF parameter vetor at the urrent frame i $f is the erene LSF parameter vetor e i i NOTE: is the omputed LSF parameter predition residual at the urrent frame i is the frame index; This predition residual is used for both halves of the frame in the quantization algorithm. The omputation of the erene LSF parameter vetor f $ is made on the basis of the quantized LSF parameters, f $ ( 1) and f $ ( 2 ), by averaging the parameters over the hangover period of 7 frames, aording to the following equation: 7 2 f$ 1 1 f$ 7 n1 2 m1 ( m) k n (3) $f ( ) m k n is the mth quantized LSF parameter vetor of one of the frames of the hangover period (n = 1,..., 7); n is the hangover period frame index (n = 1,..., 7); m is the LSF parameter index within a frame (1 or 2); k is the frame index. For eah omfort noise insertion period, the omputation of the erene LSF parameter vetor $ f at the end of the hangover period and for the rest of the omfort noise insertion period $ f erene LSF parameter vetor $ f is done only one will be frozen. The is evaluated in the deoder in the same way as in the enoder, beause during the hangover period the same LSF parameter vetors f $ ( m) are available at the enoder and deoder. An exeption to this are the ases when transmission errors are severe enough to ause the parameters to beome unusable, and the frame substitution proedure is ativated (see GSM [4]). In these ases, the modified parameters obtained from the frame substitution proedure are used instead of the reeived parameters. The fixed odebook gain values shall be averaged and updated in every subframe aording to the equation: g mean 28 1 gn 29 n0 (4) g 0 is the (unquantized) fixed odebook gain in the urrent subframe (n=0); g n is the (quantized or unquantized) fixed odebook gain in one of the past subframes (n = 1,..., 28); n is the averaging period index (n = 0, 1,..., 28);

10 10 EN V8.0.1 ( ) NOTE: When the averaging is started at the end of the hangover period (first SID update), all of the fixed g n of the 28 (n=1,..., 28) previous subframes (the hangover period) have quantized odebook gains values, while the fixed odebook gains g 0 of the urrent subframe has an unquantized value. In the subsequent SID updates, the fixed odebook gain values of the SID averaging period in the subframes overlapping with the hangover period have quantized values, while the parameter vetors of the more reent subframes of the SID averaging period have unquantized values. Sine most parts of the subframe proessing lause in the enoder are swithed off when the SP flag = "0" (to minimize the average omplexity of the speeh enoder algorithm), the unquantized fixed odebook gain is not diretly available for gain averaging. Due to this, the unquantized fixed odebook gain is separately omputed, based on the energy of the LP residual signal in eah subframe, aording to the following equation: g j 40 l1 e j l LP 10 2 (5) j g is the unquantized fixed odebook gain of the urrent subframe j; jl elp is the lth sample of the LP residual in the urrent subframe j; j is the subframe index (j = 1,..., 4); l is the sample index (l = 1,..., 40). NOTE: The omputed energy of the LP residual signal is divided by the value of 10 to yield the energy for one exitation pulse, sine during omfort noise generation, the subframe exitation signal (pseudo noise) has 10 non-zero samples, whose amplitudes an take values of +1 or -1. The averaged fixed odebook gain value g mean of the urrent subframe is enoded using the non-averaged fixed odebook gain values in ordinary speeh enoding mode, but the quantization algorithm is modified in order to support omfort noise quantization. The fixed odebook gain orretion fator to be quantized is obtained aording to the following equation: g mean g$ (6) g mean is the averaged fixed odebook gain value in the urrent subframe; $g is the erene fixed odebook gain; The omputation of the erene fixed odebook gain $g is made on the basis of the quantized fixed odebook gain parameters $g, by averaging the parameter values over the hangover period of 7 frames aording to the following equation: g$ g$ k n j 7 n1 4 j 1 (7) $g k n j is the quantized fixed odebook gain parameter value in subframe j of one of the frames of the hangover period (n = 1,..., 7); n is the hangover period frame index (n = 1, 2,..., 7);

11 11 EN V8.0.1 ( ) k is the frame index; j is the subframe index (j = 1,..., 4); For eah omfort noise insertion period, the omputation of the erene fixed odebook gain $g at the end of the hangover period and for the rest of the omfort noise insertion period $g erene fixed odebook gain $g is done only one will be frozen. The an be evaluated in the deoder in the same way as in the enoder, beause during the hangover period the same quantized fixed odebook gain values $g are available at the enoder and deoder. An exeption to this are the ases when transmission errors are severe enough to ause the parameters to beome unusable, and the frame substitution proedure is ativated (see GSM [4]). In these ases, the modified parameters obtained from the frame substitution proedure are used instead of the reeived parameters. The hangover period is defined in GSM [5]. It is a period added at the end of a speeh burst in whih no voie ativity is deteted (VAD flag = "0"), but the speeh enoder stays for the proessing of 7 speeh frames in speeh enoding mode (SP flag = "1"). This hangover period and the first SID frame are used for averaging the omfort noise parameters ontained in the first SID frame. 5.2 Modifiation of the speeh enoding algorithm during SID frame generation When the SP flag is equal to "0" the speeh enoding algorithm is modified in the following way: H z and - The non-averaged LP parameters whih are used to derive the filter oeffiients of the filters of the speeh enoder are not quantized. - The open loop pith lag searh is performed, but the losed loop pith lag searh is inativated. The adaptive odebook gain is set to zero. Wz - No fixed odebook searh is made. In eah subframe the pulse positions and signs of the fixed odebook exitation are loally generated using uniformly distributed pseudo random numbers. The exitation pulses take values of +1 and -1 when omfort noise is generated. The fixed odebook omfort noise exitation generation algorithm is defined in lause The memory of weighting filter Wz is set to zero, i.e., the memory of Wz is not updated. - The ordinary LP parameter quantization algorithm is inative. At the end of the hangover period the erene LSF parameter vetor f $ is alulated as defined in lause 5.1. For the rest of the omfort noise insertion period $ f is frozen. The averaged LSF parameter vetor f mean is alulated eah time a new SID frame is to be sent to the Radio Subsystem. This parameter vetor is enoded into the SID frame as defined in lause The ordinary fixed odebook gain quantization algorithm is inative. At the end of the hangover period the erene fixed odebook gain $g is alulated as defined in lause 5.1. For the rest of the omfort noise insertion period $g is frozen. The averaged fixed odebook gain value g mean is alulated eah time a new SID frame is to be sent to the Radio Subsystem. This gain value is enoded into the SID frame as defined in lause The preditor memories of the ordinary LP parameter quantization and fixed odebook gain quantization algorithms are reset when SP flag = "0", so that the quantizers start from their initial states when the speeh ativity begins again. - The omputation of the unquantized fixed odebook gain is performed based on the energy of the LP residual signal.

12 12 EN V8.0.1 ( ) 5.3 SID-frame enoding The SID-frame enoding algorithm exploits the fat that only some of the 244 bits in a frame are needed to ode the omfort noise parameters. The other bits an then be used to mark the SID-frame by means of a fixed bit pattern, alled the SID ode word. The quantization indies of the LP parameters are replaed by the quantization indies derived from the averaged LSF parameter vetor f mean. The enoding of the quantization indies is defined in lause 5.1. The fixed odebook gain quantization indies are replaed by the quantization index derived from the averaged fixed odebook gain value g mean, enoded as defined in lause 5.1, repeated four times inside the frame. The SID ode word onsists of 95 bits whih are all set to one. The bits of the SID ode word are inserted in the SID field as defined in table 1. All of the bits in the SID field are in the error protetion Class I. The remaining bits in the SID frame are set to zero. The use of these bits is for further study. Table 1: SID odeword Parameter Number of bits Bit positions (b0 = LSB) LTP LAG 1 2 b0, b1 LTP LAG 2 3 b0, b1, b2 LTP LAG 3 2 b0, b1 LTP LAG 4 4 b0, b1, b2, b3 LTP GAIN 1 3 b0, b1, b2 LTP GAIN 2 3 b0, b1, b2 LTP GAIN 3 4 b0, b1, b2, b3 LTP GAIN 4 4 b0, b1, b2, b3 PULSE 1 of 1st subfr. 4 b0, b1, b2, b3 PULSE 2 of 1st subfr. 4 b0, b1, b2, b3 PULSE 3 of 1st subfr. 4 b0, b1, b2, b3 PULSE 4 of 1st subfr. 4 b0, b1, b2, b3 PULSE 5 of 1st subfr. 2 b2,b3 PULSE 1 of 2nd subfr. 4 b0, b1, b2, b3 PULSE 2 of 2nd subfr. 4 b0, b1, b2, b3 PULSE 3 of 2nd subfr. 4 b0, b1, b2, b3 PULSE 4 of 2nd subfr. 4 b0, b1, b2, b3 PULSE 5 of 2nd subfr. 2 b2,b3 PULSE 1 of 3rd subfr. 4 b0, b1, b2, b3 PULSE 2 of 3rd subfr. 4 b0, b1, b2, b3 PULSE 3 of 3rd subfr. 4 b0, b1, b2, b3 PULSE 4 of 3rd subfr. 4 b0, b1, b2, b3 PULSE 5 of 3rd subfr. 2 b2,b3 PULSE 1 of 4th subfr. 4 b0, b1, b2, b3 PULSE 2 of 4th subfr. 2 b2, b3 PULSE 3 of 4th subfr. 4 b0, b1, b2, b3 PULSE 4 of 4th subfr. 4 b0, b1, b2, b3 PULSE 5 of 4th subfr. 2 b2,b3 The parameters in table 1 are defined in GSM [3]. 6 Funtions on the reeive (RX) side The situations in whih omfort noise shall be generated on the reeive side are defined in GSM [5]. Generally speaking, the omfort noise generation is started or updated whenever a valid SID frame is reeived.

13 13 EN V8.0.1 ( ) 6.1 Averaging and deoding of the LP and fixed odebook gain parameters When speeh frames are reeived by the deoder the LP and the fixed odebook gain parameters of the last seven speeh frames shall be kept in memory. The deoder ounts the number of frames elapsed sine the last SID frame was updated and passed to the RSS by the enoder. Based on this ount, the deoder determines whether or not there is a hangover period at the end of the speeh burst (if at least 31 frames have elapsed sine the last SID update when the first SID frame after a speeh burst arrives, the hangover period has existed at the end of the speeh burst). As soon as a SID frame is reeived, and the hangover period is deteted at the end of the speeh burst, the stored LP and fixed odebook gain parameters shall be averaged to obtain the erene LSF parameter vetor $ f and the erene fixed gain odebook value g. The erene LSF parameter vetor and the erene fixed odebook gain value shall be frozen and used for the atual omfort noise insertion period. The averaging proedure for obtaining the erene parameters is as follows: - when a speeh frame is reeived, the LSF and fixed odebook gain parameters are deoded and stored in memory; - when the first SID frame is reeived, and the hangover period is deteted at the end of the speeh burst, the stored LSF and fixed odebook gain parameters are averaged in the same way as in the speeh enoder as follows (see also lause 5.1): 7 2 f$ 1 1 f$ 7 n1 2 m1 ( m) k n (8) $f ( ) m k n is the mth quantized LSF parameter vetor of one of the frames of the hangover period (n = 1,..., 7); n is the hangover period frame index (n = 1, 2,..., 7); m is the LSF parameter index within a frame (1 or 2); k is the frame index. and g$ g$ k n j 7 n1 4 j 1 (9) $g k n j is the quantized fixed odebook gain parameter value in subframe j of one of the frames of the hangover period (n = 1,..., 7); n is the hangover period frame index (n = 1,..., 7); k is the frame index; j is the subframe index (j = 1,..., 4). One the erene LSF parameter vetor has been omputed, the averaged LSF parameter vetor $f mean i of the frame i (enoded into the SID frame) an be reprodued at the deoder eah time a SID update frame is reeived, aording to the equation: i i $ mean f e $ f $ = (10)

14 14 EN V8.0.1 ( ) $f mean i is the quantized, averaged LSF parameter vetor at the urrent frame i to be used for omfort noise generation; $f is the erene LSF parameter vetor; $e i is the reeived quantized LSF parameter predition residual at the urrent frame i; i is the frame index. mean The averaged fixed odebook gain $g i of the frame i (enoded into the SID frame) an be similarly reprodued at the deoder eah time a SID update frame is reeived, aording to the following equation: mean g$ i g$ $ i (11) mean $g i is the averaged fixed odebook gain value in the urrent frame i to be used for omfort noise generation; $g is the erene fixed odebook gain; $ i is the reeived quantized fixed odebook gain orretion fator in the urrent frame i; i is the frame index. 6.2 Comfort noise generation and updating The omfort noise generation proedure uses the GSM Enhaned Full Rate speeh deoder algorithm defined in GSM [3]. When omfort noise is to be generated, the various enoded parameters are set as follows: In eah subframe, the pulse positions and signs of the fixed odebook exitation are loally generated using uniformly distributed pseudo random numbers. The exitation pulses take values of +1 and -1 when omfort noise is generated. The fixed odebook omfort noise exitation generation algorithm works as follows: for (i = 0; i < 40; i++) ode[i] = 0; for (i = 0; i < 10; i++) { } j = random(4); idx = j * 10 + I; if (random(2) == 1) ode[idx] = 1; else ode[idx] = -1; ode[0..39] fixed odebook exitation buffer; random(4) generates a random integer value, uniformly distributed between 0 and 3; random(2) generates a random integer value, uniformly distributed between 0 and 1. The fixed odebook gain values are those reeived in the SID frame.

15 15 EN V8.0.1 ( ) The adaptive odebook gain values in eah subframe are set to 0. The pith delay values in eah subframe are set to 40. The LP filter parameters used are those reeived in the SID frame. The preditor memories of the ordinary LP parameter and fixed odebook gain quantization algorithms are reset when SP flag = "0", so that the quantizers start from their initial states when the speeh ativity begins again. With these parameters, the speeh deoder now performs the standard operations desribed in GSM [3] and synthesizes omfort noise. Updating of the omfort noise parameters (fixed odebook gain and LP filter parameters) ours eah time a valid SID frame is reeived, as desribed in GSM [5]. When updating the omfort noise, the parameters above should perably be interpolated over the SID update period to obtain smooth transitions. 7 Computational details A low level desription has been prepared in form of an ANSI C soure ode whih is part of GSM [2].

16 16 EN V8.0.1 ( ) Annex A (informative): Change Request History Change history SMG No. TDo. No. CR. No. Clause affeted New version Subjet/Comments SMG# Publiation SMG# Release 1996 version SMG# Release 1997 version SMG# Release 1998 version Version update to for Publiation SMG# Release 1999 version Update to Version for Publiation

17 17 EN V8.0.1 ( ) History Doument history V8.0.0 July 2000 One-step Approval Proedure OAP : to V8.0.1 November 2000 Publiation