Quantization Noise Power Injection In Subband Audio Coding Using Low Selectivity Filter Banks

Transcription

1 Quantzaton Nose Power Injecton In Subband Audo Codng Usng Low Selectvty Flter Banks D. ARTÍNEZ -UÑOZ, N. RUIZ-REYES, P. VERA-CANDEAS, P.J. RECHE-LÓPEZ, J. CURPIÁN-ALONSO Departamento de Electrónca Unversdad de Jaén C/ Alfonso X El Sabo, 8; 3700 Lnares SPAIN Abstract: - The ISO-PEG standards are wdely used for hgh qualty audo codng. They use perceptual models that requre hgh frequency resoluton. Ths mples two characterstcs: they use hgh selectvty flter bank to decompose the audo sgnal, and the FFT frame length for the frequency maskng threshold calculatons s long. Both characterstcs carred out a very hgh delay codng. In applcatons where the delay s a crtcal parameter,.e. when the use of a feedback channel s mportant, the ISO-PEG algorthms are not sutable. In ths paper we present how to overcome the frst handcap: to compatblze the use of short mpulse flter bank response and perceptual model applcaton. These flter banks present low selectvty and so the non-deal flters frequency responses should be taken nto account. Key-Words: Subband audo codng, Nose njecton, Flter bank, Psycho-acoustc model Introducton In last years, a lot of work has been developed n the dffcult ssue of data compresson ether for vdeo or audo [][]. The man objectve that has guded the mentoned projects has been the maxmzaton of the coder effcency. Other aspects, such as codng delay, have been consdered as mnor goals. The most mportant are the ISO-PEG algorthms for vdeo and audo. PEG s a workng group n a subcommttee of ISO/IEC (Internatonal Organzaton for Standardzaton/ Internatonal Electrotechncal Commsson) n charge of developng nternatonal standards for compresson, decompresson, processng, and coded representaton of movng pctures, audo, and ther combnaton. In partcular, PEG defnes the syntax of low btrate vdeo and audo btstreams of synthetc and natural sources, descrptons of ther structure and content, and the operaton of conformant decoders of these btstreams. The encoder algorthms are not defned by PEG. Ths allows for contnuous mprovement of encoders and ther adaptaton to specfc applcatons, wthn the btstream syntax defnton. Along wth the vdeo and audo codng, PEG also defnes means to multplex several vdeo, audo and nformaton streams synchronously n one sngle btstream, descrbes methods to test conformance of btstreams and decoders to the standard, and publshes techncal reports contanng software descrbng the decoder operaton and software descrbng examples of encoder operaton. PEG works n phases. These phases are normally denoted by Arabc numbers (PEG-, PEG-, PEG-4, PEG-7). These phases do not descrbe dfferent versons of one standardsaton effort, but are rather completely dfferent coexstng standards that all handle dfferent aspects of ultmeda communcaton. For audo, the coders ncluded n the ISO-PEG standards apply perceptual models that requre hgh frequency resoluton. Ths mples two characterstcs: ) Coders use hgh selectvty flter bank (wth long mpulse response length) to decompose the audo sgnal. ) The frame length s hgh n order to compute the frequency maskng threshold by FFT. Both characterstcs carred out a very hgh delay codng. Recently, a new hgh qualty audo PEG standard has been proposed: PEG-AAC (Advanced Audo Codng). As n the prevous PEG standards, t acheves hgh qualty but the delay s also hgh. However, ths coder presents a

2 named low-delay mode that ntroduces around 40 ms of codng delay. In full-duplex communcatons, the overall delay wll be at least 80 ms. The propagaton and the mplementaton delay should also be consdered. The delay n some applcatons may be a crtcal requrement. For example, a sport commentator whose speakng s coded and transmtted to the base, where t s mxed wth musc before returnng to hs workng place n order to be heard by hmself. A long delay (greater than 60 ms.) may force the commentator to stutter. Consequently, nowadays there s not a standard for audo codng that offers hgh qualty and low delay. Actually, n applcatons where a feedback channel s requred, the ITU-T standard for 7 khz audo (G.7, 64 kbps) s appled. It s based on a two-band sub-band coder, wth ADPC codng of each subband. The low and hgh frequency subbands are quantzed usng 6 and bts per sample, respectvely. The flter banks used for analyss and synthess acheve a communcatons delay of about.5 ms. Ths algorthm was developed prmarly for ISDN teleconferencng and loudspeaker telephony [3]. Because of the 64 kbps capablty, a sngle voce-grade channel on a dgtal or analog, publcswtched telephone network (PSTN) can transport an A-qualty sound program over any dstance and yeld a broadcast-grade voce program at the recevng end. Its subband-adpc codng scheme attans acceptable qualty for speech sgnals but the features for muscal sgnals are not so good. In ths work, we present how to compatblze the applcaton of a psycho-acoustc model n a subband coder whch uses a low selectvty flter bank, n order to reduce the delay codng. In ths case, the control of quantzaton nose ntroduced n one subband s a dffcult task, because t s spread over dfferent frequency bands. Quantzaton nose ntroduced n any subband may ntroduce artfacts nto any dfferent frequency band. The other mportant problem s the ncorporaton of psycho-acoustc nformaton: psycho-acoustc models need the estmaton of the power spectral densty of the audo frame. Therefore, hgh delay s added due to the necessty of large number of samples n the frame to acheve an accurate spectral estmaton. However, ths problem s not consdered n ths paper. The overall problem may be formulated as follow:. Flters n the flter bank are not selectve.. Quantzaton nose nto any subband spreads over other frequency bands of the reconstructed sgnal. In next secton we descrbe the proposed strategy. Fnally, we show an example of the prevously presented strategy. Proposed strategy In perceptual subband audo coder, the psychoacoustc model output s used to compute the maskng to nose rato (SR) for each band over any frame. Bt allocaton s realzed accordng to the obtaned SR value. We need to calculate both the energy sgnal level and the maskng threshold level for every subband n order to fnd the SR. In the calculus of both values we must have nto account the low selectvty of the flter bank. From here on, we present the calculus we do for every subband.. askng Threshold Over each frame we calculate the maskng threshold n the frequency doman applyng any psychoacoustc model. The obtaned maskng threshold must be modfed to consder the low selectvty of the flter bank. Once t s known the maskng threshold, we must concentrate the study n each band. We need to nsure that the overall dstorton n any subband due to the spreadng of the quantzaton nose ntroduced n each subband s below the maskng threshold. Next, we explan how to determne the maxmum quantzaton nose level that can be njected n each band. We denote the band number of the flter bank as, the frequency responses n the DFT doman of the analyss and synthess flter bank as H[k] and F[k], respectvely, and the maskng threshold n the DFT doman as U[k]. Both, H[k] as F[k], are matrxes and U[k] s a vector. The steps that follow are:. For each band,, we search for the mnmum of the maskng threshold modfed by the frequency response of the synthess flter bank branch, as follows: [ ] [ ] k U k Umn = mnmum () F In ths way, we take nto account how much nose can be njected n each subband wthout perceptual qualty loss, consdered n solaton. The frequency ndex where Umn was found s also obtaned (Kmn ).. Now we consder the contrbuton of all the subbands. The overall dstorton must be below the maskng threshold, U[k]. To calculate the maxmum nose power that can be njected n

3 each band, we consder the followng suppostons: Quantzaton nose s whte: we suppose that the nose produced n each band go nto the other accordng to frequency response of the correspondng flter of synthess flter bank. As the number of bts assgned to the band s hgher, ths supposton become true. However, at ths pont we stll do not know the fnal bt allocaton. By applyng ths supposton, we must guarantee that = [ k ] U [ k] for all k U F () where U s the nose power njected n the - th subband, whch must be calculated. The frequency ndex of the mnmum of the overall dstorton nsde a band s close to that prevously found when the band was consdered n solaton (Kmn ). By applyng ths second supposton, we obtan the followng system of equatons Umn = U Umn Umn = U [ Kmn k Fk [ Kmn ] [ Kmn k Fk [ Kmn ] = U K [ mn k Fk [ Kmn ] 3. Once U s calculated, these values wll be used to compute the sgnal to mask rato. (3). Sgnal level The subband sgnal level for each band, L, s calculated accordng to expresson (4). { E[ k ] H [ k ] } L = maxmum (4) Where E[k] represents the power spectral densty of the nput samples frame and H [k] s the frequency response (n the DFT doman) of the -th flter of the analyss flter bank..3 Sgnal to mask rato Once the maskng and the sgnal levels have been calculated for each subband, the sgnal to mask rato, SR(), s obtaned as: L SR( ) = 0 * log 0 (5) U 3 Example of applcaton The coder consdered responds to the structure shows n fgure. It presents a sub-band structure wth dynamc bt allocaton accordng to a psychoacoustc model and adaptve quantzaton. Input Sgnal Flter Flter Flter 3 Psycho-acoustc analyss APC APC APC3 Bt Allocaton Fg.. Block dagram of the consdered coder. The nput sgnal s decomposed n 3 subbands. Each subband s decmated and quantzed usng adaptve quantzaton. The number of quantzaton steps correspondng to each subband s defned accordng to a psycho-acoustc model. Psychoacoustc nformaton s estmated usng frames of 5 samples each one. 3. Pseudo-QF Flter Bank We have used pseudo-qf conventonal cosnemodulated flter bank [4,5] for dvdng the ncomng sgnal nto 3 separate subband sgnals. Ths flter banks based on cosne modulaton, demand only to desgn a unque prototype flter p[n]. Several methods have been proposed to facltate the desgn of the prototype flter and to mprove the characterstcs of the resultant system [6,7]. We have desgned a 56-length prototype flter by usng a method that controls the poston of the 3 cutoff frequency and sets t at π/ [8]. The magntude response plot for the analyss flter s showed n fgure. The stopband attenuaton s around 50. Ths poor stopband attenuaton must be taken nto account because t can become a sgnfcant source of perceptual dstorton due to alasng of quantzaton nose Normalzed Frequency Fg.. Flter bank magntude frequency response.

4 3. Psycho-acoustc Analyss Once a frame s completed, we apply a psychoacoustc model to the last nput samples. The maskng to nose rato (SR) s calculated for each band over ths frame. Ths value s used by the bt allocaton block. We calculate the SR values accordng to the strategy presented n the prevous secton. 3.. askng Threshold Over the frame of 5 samples we obtan the maskng threshold n the frequency doman applyng the psycho-acoustc model smlar to the psychoacoustc model number of the ISO-PEG standard. Ths threshold has been modfed to consder the low selectvty of the flter bank. In fgure 3 s showed the maskng threshold (rectangular lne), the U value obtaned (star pont) and the overall dstorton (flexble lne) when the nose power njected n each band s U. Fgure 4 s a detal of fgure 3. The U value s lower than mnmum maskng threshold for the band. In ths way, the overall dstorton obtaned when the nose power njected n the band s U, s lower than the maskng threshold Frequency ndex Fg. 3. askng threshold, U and overall dstorton Frequency ndex Fg. 4. Fgure 3 detal. 4 Results and conclusons In applcatons where the delay s a crtcal parameter the standards PEG for audo codng are not sutable. They use long mpulse flter bank response and the frame length for apply the FFT transform s also long. Both characterstcs ntroduces hgh delay. In ths paper we have presented a strategy to compatblze the short mpulse flter bank response use and the psychoacoustc applcatons. Due to the low selectvty of the flter bank, we must have nto account the overlappng dstorton. The spread of quantzaton nose ntroduced n each subband over the others s consdered n order to determne the maxmum quantzaton nose power that can be ntroduced n every subband sgnal. We appled ths strategy to a subband coder that use a cosne-modulated flter bank wth length of the flters mpulse response of 56 and adaptve quantzaton. Ths low number of taps produces delay as low as 5.8 ms but the flter bank has low selectvty. Applyng the presented strategy we get a better value for the nose power that we can nject n each band wthout overcomng the maskng threshold. References: [] K.H. Brandenburg, G. Stoll: "ISO-PEG- audo: A generc standard for codng of hghqualty dgtal audo ". J. Audo Eng.Soc. Oct [] D. artínez, F. López,. Rosa, N. Ruíz: Recent Advances n Sgnal Processng and Communcatons. World Scentfc Engneerng Socety, pp , 999. [3] N. Jayant: Sgnal compresson: Technology targets and research drectons, IEEE J. Select. Topcs Commun., vol. 0, pp , June 99. [4] P. P. Vadyanathan: ultrate Systems and Flter Banks. New Jersey, Prentce-Hall, 993. [5] J. H. Rothweler: Polyphase Quadrature Flters A New Subband Codng Technque, n Proc. of Int. Conf. ASSP 83, Boston (USA), pp , Aprl 983. [6] C. D. Creusere and S. K. tra: A Smple ethod for Desgnng Hgh-Qualty Prototype Flters for -Band Pseudo QF Banks, IEEE Transactons on Sgnal Processng, vol. 43, No. 4, pp , Aprl 995.

5 [7] Y.-P- Ln and P. P. Vadyanathan: A Kaser Wndow Approach for the Desgn of Prototype Flters of Cosne odulated Flters Banks, IEEE Sgnal Proc. Letters, vol. 5, No. 6, pp. 3-34, June 998. [8] F. Cruz, P. artín, F. López, P. L. López; A. artín: Nearly Perfect Transmultplexers Usng Cosne-odulated Flters Banks. Proc. of 00 IEEE-EURASIP Workshop on Nonlnear Sgnal and Image Processng (NSIP- 0), Baltmore-aryland (USA), Juno 00. [9] D. artínez,. Rosa, F. López, N. Ruíz: Low delay audo coder usng adaptve quantzaton, n Proc. of X European Sgnal Processng Conference (EUSIPCO 000), Tampere (Fndland), September 000. [0]. Rosa, D. artínez, F. Cruz, F. López: Low delay audo codng ncorporatng Psychoacoustc Informaton, n Proc. of 0 th Audo Engneerng Socety Conventon (0 th AES Conventon), Amsterdam (The Netherlands), ay 00.