Distributed Echo Cancellation in Multimedia Conferencing System

Transcription

1 Disribued Echo Cancellaion in Mulimedia Conferencing Sysem Balan Sinniah 1, Sureswaran Ramadass 2 1 KDU College Sdn.Bhd, A Paramoun Corporaion Company, 32, Jalan Anson, Penang, Malaysia. sbalan@kdupg.edu.my hp:// 2 Nework Research Group, School Of Compuer Science, Universii Sains Malaysia, Minden, Penang, Malaysia. sures@cs.usm.my hp://nrg.cs.usm.my Absrac. As qualiy of video and audio frames ransmission via inerne/lan is vial, numerous mehods and echniques are employed o susain a beer performance of mulimedia sreaming. Ye, echo cancellaion for speech and audio a sofware level sill under research. The prime objecive of echo cancellaion is o improve clariy of audio/speech signal. Echo cancellaion is a digial signal processing echniques for removing unwaned signal from speech/audio. Many echniques have been implemened o reduce he echo during conferencing; however here is more space o refine and enhance he exising echniques. This proposal suggess a sofware approach o achieve echo-cancellaion in poin-o-poin mulimedia conferencing sysem. The proposed echnique use phase shifing mehod o eliminae any exising echo in he audio daa received from he nework. The soluion will also enhance incoming audio qualiy by reducing background noise. The filered or process audio daa will provide high qualiy speech and hopefully will improve he qualiy of mulimedia conferencing sysem one sep furher. 1 Inroducion An echo is he repeiion of a sound caused by he reflecion of sound waves [1]. In mulimedia conferencing sysem, paricularly audio conferencing echoes are problemaic if he speakers hear a delayed version of he same signal (voice). Numerous researches have been conduced by he elecommunicaions indusry o conrol/eliminae unwaned signal (echo) few decades ago. Echo is becoming he facor of producing low audio qualiy signal when he Round Trip Delay (he ime aken o reflec an echo) is more han 30 milliseconds [1]. Echo or Acousic echo is caused by acousic coupling problems beween an audio conferencing speaker and is microphone. The endency o produce echoes is roughly inversely proporional o he disance beween he speaker and microphone. Usually in video conferencing sysems, he use of earphones eliminaes his problem. However, i is only applicable for deskop video conferencing unis. The problem sill exiss in all he boardroom video conferencing unis.

2 This paper will focus on he sofware based phase shifer echnique o remove he echo in audio conferencing ool. Phase shifing is a echnique where he original signal (noised signal) would be added wih he shifed signal, which needs o be removed from he original signal by using he 180º phase shifer. The paper will no address he implemenaion procedure or echniques for he phase shifer mehod bu raher discuss more on heoreical aspecs of designing approach o eliminae his problem. This would be an overall discussion of he proposed mehod o remove echo in audio conferencing. I is believed ha his mehod would be able o provide a hardware independen soluion for acousic echo cancellaion. 2 Lieraure Review The process ha is described by he proposed echo cancellaion echnique involved Digial Signal Processing (DSP) approach, which is concerned wih he digial represenaion of analogue signals and he use of digial processors o analyse, modify or exrac informaion from signals. Analogue signals are sampled a regular inervals and convered o digial form. Processing a digial signal would be able o guaranee he accuracy (number of bis used), reproduce he signal perfecly and give a space for reprogramming. I is known ha human brain does very well in speech recogniion while compuers failed o compee wih he human brain. Even compuers are able o sore and recall vas amouns of daa, perform mahemaical calculaions a high speeds and do repeiive ask wihou failing or geing bored, ye hey perform very poorly when faced wih raw sensory daa [2]. In speech recogniion, each word in he incoming audio signal is isolaed and hen analyzed o idenify he ype of exciaion and resonae frequencies. The Phase Shifing mehod, which has idenified o eliminae he echo indeed, would be using digial filers o accomplish he ask. Digial filers were designed o provide high performance in DSP. Digial filers main uses would be signal separaion and signal resoraion [2]. A digial filer is jus a filer ha operaes on digial signals, such as sound represened inside a compuer. I is a compuaion, which akes one sequence of numbers (he inpu signal) and produces a new sequence of numbers (he filered oupu signal) [3]. A real digial filer T n is defined as any real-valued funcion of a signal for each ineger n Є Z. Thus, a real digial filer maps every real, discree-ime signal o a real, discreeime signal. A complex filer, on he oher hand, may produce a complex oupu signal even when is inpu signal is real [3]. Phase is always measured relaive o a reference, which if known, permis absolue phase measuremen and if no known, permis only relaive phase measuremen [4]. The following diagrams illusrae his concep furher.

3 Figure 1.0 Signal wih 0 phase which is used as a reference for oher phase measuremen. Figure 2.0 Signal wih +90 phase wih reference o he signal from figure 1.0. I is also known as phase lead. Figure 3.0 Signal wih -90 phase wih reference o he signal from figure 1.0. I is also known as phase delay. 2.1 Previous Work A very less sofware based approaches have been aken o reduce he echo cancellaion problem. Moreover, mos of he echniques do no concenraing any specific applicaion area such as mulimedia conferencing area. Ocasic s Advance processors (Ocasic OCT6100) use a deerminisic echnique o deliver good qualiy in a reliable and deerminisic manner [5]. The OCT6100 series employed use of inexpensive memory and high processing power. I uses heir own predefined Leas Squares algorihm, which works based on minimizing he energy of he echo. The algorihm ensures he improved handling of double alk and background noise. Speaker idenificaion echnology using Leas Mean Square (LMS) algorihm is anoher effor in handling he echo problem [7]. The approach is for canceling he echo in long disance elephone conversaion due o he irregulariies of he analog elephone nework. The implemenaion of he echo canceller has been opimized for wo-way elephone conversaion and has been esed on he SWITCHBOARD corpus. 3 The Proposed Sysem Archiecure The proposed design would be purely based on a sofware approach. The Phase Shifer is a sofware componen ha would work on audio signal cancellaion in a machine, which receives he acousic echo. Figure 4.0 shows he illusraed diagram of he proposed design.

4 m(n) Phase Shifer m(n) + m(n) Microphone Speaker Figure 4.0 In audio (boardroom) conferencing sysem, when a near-end speaker speaks he voice signal would be added o he far-end speaker s original voice and i produces acousic echo for he near-end speaker. The process is similar o he far-end speaker. The above diagram shows he near-end speaker s overall echo cancellaion archiecure using Phase Shifer algorihm. In a normal (wihou echo cancellaion) boardroom conferencing environmen, when a user speaks hrough microphone he signal is capured, m(n). While his signal is ransmiing o he far-end user, he original signal (voice) is added wih he signal from he speaker. Thus he overall signal received by he far-end speaker would be m(n) + which in urn produces he echo for he far-end speaker. Considering he acousic echo which is solely produced by he speaker signal, he Phase Shifer approach is inroduced o eliminae he addiional signal or echo. This shifer would synhesis a signal ha has a differen phase (180 0 ) as compare o he signal produced by he speaker. Thus he newly synhesized signal would be. In order o remove he echo produced by he speaker, his signal (-) would be added o he echoed signal (m(n) + ). This addiion operaion will produce only he signal from near-end speaker voice (m(n)), ha is :-

5 m(n) + + (-) = m(n) Echoed signal Newly synhesized signal The signal addiion process would involve he implemenaion of digial filers. The Finie Impulse Response (FIR) filer is chosen o complee he above-menioned ask. FIR is a filer srucure ha can be used o implemen almos any sor of frequency response digially. 3.1 The Phase Shifer Algorihm Below is he brief descripion of he proposed phase shifer algorihm. capure_audio_daa send_audio_daa receive_audio_daa do_phase_analysis if phase NOT Consan hen do_phase_averaging else ge_phase end if synhesis_new_audio_daa_wih_phase_shifer while(capure_audio_daa) do add_echoed_audio_daa_wih_synhesized_daa_using_fir_filer send_audio_daa clear_echoed_audio_daa_buffer clear_synhesis_audio_daa_buffer done end while I is known ha for fas and high bandwidh ransmission, User Daagram Proocol (UDP) would be he bes opion as he mode of communicaion. However, he roue aken by he UDP packes are no consisen and herefore he phase of audio daa ha arrives mos likely would be varied. Thus, i is vial o do a phase analysis on he received audio daa before synhesizing new audio daa for canceling he echo.

6 Figure 5.0 Figure 5.0 shows wo analog audio signals wih same ampliude bu differen phases. These wo signals would be able o cancel each oher when hey are added wih a digial filer. Thus, produce a silence audio daa, which evenually cancel he echo. 4 Problems and Consrains The proposed mehod would no be an ideal soluion for pure echo cancellaion in mulimedia conferencing. There are some oher exernal elemens could caused he accomplishmen of he proposed mehod being disruped. a) Exernal Environmenal Noises The proposed phase shifer mehod would no be able o eliminae he exernal environmenal noises due o he fac ha hese disorions are negligible as is ampliude usually is low. The impac of hese elemens would be serious o he soluion if he ampliude of hese noises is high. Thus, addiional unnecessary daa is sen. So inclusion of proper eliminaion mehod for hese noises would be essenial. b) Elecrical Echo Elecrical echo is caused by impedance mismaches in he analog local loop [5]. Elecrical echo would no be an obvious problem in an inrane audio conferencing sysem where he disance is relaively shor and does no produce significan delays. However, he elecrical echo has o be conrolled especially in dial up connecion, which is using analog line as a medium of communicaion. c) UDP Packes ariable Roue UDP is an ideal proocol for audio conferencing since hey place a higher prioriy on coninuous media sreaming [6]. However he roues aken by UDP packes are always varied and no predicable. Consequenly, he audio packes may arrive in

7 differen phase. Due o he inconsisen roues, which have caused he differen phases of audio daa, i would be much complicaed in predicing he phase of incoming audio daa. The averaging process of differen phases is believed o give a soluion for his problem, ye i does no guaraneed an excepional qualiy in delivering he echoless audio daa. Implemenaion of audio conferencing in IPv6 is able o eliminae he occurrences of variable phase problem as he flow label and raffic class in IPv6 prioriized he UDP packes a he rouer level. 5 Conclusion and Fuure Works The proposed idea for echo cancellaion in mulimedia conferencing sysem uses an 180 o phase shifer echnique. The approach is more focusing on he aspec of digial signal processing as well as he phase manipulaion. Pracical implemenaion of his echnique is ye o complee bu he heoreical idea is believed o give a reasonable resul in canceling he echo. Besides, i is a purely hardware independen approach o eliminae he criical echo problem in mulimedia conferencing. Even he proposed idea is lacking in purifying he audio signal, i is able o provide a beer soluion for he primary problem. The idea can be improved wih he implemenaion of mulimedia conferencing sysem in IPv6 as well as inroducing exernal noise reducion echnique. References 1 Ocasic Semiconduor, Echo Cancellaion and oice Qualiy in Today s Circui Swiched, Digial Wireless, and oice over Packe Neworks, Augus Seven W. Smih, The Scienis and Enginners Guide o Digial Signal Processing, Second Ediion, ``Inroducion o Digial Filers'', by Julius O. Smih III, 2003 DRAFT. 4 Andrew Baeman, Iain Paerson-Sephens, The DSP Handbook, Prenice Hall, Ocasic, Echo Cancellaion and oice Qualiy in Today s Circui Swiched, Digial Wireless, and oice Over Packe Neworks, Josh Beggs & Dylan Thede, Designing Web Audio, Aravind Ganapahiraju & Joseph Picone, Echo Cancellaion for Evaluaing Speaker Idenificaion Technology,