An Error – Concealment Technique for Wireless Digital Audio Delivery N. Tatlas, A. Floros, T. Zarouchas and J. Mourjopoulos

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University Introduction Digital audio technology evolution –… from analog to digital –Full digital audio chains –All subsystems are digitally interconnected Interconnections types –Synchronous (S/PDIF, HDMI) –Asynchronous (Packet based networks) –Wired –Latest trends: Wireless! Bluetooth, USB, WLANs

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University Packet – based digital audio distribution (DADi) Large scale applications –Real-time streaming over the network –Network-centric live performances Short scale applications –Audio distribution in home environments –Numerous distribution scenarios can be established From any digital audio source To any networked receiver/playback device Wireless distribution (WiDADi) –Flexible, cable-free, low-cost packet-based audio distribution

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University WiDADi for Home Environments Application #1 –Point-to-point home audio delivery Audio server Audio player 2 Audio player 1 Audio player 3

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University WiDADi for Home Environments (cont’d) Application #2 –Point-to-multipoint home audio delivery (R) Speaker(L) Speaker (RR) Speaker(RL) Speaker Audio source (C) Speaker (Sub) Speaker

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University WiDADi for Home Environments (cont’d) Current technologies –Standardized or proprietary wireless protocols –Analog transmission in the range MHz –High-rate digital transmission protocols Operating in the range of 2.4 and 5GHz Bluetooth IEEE802.11x (WLANs) Main technology drawbacks –Audio compression is usually required –Lack of in-time delivery guarantees –Conversion of the asynchronous packet-based data delivery to synchronous Synchronize the remote audio receivers

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University Aim of our work Distortions introduced during DADi over WLANs –playback gaps due to packet losses –relative channel phase shifting causes loss of the sound spatial information –In both cases, the distortion presence is audible! Improve the audible effect of temporal playback muting during the DADi over WLANs –QoS WLAN support (IEEE e) –A playback synchronization correction scheme Consistent Delay Synchronization (CoDeS) –minimizes the variable channel phase shift and keeps all the wireless audio receivers synchronized –An error concealment mechanism Fading Pattern Repetition (FPR) strategy

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University FPR strategy description Based on the well-known Pattern-Repetition (PR) algorithm –widely used in speech transmission applications –“When a packet is lost, it is substituted by inserting a previous, correctly received data segment” PR advantages –Low implementation complexity and computational load PR drawbacks –amplitude and phase mismatch between the audio stream and the segment to be repeated –cause audible clicking sounds

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University FPR strategy description (cont’d) Employ time-domain windowing functions to minimize PR distortions –Effect similar to fade-in and out process Original frame sequence Lost packet silence Legacy PR strategy FPR strategy

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University FPR strategy implementation FPR parameters –Window type Linear or exponential fading –Window length Limited to the transmitted packet length (L p ) A length of 50 samples was selected in this work –Low computational load –The number of packets (N p ) to be used for possible substitution Stored in a “substitution” buffer 5 packets were stored Current implementation requires only 5KB of memory

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University Test methodology CD-quality wireless audio playback was simulated –No audio compression (stereo 16bit, 44.1kHz LPCM) –Total pure digital audio data rate: Mbps (R) Speaker (L) Speaker

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University Test methodology (cont’d) 48 header bytes added –8 bytes for the UDP transport protocol –40 bytes for CoDeS and reserved Transmission packet length –294 and 882 bytes Total bandwidth required –1.6416Mbps and 1.488Mbps The low-cost IEEE b protocol was employed –PHY rate equal to 11Mbps –Combined with the IEEE802.11e polling-based QoS mechanism

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University Simulation platform description Input Stereo Wave File Wave2Trace L-TraceR-Trace R-Trace’ L-Trace’ Output Stereo Wave File Distortion analysis Simulation parameters Trace2Wave Transmission parameters

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University Simulation platform description (cont’d) Distortions audibility effect was assessed via perceptual criteria –Noise-to-Mask Ratio (NMR) Employs masking functions of the human ear determines the distance of the distortions imposed from the instantaneous audibility threshold The original PCM signal prior transmission is used as reference for perceptually insignificant distortions, the values of NMR must be as low (and negative) as possible –Audibility tests

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University Results Measured NMR vs transmitted packet length

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University Results (cont’d) (R) Speaker (L) Speaker Initial Signal CoDeS Only CoDeS + PR CoDeS + FPR Audibility tests No concealment

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University Conclusions WLANs represent a very promising and flexible mean for audio delivery –Recent advantages on QoS support Variable link quality degradations lead to –excessive packet delivery delays … and equivalent data losses –relative audio channels phase shifts in typical multichannel audio playback setups

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University Conclusions (cont’d) The FPR error concealment strategy is proposed –Combined with the CoDeS Synchronization scheme Aims to minimize the perceptual effect of muting introduced by synchronization scheme –By replacing the silence gaps with appropriately processed data samples derived from successfully received packets Low design complexity and computational cost Easy-to-implement algorithm Future work –include the FPR enhancement in IEEE e scheduling schemes –perceptually optimize shaping techniques of the audio samples used for packet replacement

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