An Error – Concealment Technique for Wireless Digital Audio Delivery

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

2. 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

3. 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

4. WiDADi for Home Environments • Application #1 • Point-to-point home audio delivery Audio server Audio player 3 Audio player 1 Audio player 2

5. WiDADi for Home Environments (cont’d) • Application #2 • Point-to-multipoint home audio delivery (L) Speaker (C) Speaker (R) Speaker Audio source (Sub) Speaker (RL) Speaker (RR) Speaker

6. WiDADi for Home Environments (cont’d) • Current technologies • Standardized or proprietary wireless protocols • Analog transmission in the range 800-900MHz • 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

7. 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 802.11e) • 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

8. 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 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”

9. 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

10. FPR strategy implementation • FPR parameters • Window type • Linear or exponential fading • Window length • Limited to the transmitted packet length (Lp) • A length of 50 samples was selected in this work • Low computational load • The number of packets (Np) to be used for possible substitution • Stored in a “substitution” buffer • 5 packets were stored • Current implementation requires only 5KB of memory

11. Test methodology • CD-quality wireless audio playback was simulated • No audio compression (stereo 16bit, 44.1kHz LPCM) • Total pure digital audio data rate: 1.4112Mbps (L) Speaker (R) Speaker

12. 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 802.11b protocol was employed • PHY rate equal to 11Mbps • Combined with the IEEE802.11e polling-based QoS mechanism

13. Simulation platform description Input Stereo Wave File Distortion analysis Output Stereo Wave File Wave2Trace Trace2Wave Simulation parameters L-Trace R-Trace L-Trace’ R-Trace’ Transmission parameters

14. 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

15. Results • Measured NMR vs transmitted packet length

16. Results (cont’d) Audibility tests No concealment CoDeS Only CoDeS + PR (L) Speaker (R) Speaker CoDeS + FPR Initial Signal

17. 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

18. 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 802.11e scheduling schemes • perceptually optimize shaping techniques of the audio samples used for packet replacement

19. Questions?

20. E-mail: floros@ionio.gr www.wcl.ee.upatras.gr/audiogroup www.ionio.gr