an error – concealment technique for wireless digital audio delivery n. tatlas, a. floros, t....

An Error – Concealment Technique for An Error – Concealment Technique for Wireless Digital Audio DeliveryWireless Digital Audio Delivery

N. Tatlas, A. Floros, T. Zarouchas and J. Mourjopoulos

AudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian UniversityAudioGroup, WCL, University of Patras - Dept. of Computer Science, Ionian University

IntroductionIntroduction

• 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


Packet – based digital audio distribution Packet – based digital audio distribution (DADi) (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


WiDADi for Home EnvironmentsWiDADi for Home Environments

• Application #1– Point-to-point home audio delivery

Audio serverAudio server

Audio player 2Audio player 2Audio player 1Audio player 1

Audio player 3Audio player 3


WiDADi for Home Environments (cont’d)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


WiDADi for Home Environments (cont’d)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


Aim of our workAim 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


FPR strategy descriptionFPR 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


FPR strategy description (cont’d)FPR strategy description (cont’d)

• Employ time-domain windowing functions to minimize PR distortions– Effect similar to fade-in and out process

t1 2 3

t1 2 3

t1 2 3 4

4

4 5 6

6

4 6

t1 2 3 4 64

Original frame sequence

Lost packet silence

Legacy PR strategy

FPR strategy


FPR strategy implementationFPR 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


Test methodologyTest methodology

• CD-quality wireless audio playback was simulated– No audio compression (stereo 16bit, 44.1kHz

LPCM)– Total pure digital audio data rate: 1.4112Mbps

(R) Speaker(R) Speaker(L) Speaker(L) Speaker


Test methodology (cont’d)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


Simulation platform descriptionSimulation platform description

Input Stereo Wave File

Wave2Tra

ce

L-Tra

ce

R-T

race

R-T

race

’

L-Tra

ce’

Output Stereo Wave File

Distortion

analysis

Simulation parameters

Tra

ce2

Wave

Transmission parameters


Simulation platform description (cont’d)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


ResultsResults

• Measured NMR vs transmitted packet length

-30 -25 -20 -15 -10 -5 0

294

882

Pac

ket

length

(byt

es)

NMR values (dB)

CoDeS+PR CoDeS+FPR CoDeS


Results (cont’d)Results (cont’d)

(R) Speaker(R) Speaker(L) Speaker(L) Speaker

Initial Signal

CoDeS Only

CoDeS + PR

CoDeS + FPR

Audibility testsAudibility tests

No concealment


ConclusionsConclusions

• 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


Conclusions (cont’d)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

Questions?Questions?

E-mail: [email protected]: [email protected]

www.ionio.grwww.ionio.grwww.wcl.ee.upatras.gr/www.wcl.ee.upatras.gr/audiogroupaudiogroup

an error – concealment technique for wireless digital audio delivery n. tatlas, a. floros, t....

Documents