A Methodology for Deriving VoIP Equipment Impairment Factors for a Mixed NB/WB Context

Wireless Access Research

Adil Raja August 2007

A Methodology for Deriving VoIP Equipment Impairment Factors for a

Mixed NB/WB Context

Adil Raja

13th August, 2007



VoIP- The Shift to WB Paradigm

• The NB and WB would have to coexist for some time. i.e. operate either in cascade or in parallel.

• This work proposes an extension to E-Model for the parallel case.



The E-Model

• An Instrumental Model initially designed for NB handset telephony.

• R= R0-Is-Id-Ie,eff+A• R0=93.2 – the basic SNR.• Is - simultaneous impairments e.g. OLR, non-

optimum sidetone.• Id - e.g e2e delay and echo.• Ie,eff Impairments due to low-bitrate codecs and

packet loss.• A – Advantage factor.



The E-Model…

• R <-> MOS



Moller’s WB extension to E-Model

• Consider Two experiments• Each experiment is composed of two sets of speech

samples.• MOS evaluation is done based on ACR paradigm.• One set is evaluated in the NB/WB context.• The other set is evaluated in the NB context, by

downsampling the WB coded speech samples.



ExampleSpeech quality evaluation on narrowband

conditions only

1

2

3

4

5

1

Su

bje

ctiv

e M

OS

C1 C2

C3 C4

C5 C6

C7 C8

C9 C10

C11 C12

C13 C14

C15 C16

C17 C18

C19 C20

C21 Reference

Speech quality evaluation for narrowband and wideband conditions

1

2

3

4

5

1 2

Su

bje

cti

ve

MO

S

C1 C2

C3 C4

C5 C6

C7 C8

C9 C10

C11 C12

C13 C14

C15 C16

C17 C18

C19 C20

C21 Reference



ExampleRelationship between MOS for narrowband conditions

and MOS for wideband conditions

1

2

3

4

5

1 2 3 4 5

Subjective NMOS

Su

bje

cti

ve

MM

OS

STUDY GROUP 12 – DELAYED CONTRIBUTION 46

Discussion on unified objective methodologies for the comparison of voice quality of narrowband and wideband scenarios



The Extension

• Rnb=a+(eRnb/wb/b-1)

• a=169.38

• b=176.32

• The R-scale is stretched by 29%

• A linear extension with somewhat similar results exists.



R-scale extension with PESQ

• Two sets of speech samples are created.

• Both contain stimuli coded by NB and WB codecs. (124 samples each).

• The first (NB/WB) set is evaluated using PESQ-WB.

• The NB coded stimuli were upsampled to 16 kHz.



R-scale extension with PESQ

• How to evaluate the 2nd part of the test?

• For the 2nd (NB) part of test three options were considered for evaluating the samples.

• In Option 1 All the files (ref|deg of NB|WB) were downsampled to 8 kHz.

• This qualifies as a NB test.



R-scale extension … Option 1

RNB=a+b*RNB/WB

Max RNB=100.48

Suggests no extension.



The outcome




All Evaluation is done by PESQ-WB.

Contracts the scale instead.




• Same as Option 2 but the Ref samples for NB stimuli are downsampled/upsampled.

• This would clearly tantamount to repeating option 1 but with PESQ-WB



An Alternative

Using Supp 23 Exp 1&3.

Mapping between PESQ-WB & -NB evaluted NB coded samples.

Max RNB=107.1786



The Codecs

• G.729

• AMR-NB – 7.4 & 12.2 kbps

• G.723.1 – 6.3 kbps

• G.722.1 24/32 kbps

• G.722.2 – all 9 modes



Other metrics

• MLR - [0,2.5,. . . , 15, 20, . . . , 40]%• CLP - 10, 50, 60, 70 and 80%. • PI – 10-60 ms.• A total of 2,820 conditions were simulated• A given condition was applied to a file for 30 times; to

negate the effect of loss location.• Thus a total of 3,38,400 speech files were created.• Evaluation was done on beowulf using PESQ-WB



Ie,wb,eff vs various codecs



Scatter plot –training data



Scatter plot-Testing data



Surface plots of the derived functions



Parameter Significance



Ie,wb,eff vs mlr and mbl



Ie,wb,eff vs mlr and PI

Engineering

A Methodology for Deriving VoIP Equipment Impairment Factors for a Mixed NB/WB Context