Standardization of the Speech Transmission Index

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Standardization of theSpeech Transmission Index

• Herman J.M. Steeneken
• Former convenor ISO TC159/SC5/WG3, CEN TC 122/WG8

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Overview

• Workgroup Activities
• Scope and Field of Application of ISO-9921
• Criteria for Speech Communication Quality
• Assessment Methods
• Prediction methods
• Conclusions

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Workgroup activities (ISO, IEC)

• Standardisation of acoustical warning signals,
• (review ISO-7731, FDIS 2002, CEN standard next)
• Standardisation of Speech Transmission Quality,
• (review ISO-9921, FDIS 2002, CEN standard next)
• Standardisation of Speech Transmission Index,
• (review IEC-60268-16, CDV 2002)
• Standardisation of Sound systems for emergency
• purposes (review IEC 60849, CDV 2002)

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Scope and Field of Applications of ISO-9921
Criteria for speech communication quality - to
be understood by designers and responsible
people - sub-divided into representative
groups of applications Assessment methods
(simple and advanced) Prediction methods
(related to assessment)
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Criteria for Speech Communication Quality
Measures Speech Intelligibility Vocal effort
of speaker Applications Alert and warning
poor loud Person-to-person (critical) fair loud
Person-to-person (relaxed) good normal
Public address in public areas fair normal
Personal comm. systems fair normal
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Assessment Methods

• Subjective Assessment
• Mean Opinion Scores simple
• Sentence Intelligibility simple
• (embedded) Word lists advanced
• Objective Assessment
• Speech Interference Level (SIL) simple
• Speech Transmission Index (STI) advanced
• Speech Intelligibility Index (SII)

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Relation between subj. and obj. intelligibility
measures
Qualif. Sent. CVC PB
STI LSA LLN SII
nonsense meaning dB Excellent 100
gt81 gt 98 gt0.75 gt21 gt0.75 Goo
d 100 70-81 93-98
0.60-0.75 15 - 21 Fair 100
53-70 80-93 0.45-0.60 10 - 15 Poor
70-100 31-53 60-80
0.30-0.45 3 - 10 lt0.45 Bad lt70 lt31
lt60 lt 0.30 lt 3
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Qualification and relation between subjective
measures and STI
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Common Intelligibility Scale, CIS
Barnett and Knight, IOA 1994
CIS not linear with SNR ? STI ? 100 -
ALcons x AI ? PB words (256 words) ? Short
Sentences ? PB words (1000 words) ? 1000
syllables
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Relation STI versus SII
r 0.93
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Relation SIL versus STI/SII

r 0.97 (STI) r 0.95 (SII)
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Criteria in ISO-9921and IEC 60849
Application Intell. SIL STI
(CIS) Vocal dB
effort Alert and warning (simple) poor
8 0.40 0.6 Loud Alert and
warning (critical) fair 11
0.50 0.7 Loud Person-to-person (critical)
fair 11 0.50 0.7
Loud Person-to-person (relaxed) good 15
0.60 0.8 Normal Public address in public
areas fair 11 0.50 0.7
Normal Personal communication fair
11 0.50 0.7 Normal
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Prediction Methods
SIL, vocal effort, noise (ISO-9921) STI, vocal
effort, gender speaker,noise, band-pass
limiting, masking, reverberation,
echoes, non-linearity, (IEC 60268-16) SII,
vocal effort, noise, band-pass limiting,
masking, (ANSI 3 5.2)
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Calibration (Proposed)

• Reference speech signals at well defined
• conditions (through internet)
• Standardized spread sheets to calculate
• objective measures
• Specific software to process digital speech
• samples for listening tests

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Conclusions

• Increasing interest in verbal warning systems
• Revised standards on ergonomic assessment
• of speech communication, alarm and warning
• systems, all STI methods
• To be used by decision makers and designers
• Criteria, Assessment methods, and Prediction
• methods included