Speech Transmission Index
Encyclopedia
Speech Transmission Index, short STI
is a measure of speech transmission quality. The absolute measurement of speech intelligibility is a complex science. The STI measures some physical characteristics of a transmission channel (a room, electro-acoustic equipment, telephone line, etc.), and expresses the ability of the channel to carry across the characteristics of a speech signal. STI is a well-established objective measurement predictor of how the characteristics of the transmission channel affects speech intelligibility.
The influence that a transmission channel has on speech intelligibility is dependent on:
in 1980.
Barnett (1995, 1999) proposed to use a reference scale, the Common Intelligibility Scale (CIS), based on a mathematical relation with STI (CIS = 1 + log (STI)).
STI predicts the likelihood of syllables, words and sentences being comprehended. As an example, for native speakers, this likelihood is given by:
If non-native speakers, people with speech disorders or hard-of-hearing people are involved, other probabilities hold.
It is interesting but not astonishing that STI prediction is independent of the language spoken - not astonishing, as the ability of the channel to transport patterns of physical speech is measured.
Another method is defined for computing a physical measure that is highly correlated with the intelligibility of speech as evaluated by speech perception tests given a group of talkers and listeners. This measure is called the Speech Intelligibility Index, or SII.
Further the following standards have, as part of the requirements to be fulfilled, integrated testing the STI and realisation of a minimal speech transmission index:
in airports and railway stations).
In STIPA testing, the preferred test signal employed is a signal with speech-like characteristics.
Speech can be described as noise that is modulated by low-frequency signals. Therefore STIPA employs a special amplitude modulation scheme to generate its test signal. At the receiving end of the communication system, the depth of modulation of the received signal is measured and compared with that of the test signal in each of a number of frequency bands. Reductions in the modulation depth are associated with loss of intelligibility.
An alternative Impulse response
method assumes that the channel is linear and requires stricter synchronization of the sound source to the measurement instrument.
in June 2011. At this time, this simplified STI derivative was still stipulated as a standard method in some industries. In this case, inferior-quality electro-acoustic components may pass testing, which would be avoided by applying the STIPA method.
STI
-Institutes:*Sega Technical Institute*Semantic Technology Institute International, a scientific and research network; also some of its members call themselves STI*Southern Technical Institute—a university in Marietta, Georgia, USA...
is a measure of speech transmission quality. The absolute measurement of speech intelligibility is a complex science. The STI measures some physical characteristics of a transmission channel (a room, electro-acoustic equipment, telephone line, etc.), and expresses the ability of the channel to carry across the characteristics of a speech signal. STI is a well-established objective measurement predictor of how the characteristics of the transmission channel affects speech intelligibility.
The influence that a transmission channel has on speech intelligibility is dependent on:
- the speech level
- frequency response of the channel
- non-linear distortions
- background noiseBackground noiseIn acoustics and specifically in acoustical engineering, background noise or ambient noise is any sound other than the sound being monitored. Background noise is a form of noise pollution or interference. Background noise is an important concept in setting noise regulations...
level - quality of the sound reproduction equipment
- echosEcho (phenomenon)In audio signal processing and acoustics, an echo is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single...
(reflectionsReflection (physics)Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
with delay > 100ms) - the reverberation timeReverberationReverberation is the persistence of sound in a particular space after the original sound is removed. A reverberation, or reverb, is created when a sound is produced in an enclosed space causing a large number of echoes to build up and then slowly decay as the sound is absorbed by the walls and air...
- psychoacoustic effectsPsychoacousticsPsychoacoustics is the scientific study of sound perception. More specifically, it is the branch of science studying the psychological and physiological responses associated with sound...
(masking effects)
History
STI was introduced by Tammo Houtgast and Herman Steeneken in 1971, and was accepted by Acoustical Society of AmericaAcoustical Society of America
The Acoustical Society of America is an international scientific society dedicated to increasing and diffusing the knowledge of acoustics and its practical applications.-History:...
in 1980.
Scale
STI is a numeric representation measure of communication channel characteristics whose value varies from 0 = bad to 1 = excellent. On this scale, an STI of at least .5 is desirable for most applications.Barnett (1995, 1999) proposed to use a reference scale, the Common Intelligibility Scale (CIS), based on a mathematical relation with STI (CIS = 1 + log (STI)).
STI predicts the likelihood of syllables, words and sentences being comprehended. As an example, for native speakers, this likelihood is given by:
| STI Value | Quality according to IEC 60268-16 | Intelligibility of Syllables in % | Intelligibility of Words in % | Intelligibility of Sentences in % |
|---|---|---|---|---|
| 0 - 0.3 | bad | 0 - 34 | 0 - 67 | 0 - 89 |
| 0.3 - 0.45 | poor | 34 - 48 | 67 - 78 | 89 - 92 |
| 0.45 - 0.6 | fair | 48 - 67 | 78 - 87 | 92 - 95 |
| 0.6 - 0.75 | good | 67 - 90 | 87 - 94 | 95 - 96 |
| 0.75 - 1 | excellent | 90 - 96 | 94 - 96 | 96 - 100 |
If non-native speakers, people with speech disorders or hard-of-hearing people are involved, other probabilities hold.
It is interesting but not astonishing that STI prediction is independent of the language spoken - not astonishing, as the ability of the channel to transport patterns of physical speech is measured.
Another method is defined for computing a physical measure that is highly correlated with the intelligibility of speech as evaluated by speech perception tests given a group of talkers and listeners. This measure is called the Speech Intelligibility Index, or SII.
Standards
STI has gained international acceptance as the quantifier of channel influence on speech intelligibility. The International Electrotechnical Commission Objective rating of speech intelligibility by speech transmission index, as prepared by the TC 100 Technical Committee, defines the international standard.Further the following standards have, as part of the requirements to be fulfilled, integrated testing the STI and realisation of a minimal speech transmission index:
- International Organization for Standardization (ISO)International Organization for StandardizationThe International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
standard for sound system loudspeakers in Fire detection and fire alarm systems - National Fire Protection Association Alarm Code
- British Standards Institution Fire detection and alarm systems for buildings
- German Institute for Standardization Sound Systems for Emergency Purposes
STIPA
STIPA (Speech Transmission Index for Public Address Systems) is a simplified version of STI designed for practical application in specific situations (amongst others measuring PA SystemsPublic address
A public address system is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a sound source, e.g., a person giving a speech, a DJ playing prerecorded music, and distributing the sound throughout a venue or building.Simple PA systems are often used in...
in airports and railway stations).
In STIPA testing, the preferred test signal employed is a signal with speech-like characteristics.
Speech can be described as noise that is modulated by low-frequency signals. Therefore STIPA employs a special amplitude modulation scheme to generate its test signal. At the receiving end of the communication system, the depth of modulation of the received signal is measured and compared with that of the test signal in each of a number of frequency bands. Reductions in the modulation depth are associated with loss of intelligibility.
An alternative Impulse response
Impulse response
In signal processing, the impulse response, or impulse response function , of a dynamic system is its output when presented with a brief input signal, called an impulse. More generally, an impulse response refers to the reaction of any dynamic system in response to some external change...
method assumes that the channel is linear and requires stricter synchronization of the sound source to the measurement instrument.
History
Historically (for example, at the time when aviation authorities first demanded sufficiently high STI measurements of electro-acoustic systems) the RASTI (Room Acoustics Speech Transmission Index) method was a practical solution, because full STI measurements were technologically not feasible. RASTI does not take the whole signal-degrading effects of electro-acoustic components into account and was declared obsolete by the IECInternational Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
in June 2011. At this time, this simplified STI derivative was still stipulated as a standard method in some industries. In this case, inferior-quality electro-acoustic components may pass testing, which would be avoided by applying the STIPA method.