ETSI TS 103 557 V1.3.1 (2020-03)

Speech and multimedia Transmission Quality (STQ); Methods for reproducing reverberation for communication device measurements

RTS/STQ-290

General Information

• Status: Published
• Publication Date: 22-Mar-2020
• Technical Committee: STQ - Speech and multimedia Transmission Quality
• Current Stage: 12 - Completion
• Due Date: 19-Mar-2020
• Completion Date: 23-Mar-2020

Standards Content (Sample)

ETSI TS 103 557 V1.3.1 (2020-03)






TECHNICAL SPECIFICATION
Speech and multimedia Transmission Quality (STQ);
Methods for reproducing reverberation
for communication device measurements

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2 ETSI TS 103 557 V1.3.1 (2020-03)



Reference
RTS/STQ-290
Keywords
quality, reverberation, simulation, speech,
terminal, testing
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3 ETSI TS 103 557 V1.3.1 (2020-03)
Contents
Intellectual Property Rights . 4
Foreword . 4
Modal verbs terminology . 4
Introduction . 4
1 Scope . 5
2 References . 5
2.1 Normative references . 5
2.2 Informative references . 5
3 Definition of terms, symbols and abbreviations . 6
3.1 Terms . 6
3.2 Symbols . 6
3.3 Abbreviations . 6
4 Rationale . 7
5 Room simulation in Sending . 7
5.1 Impulse Response Measurement . 7
5.1.1 Microphone setup . 7
5.1.1.1 Fixed Microphone setup . 7
5.1.1.2 Flexible Microphone setup . 7
5.1.2 Sound source . 7
5.1.3 Measurement procedure . 8
5.2 Loudspeaker setup for reproducing reverberation based on the fixed microphone setup . 8
5.2.1 Introduction and System Overview . 8
5.2.2 Preparations . 9
5.2.2.1 Separation of impulse responses . 9
5.2.2.2 Delay and level adjustment . 10
5.2.3 Test room requirements . 11
5.2.4 Equalization and calibration . 11
5.2.5 Accuracy of the reproduction arrangement . 12
5.2.5.1 Evaluation parameters . 12
5.2.5.2 Reverberation time . 12
5.2.5.3 Clarity . 12
5.2.5.4 Direct-to-Reverberant Energy Ratio . 12
5.2.5.5 Coherence. 12
5.3 Loudspeaker setup for reproducing reverberation based on the flexible microphone setup . 13
5.4 Impulse response database and signal generation . 13
5.5 Validation and examples . 13
5.5.1 Reproduction of room acoustical parameters . 13
5.5.2 Application examples . 17
Annex A (normative): Impulse Response Database . 19
A.1 Fixed microphone setup . 19
A.2 Impulse responses in a home-like test environment . 20
Annex B (informative): Home-like test environment . 23
History . 25


ETSI

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4 ETSI TS 103 557 V1.3.1 (2020-03)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (https://ipr.etsi.org/).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Trademarks
The present document may include trademarks and/or tradenames which are asserted and/or registered by their owners.
ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
Foreword
This Technical Specification (TS) has been produced by ETSI Technical Committee Speech and multimedia
Transmission Quality (STQ).
The present document is to be used in conjunction with:
• ETSI TS 103 224 [1] series: "A sound field reproduction method for terminal testing including a background
noise database".
The present document describes a sound field recording and reproduction technique which can be applied for all types
of terminals but is especially suitable for modern multi-microphone terminals including array techniques. While ETSI
TS 103 224 [1] focuses on background noise, the present document considers the reproduction of reverberation.
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
Introduction
Many devices that employ microphones to pick up speech signals are used in a hands-free manner. Since there is
usually a larger distance between the talker and the device, the microphone signals contain a significant amount of noise
and reverberation.
This includes, e.g. phones in hands-free mode, group-audio terminals or smart speakers with speech recognition
capabilities as well as terminals in handset or headset mode. Note that the same issues can also arise for hand-held
devices depending on the acoustic conditions, see [i.1].
Testing of these devices requires a realistic reproduction of both the noise as well as the reverberation in a defined and
reproducible manner. For background noise reproduction, ETSI has standardized a reproduction method (with an
accompanying database of background noise signals) in ETSI TS 103 224 [1].
ETSI

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5 ETSI TS 103 557 V1.3.1 (2020-03)
1 Scope
The present document describes a methodology for recording and reproducing different room characteristics and
realistic reverberation under conditions that are well-defined and tailored for a calibrated setup in a lab environment.
The individual aspects of the description are:
• Measurement of room impulse responses.
• Processing of test signals.
• Loudspeaker setup, calibration and equalization.
The methodology is fundamentally designed for use without access to internals of the Device Under Test (DUT),
e.g. the exact positions and orientations of the device's microphones or the unprocessed microphone signals. The
methodology is intended to be used for performance evaluation of all types of devices where the room characteristics
may impact the performance.
2 References
2.1 Normative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found at
https://docbox.etsi.org/Reference/.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
The following referenced documents are necessary for the application of the present document.
[1] ETSI TS 103 224: "Speech and multimedia Transmission Quality (STQ); A sound field
reproduction method for terminal testing including a background noise database".
[2] Recommendation ITU-T P.58: "Head and torso simulator for telephonometry".
[3] N. Xiang: "Evaluation of reverberation times using a nonlinear regression approach" in The
Journal of the Acoustical Society of America Vol. 98, 1995.
[4] Recommendation ITU-T P.56: "Objective measurement of active speech level".
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] M. Jeub, M. Schäfer, H. Krüger, C. Nelke, C. Beaugeant and P. Vary: "Do We Need
Dereverberation for Hand-Held Telephony?", International Congress on Acoustics (ICA), Sydney,
2010.
ETSI

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6 ETSI TS 103 557 V1.3.1 (2020-03)
[i.2] Recommendation ITU-T P.341 (03/2011): "Transmission characteristics for wideband digital
loudspeaking and hands-free telephony terminals".
[i.3] ISO 3382-1: "Acoustics -- Measurement of room acoustic parameters -- Part 1: Performance
spaces".
[i.4] Recommendation ITU-T P.501: "Test signals for use in telephonometry".
[i.5] ETSI TS 103 738: "Speech and multimedia Transmission Quality (STQ); Transmission
requirements for narrowband wireless terminals (handsfree) from a QoS perspective as perceived
by the user".
[i.6] ETSI TS 103 740: "Speech and multimedia Transmission Quality (STQ); Transmission
requirements for wideband wireless terminals (handsfree) from a QoS perspective as perceived by
the user".
3 Definition of terms, symbols and abbreviations
3.1 Terms
Void.
3.2 Symbols
For the purposes of the present document, the following symbols apply:
Clarity with cut-off time
Sampling frequency
Reverberant components of the impulse response to microphone
ℎ Impulse response to microphone
ℎ Impulse response to the microphone closest to the HATS
ℎ Direct path component of the impulse response to microphone ,
Length of the impulse response
Number of microphones
MM-# Microphone number # of MM
MSA-# Microphone number # of MSA
MU-# Microphone number # of MU
Reverberation time

Source signal
Cut-off time for clarity (50 ms for speech, 80 ms for music)

T Delay difference between HATS and reverberation reproduction system
DIFF
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
DRR Direct-to-Reverberant energy Ratio
DUT Device Under Test
FFT Fast Fourier Transform
HATS Head And Torso Simulator
LRC Lip Ring Centre
MLS Maximum Length Sequence
MM Measurement Microphone(s)
MRP Mouth Reference Point
MSA Microphone Sound Array
MU Mock-Up phone
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SFR Send Frequency Response
SLR Send Loudness Rating
SNR Signal-to-Noise Ratio
4 Rationale
Including reverberation in a realistic manner is of paramount importance for accurate testing of, e.g. phones in hands-
free mode, group-audio terminals or smart speakers with speech recognition capabilities. While it is possible to test this
simply by using the device in a reverberant room, the present document introduces an alternative approach that is based
on the explicit reproduction of the reverberant sound field at several microphone positions.
The present method does not require many rooms or variable acoustics setups for testing multiple acoustic conditions.
The reverberant sound field can both be measured directly in a reverberant room or it can be calculated based on a
database of impulse responses that is provided in combination with the present document.
NOTE 1: The room acoustics has an impact on three transmission paths that could be considered from a
measurement perspective:
 from the mouth of the user to the microphone(s) of the DUT (Sending);
 from the loudspeaker(s) of the DUT to the ears of the user (Receiving);
 from the loudspeaker(s) of the DUT to the microphone(s) of the DUT (Echo-path).
NOTE 2: The methodology described in the present document is intended for the first scenario. Although it might
be applicable to the other scenarios as well (possibly with modifications), this has not been verified yet
and is subject to further study.
5 Room simulation in Sending
5.1 Impulse Response Measurement
5.1.1 Microphone setup
5.1.1.1 Fixed Microphone setup
A fixed microphone setup should be used for DUTs with smaller form factors (e.g. mobile phones in hands-free
operation). The microphone setup shall consist of =8 microphones and conform to the description in ETSI
TS 103 224 [1], clause 5. Since this setup is device-independent, no new impulse response measurements are necessary
when testing a new device.
5.1.1.2 Flexible Microphone setup
For larger DUTs, a flexible microphone setup with a use-case dependent number of microphones shall be used. The
microphone setup shall conform to the description in ETSI TS 103 224 [1], clause 7. Since this setup is device-
dependent, testing a new device requires new impulse response measurements.
5.1.2 Sound source
Since the testing scenario consists of a human talker in a reverberant environment, a HATS with an equalized artificial
mouth according to Recommendation ITU-T P.58 [2] shall be used for sound generation. This applies to measurement
of impulse responses as well as to recording of reverberant speech signals.
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5.1.3 Measurement procedure
There exist different possibilities for measuring impulse responses, e.g. using Maximum Length Sequences (MLS) or
using swept-sines (sweeps). The advantage of sweeps is that non-linearities can easily be observed and that the SNR in
lower frequencies is higher than with MLS. Using logarithmic sweeps is therefore recommended for system
identification. The sweep should cover a frequency range from 20 Hz to 20 kHz and the length of the sweep should be
chosen in such a way that no significant components of the impulse response are truncated. Accordingly, a sweep length
of at least 2 s should be used for typical rooms while larger rooms might need longer sweeps. While the sweep can be
used directly as the measurement signal, it is recommended to construct the measurement signal from the individual
sweep by repeating it at least five times. If the repetition is used, the determination of the impulse response should be
based on an averaging of all but the first sweep period. The first period is discarded to avoid transient effects and the
averaging should be performed in the time domain.
5.2 Loudspeaker setup for reproducing reverberation based on
the fixed microphone setup
5.2.1 Introduction and System Overview
In order to correctly reproduce the sound field in a reverberant room, the setup described in ETSI TS 103 224 [1] is not
sufficient. Direct sound as well as the reverberant components shall be considered. A combination of the multi-channel
loudspeaker setup with the artificial mouth of a HATS is used for the reproduction.
The multi-channel loudspeaker setup shall follow the description in ETSI TS 103 224 [1]. If possible, the HATS should
be positioned at the same position (distance, angle, mouth direction, etc.) for the sound field reproduction with respect
to the microphone arrangement as in the original reverberant room. If this is not possible, the HATS shall be positioned
at the same angle with respect to the microphone arrangement as in the original reverberant room and the change in
distance shall be considered when adjusting levels and delays between the direct path and the reverberant components.
An overview of the signal processing is given in Figure 1. The two paths are visible here as well: The signal for the
HATS is only subject to a level (multiplication by coefficient a) and delay (delay element of length T ) adjustment
DIFF
which is covered in clause 5.2.2.2. The reverberant signal components are reproduced by removing the direct path from
the impulse responses (see clause 5.2.2.1) and generating the target signals for the system according to ETSI
TS 103 224 [1] by convolving the source signal with the remaining parts of the impulse responses.

Figure 1: Overview of the signal processing for the reproduction setup
For the fixed microphone setup, the impulse responses can be taken directly from the provided database. For the
flexible microphone setup, individual measurements have to be carried out to use the reproduction system.
ETSI

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5.2.2 Preparations
5.2.2.1 Separation of impulse responses
The reproduction system needs two signal parts: the direct sound (single channel that is played over the HATS) and the
reverberant components (eight target signals that are fed into the reproduction system according to ETSI
TS 103 224 [1]). An example impulse response ℎ is shown in Figure 2 with the direct path component ℎ in ,
orange and the remaining reverberant components in blue. These two components constitute the entire impulse
response according to:
ℎ ℎ
,

Figure 2: Time domain representation of an example impulse response
An overview of the necessary signal processing is depicted in Figure 5. From the measured impulse responses
ℎ… ℎ , the reverberant components … are extracted by removing the first few milliseconds of the
impulse response, i.e. the direct path. These reverberation filters … are then used to calculate the input
signals for the reproduction system according to ETSI TS 103 224 [1].
The extraction of the direct path from the impulse response is done by searching for the largest absolute amplitude in
the impulse response and selecting a window of ±2,5 ms around this position. To avoid signal processing artefacts,
squared sinusoidal sections shall be used for fading in and out. The entire 5 ms section shall have 0,25 ms of squared
sine fade-in in the beginning and 0,25 ms of squared sine fade-out in the end.
The resulting window function is depicted in Figure 3 and an enlarged view of the fade-in is presented in Figure 4. If
the largest amplitude in the impulse response is closer than 2,5 ms to the beginning of the impulse response, the fade-in
shall be omitted and the first part of the impulse response (from the start to 2,5 ms after the largest amplitude) shall be
used for the direct path. The remainder of the impulse response contains all the reverberant components.
ETSI

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Figure 3: Window function for extracting the direct path from the impulse responses
(time scale relative to maximum position of impulse response)

Figure 4: Fade-in of the window function
The remainder of the impulse response contains all the reverberant components.
All channels of the impulse response shall be separated according to the given procedure. As described, the direct paths
are used for determining the delay and level differences between the two system components. For calculating the
reproduction targets for the reproduction system according to ETSI TS 103 224 [1], however, the reverberant
components are used.
5.2.2.2 Delay and level adjustment
For the aforementioned level and delay compensation, the signal from the artificial mouth needs to be adjusted to the
signal from the reproduction system. This shall be achieved by comparing the level of and the delay between two
signals. A linear or logarithmic sweep signal of at least 2 s covering a frequency range from ≤ 100 Hz to ≥ 4 000 Hz
shall be both played by the artificial mouth and convoluted with the direct path components to get target signals for the
reproduction system.
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11 ETSI TS 103 557 V1.3.1 (2020-03)
As shown in Figure 5, this leads to two recorded signals: y (k) and y (k). The delay difference between the two
HATS REP
systems shall be estimated by calculating the cross-correlation function Φy y ( λ) between the two signals and
HATS REP
searching for the peak value of its envelope E( λ). The envelope E( λ) is calculated using the Hilbert transformation
H{Φy y }( λ) of the cross-correlation:
HATS REP

∑
Φ ⋅
 Φ Φ
The position on the time axis of the peak value is the delay difference T between the two system paths. The signal
DIFF
that arrives earlier (this will usually be the signal of the artificial mouth) shall be delayed by T to temporally align
DIFF
the two paths.
The level alignment can be carried out in similar fashion. The levels of both signals shall be calculated according to
Recommendation ITU-T P.56 [4] and the attenuation or amplification a in the path of the HATS shall be set to
compensate the level difference between the two signals.
NOTE: In case the distance between HATS and DUT is identical to the one included in the impulse responses, the
result of the level adjustment is expected to be 0 dB. However, it is recommended to apply the determined
level adjustment anyway, in order to compensate for small differences between simulated and real setup.

Figure 5: Overview of the signal processing for the delay compensation
5.2.3 Test room requirements
The requirements are identical to the requirements in ETSI TS 103 224 [1]. In addition, the reverberation time of the
measurement chamber that is used for the reproduction imposes a lower bound on the range of rooms that can be
reproduced. Thus, an anechoic room is recommended. If semi-anechoic rooms or measurement chambers are used, the
effects of additional reflections of the reproduction room have to be considered.
5.2.4 Equalization and calibration
The calibration of the microphone array and the equalization of the loudspeaker system are identical to ETSI
TS 103 224 [1]. The artificial mouth of the HATS shall conform to Recommendation ITU-T P.58 [2], no additional
equalization is needed.
ETSI

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5.2.5 Accuracy of the reproduction arrangement
5.2.5.1 Evaluation parameters
The accuracy of the reproduction shall be verified by measuring the following four parameters and asserting that they
are within the given tolerance range around the true value for each dataset.
The first three parameters are based on the impulse response h(k) of the entire reproduction system. The impulse
response of the reproduction system should be determined in the same manner as described in clause 5.1.3. Note that
the measurement signal (usually a sweep) has to be fed into the entire reproduction system and not only emitted by the
artificial mouth, i.e. the measurement signal is the source signal s(k) in Figure 1. The impulse response and all the
derived parameters shall be determined for all eight microphones in the fixed microphone setup and for all microphones
in the flexible microphone setup, respectively.
5.2.5.2 Reverberation time
Probably the most
...