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Timestamp: 2019-04-25 18:47:39+00:00

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small impairments in audio systems including multichannel sound systems (with or without picture).
Experts Group (ISO/MPEG) – Audio ad hoc Group.
11. Contents of test reports.
given to the test subjects.
A number of common words are used with technical meanings. A Glossary of these is given in Appendix 4.
subjective assessment of small impairments in audio systems, the most formal experimental methods shall be used.
secondly by quantitative data from human observers.
arranged in a way that reveals the effects of the independent factors, and only of these factors.
distributed randomly, both within and between sessions.
assessment of audio systems is carried out without accompanying pictures.
are actual assessments of the impairments.
3.2 Criteria for selecting subjects The outcome of subjective tests of sound systems with small impairments utilizing a selected group of listeners is not primarily intended for extrapolation to the general public. include methods such as audiometric tests. In some cases both types of rejection might be used.2. This must however be balanced against the risk of limiting the relevance of the result too much. 3. a larger number of subjects will be required to ensure that a sufficiently large number pass the post-screening test. A suggested statistical method for doing this is described in Appendix 1.1 Expert listeners It is important that data from listening tests assessing small impairments in audio systems should come exclusively from subjects who have expertise in detecting these small impairments. Any type of rejection technique which is not carefully analysed and applied may lead to a biased result. ITU-R BS. Rec.1116-1 3 3 Selection of listening panels 3. The application of a post-screening method may clarify the tendencies in a test result. bearing in mind the variability of subjects’ sensitivities to different artefacts. Here. The training procedure might be used as a tool for pre-screening.1 Pre-screening of subjects Pre-screening procedures. 3. experience has shown that data from 20 subjects is often sufficient for drawing appropriate conclusions from the test. There is sometimes a reason for introducing a rejection technique either before (pre-screening) or after (post-screening) the real test. The demanding nature of the test procedure is intended to reveal those problems that may be revealed during the extensive period of exposure under different conditions which occur in real life once a system has been introduced to the consumer. 3. . are able to perceive relatively subtle degradations but also to produce a quantitative estimate of the introduced impairments. tight experimental control cannot be achieved. then larger numbers of subjects might be needed to attain the required resolution.3 Size of listening panel The adequate size for a listening panel can be predicted if the variance can be estimated and the required resolution of the experiment is known. for any reason. the more important it is to have expert listeners. elimination is referred to as a process where all judgements from a particular subject are omitted. Whenever a subjective listening test is performed with the test method recommended here. However. It is therefore extremely important that. If analysis of the data can be carried out as the test proceeds.2. whenever elimination of data has been made. one is based on inconsistencies compared with the mean result and another relies on the ability of the subject to make correct identifications. then no further subjects need be processed when an adequate level of statistical significance for drawing appropriate conclusions from the test has been reached. the test report clearly describes the applied criterion so that the reader can make his own judgement. under certain conditions. Where the conditions of a listening test are tightly controlled on both the technical and behavioural side. The higher the quality reached by the systems to be tested. caution should be exercised.2 Post-screening of subjects Post-screening methods can be roughly separated into at least two classes. If some of the systems under test are expected to be nearly transparent. The first class is never justifiable. The methods are primarily used to eliminate subjects who cannot make the appropriate discriminations. Normally the aim is to investigate whether a group of expert listeners. The major argument for introducing a pre-screening technique is to increase the efficiency of the listening test. selection of subjects based on their previous experience and performance in previous tests and elimination of subjects based on a statistical analysis of pre-tests. the required information for the second class of post-screening is automatically available. If.
The following equation may be used to achieve such normalisation whilst retaining the original scale. Any perceived differences between the reference and the other stimuli must be interpreted as an impairment. As soon as the subject. The excerpt may be repeated until the subject has made an assessment. it is necessary to select an appropriate method. The use of scales without intermediate anchor points also precludes the interpretation of results in absolute terms. by increasing the size of the listening panel.0 Very annoying 1. only valid for precisely that group of expert listeners actually involved in the test. This may help to overcome translation problems in comparisons of tests carried out in different languages. It is possible to use the number scales without descriptions of anchor points. one subject at a time is involved and the selection of one of three stimuli (“A”.0 Perceptible. The size of the listening panel may also need to be increased to allow for the probability that subjects vary in their sensitivity to different artefacts. Therefore. Zi = (xi − x si ) ⋅ ss + x s ssi where: Zi : normalised result xi : score of subject i xsi : mean score for subject i in session s xs : mean score of all subjects in session s ss : standard deviation for all subjects in session s ssi : standard deviation for subject i in session s. ITU-R BS. stable and to permit accurate detection of small impairments. TABLE 1 Impairment Grade Imperceptible 5.4 Rec. in the preferred method.1284 and in Table 1. Thus. The “double-blind triple-stimulus with hidden reference” method has been found to be especially sensitive. The hidden reference and the object are simultaneously available but are “randomly” assigned to “B” and “C”. 1992]. the result can be claimed to hold for a more general group of expert listeners and may therefore sometimes be considered more convincing. according to the continuous five-grade impairment scale.0 Annoying 2. should be indiscernible from stimulus “A”. In the preferred and most sensitive form of this method.0 NOTE 1 – It has been shown that the use of pre-defined intermediate anchor points may introduce bias [Poulton. it should be used for this kind of test. The known reference is always available as stimulus “A”. the intended orientation of the scales must be indicated.0 Slightly annoying 3. has completed the grading of a trial. depending on the trial. In such cases. The grading scale shall be treated as continuous with “anchors” derived from the ITU-R five-grade impairment scale given in Recommendation ITU-R BS. “C”) is at the discretion of this subject. The result from the type of experiment dealt with in this Recommendation is. “B”. and “C” compared to “A”. “B” or “C”. . but not annoying 4. the other one may reveal impairments. in principle.1116-1 The size of a listening panel is not solely a consideration of the desired resolution. it should be possible to proceed directly on to the next trial. In this way the test procedure is self pacing. 4 Test method To conduct subjective assessments in the case of systems generating small impairments. The subject is asked to assess the impairments on “B” compared to “A”. One of the stimuli. If intermediate anchor points are not used it is essential that the results for individual subjects are normalised with respect to mean and standard deviation.
1 Familiarization or training phase Prior to formal grading. “instructions to listeners”. By the end of the familiarization process. subjects should have arrived at a stable sense of the scale that will be used in the formal grading phase which will follow familiarization or training. Only in this way can the subject exercise complete individual freedom to switch among the stimuli in the triple stimulus method. This is best done if a near-instantaneous switching (see Note 1) method is used in conjunction with a triple stimulus system as described in Appendix 3. “clicks”) of the switching system. . the grading scales and the methods of their use. The test method consists of two parts: a familiarization or training phase. although the self-paced character of trials advocated here will introduce uncontrolled variability among subjects. Properly carried out. The potential problem with having subjects try to assess more than one attribute on each trial is one of response burden. It is preferred that the attribute “basic audio quality” is evaluated in each case.g. then this might produce unreliable gradings for all the questions. 4. the test environment.2 Grading phase At the start of the first formal grading session of the day. There should be no audible artefacts (e. the test procedure should rely exclusively on short-term memory. Rec. since such artefacts can seriously interfere with the assessment process. familiarization can transform some subjects with initially low ability into experts for the purposes of the test. For the most sensitive tests they should be exposed to all the material they will be grading later in the formal grading sessions. Subject fatigue may become a major factor which would seriously interfere with the validity of judgements. subjects must be allowed to become thoroughly familiar with the test facilities. Such freedom is essential so that the subject can use his own discretion to fully explore the detailed comparisons among the stimuli of each trial. A grading session should not last for more than 20-30 min. global attribute is used to judge any and all detected differences between the reference and the object. ITU-R BS.1 Monophonic system Basic audio quality – This single. and a grading phase. For the most critical assessments. 5 Attributes Listed below are attributes specific to monophonic. To avoid this. During familiarization or training. Several illustrative comparisons might be presented just before formal grading presentations are begun. Experimenters may choose to define and evaluate other attributes. Such switching demands close time alignment among the stimuli. as a model. rest periods equal to a duration no less than the session length should be scheduled between successive sessions for each subject. 5. Subjects should also become thoroughly familiar with the artefacts under study. the eyes might remain closed for better concentration under conditions of minimal distraction. Those instructions include a description of the “double-blind triple-stimulus with hidden reference” technique of stimulus presentation. Since long. Experience suggests that no more than 10 to 15 trials per session should be scheduled to achieve the desired session length. If subjects are overburdened or confused by trying to answer multiple questions about a given stimulus event.1116-1 5 It is recommended that the scale be used to a resolution of one decimal place. Preferably. the grading process. stereophonic and multichannel evaluations.and medium-term aural memory is unreliable. an oral presentation of the test instructions should be made to each subject. if the subject wishes. subjects should be preferably together in groups (say. NOTE 1 – Exact instantaneous switching can produce artefacts if the waveforms of successive stimuli are not identical. the subject should be able to switch between stimuli without visual guidance. so that. one subject should be processed at a time. near-instantaneous switching with about 40 ms in total for fade-down/change-over/fade-up is preferred. preferably supplemented by written material. 4. For example. so that they can interact freely and discuss the artefacts they detect with each other. An example set of instructions is given in Appendix 3. consisting of three subjects).
then. this may be used as evidence that the listener’s expertise was acceptable and that there were no sensitivity problems in other aspects of the experimental situation. either embedded unpredictably within the context of apparently transparent items or else in a separate test.g. however. The search for good material is usually time-consuming. sufficient research has not yet been done to indicate whether a separate rating for stereophonic image quality as distinct from basic audio quality is warranted. ITU-R BS. unless truly critical material is found for each system. Accordingly. then it may be necessary to program special trials with low or medium anchors for the explicit purpose of examining subject expertise (see Appendix 1). The following additional attributes may be of interest: Front image quality – This attribute is related to the localization of the frontal sound sources. . from previous research). or any other weak aspects of the experiment. NOTE 1 – Up to 1993.1116-1 5. to be detectable to expert listeners but not to inexpert listeners. These anchors must be known. Critical material is that which stresses the systems under test. for items or systems where those listeners cannot differentiate coded from uncoded versions. There is no universally “suitable” programme material that can be used to assess all systems under all conditions.3 Multichannel stereophonic system Basic audio quality – This single. It includes stereophonic image quality and losses of definition. If these anchors. Thus the attribute stereophonic image quality was either implicitly or explicitly included within basic audio quality as a global attribute in those studies. 6 Programme material Only critical material is to be used in order to reveal differences among systems under test. experiments will fail to reveal differences among systems and will be inconclusive. It must be empirically and statistically shown that any failure to find differences among systems is not due to experimental insensitivity because of poor choices of audio material. critical programme material must be sought explicitly for each system to be tested in each experiment. most small impairment subjective evaluation studies of two-channel stereophonic systems have used the attribute basic audio quality exclusively. or special directional surround effects. are correctly identified by all listeners in a standard test method (§ 3 of this Annex) by applying the statistical considerations outlined in Appendix 1. findings of apparent transparency by these listeners is evidence for “true transparency”. The following additional attribute may be of interest: Stereophonic image quality – This attribute is related to differences between the reference and the object in terms of sound image locations and sensations of depth and reality of the audio event. In this case. global attribute is used to judge any and all detected differences between the reference and the object. (e. These anchors are introduced as test items to check not only for listener expertise but also for the sensitivity of all other aspects of the experimental situation. 5.2 Two-channel stereophonic system Basic audio quality – This single. In the extreme case where several or all systems are found to be fully transparent. global attribute is used to judge any and all detected differences between the reference and the object. ambience. Impression of surround quality – This attribute is related to spatial impression. before a “null” finding can be accepted as valid. Although some studies have shown that stereophonic image quality can be impaired.6 Rec.
ITU-R BS. The performance of a multichannel system under the conditions of two-channel playback shall be tested using a reference downmix.5 (number of objects). A tone burst (for example 1 kHz. the group of skilled subjects shall convene and come to a consensus on the relative sound levels of the individual test excerpts. even if these come from less easily accessible sources. therefore. The artistic or intellectual content of a programme sequence should be neither so attractive nor so disagreeable or wearisome that the subject is distracted from focusing on the detection of impairments. the apparent transparency of systems cannot be properly interpreted. This will allow subsequent use of the test media at a fixed gain setting for all programme items. – 18 dBFS) (FS: full scale) at alignment signal level should be included at the head of each recording to enable its output alignment level to be adjusted to the input alignment level required by the reproduction channel (see § 8.1116-1 7 On the other hand. the object(s) should be available. it is undoubtedly the most sensible option for use by broadcasters in the long run.71 C + 0.00 L + 0. or both. subject to a minimum value of 5 excerpts. For the purpose of preparing subjective comparison test tapes. it would be advantageous if excerpts were selected from easily accessible sources so that the prepared test tapes could be readily checked. Note. In that case. it is important that the attributes which are to be assessed are precisely defined. The SQAM compact disc is an example of such a source. For monophonic and stereophonic system evaluation.00 R + 0. The feasible number of excerpts to include in a test varies: it shall be equal for each object. In future.71 C + 0. However. the alignment level should correspond to – 18 dB with respect to the maximum possible coding level of the digital system [EBU. it is more important that truly critical excerpts be used. the experts should come to a consensus on the absolute reproduced sound pressure level for the sequence as a whole relative to the alignment level. . In addition. A reasonable estimate is 1. A successful selection can only be achieved if an appropriate time schedule is defined. For all test sequences. For test material recorded digitally. The responsibility of selecting material shall be delegated to a group of skilled subjects with a basic knowledge of the impairments to be expected. Although the use of a fixed downmix may be considered to be restricting in some circumstances. The equations for the reference downmix (see Recommendation ITU-R BS. if these anchors fail such correct identification by any listeners. In the search for critical material.71 Ls R0 = 1. it should be understood that the nature of broadcast material might change in time with future changes in musical styles and preferences.4. against the original sources.775) are: L0 = 1. Synthetic signals deliberately designed to break a specific system should not be included. The sound-programme signal should be controlled so that the amplitudes of the peaks only rarely exceed the peak amplitude of the permitted maximum signal defined in Recommendation ITU-R BS. under these conditions a peak programme meter will indicate levels not exceeding the level of the permitted maximum signal. Their starting point shall be based on a very broad range of material.645 (a sine-wave 9 dB above the alignment level). 300 ms. any stimulus that can be considered as potential broadcast material shall be allowed.1). The expected frequency of occurrence of each type of programme material in actual broadcasts should be taken into account. The tone burst may also be useful for the time-alignment of reference and test stimuli. if ever necessary.71 Rs The pre-selection of suitable test excerpts for the critical evaluation of the performance of reference two-channel down- mix should be based on the reproduction of two-channel down-mixed programme material. the loudness of each excerpt needs to be adjusted subjectively by the group of skilled subjects prior to recording it on the test media. and the experiment will need to be run again with new listeners to replace the ones who failed this additional test. 1992]. Audio excerpts will be typically 10 to 25 s long. The range can be extended by dedicated recordings. and with any other changes that may increase experimental sensitivity. objective perceptual models might aid in selecting critical material. When selecting the programme material. Due to the complexity of the task. then this suggests that either these listeners lacked sufficient expertise. Rec. or else that there were sensitivity flaws in the situation. However.
Certain quality shortcomings are more clearly perceptible in the case of headphone reproduction. however other quality shortcomings are more clearly perceptible in the case of loudspeaker reproduction.1 Amplitude versus frequency response For the pre-selection of loudspeakers.2. Frequency response curves measured at directional angles ± 10° should not differ from the main axis frequency response by more than 3 dB. .2.2. Alternatively. In all cases. NOTE 1 – The operational room response curve mentioned in § 8. The frequency response of different loudspeakers should be matched. should preferably fall within a tolerance band of 4 dB. The differences should preferably not exceed the value of 1. The electro-acoustic characteristics should fulfil the following minimum requirements. will enable the audibility of an effect to be correlated with the transducer in use.1 General “Reference monitor loudspeaker” means high-quality studio listening equipment. one central loudspeaker and/or headphones may be used. for individual trials or groups of trials. For assessing monophonic sound systems. Therefore it would be necessary to determine the appropriate kind of reproduction device by subjective pre-tests. comprising an integrated unit of loudspeaker systems in specifically dimensioned housing.2 Electro-acoustic requirements 7.3. unless otherwise specified. if the subjects are able to switch at will between loudspeakers and headphones it will not be possible to correlate the audibility of an effect with the transducer in use.or more channel stereophonic sound systems.1 General Reference monitor loudspeakers or headphones should be chosen with the aim that all sound-programme signals or other test signals can be reproduced in an optimum way.1116-1 7 Reproduction devices 7. combined with special equalization.4 describes the frequency characteristic within the sound field in the listening room. but the effective number of subjects will be reduced. Choice of either loudspeakers or headphones.2. ITU-R BS. and at directional angles ± 30° (in the horizontal plane only) by more than 4 dB. measured under free field conditions.0 dB in the frequency range of at least 250 Hz to 2 kHz.2 Reference monitor loudspeaker 7. they should provide neutral sound for any type of reproduction and should be usable for monophonic assessment as well as for two. use of both stereo loudspeakers and headphones may be necessary.8 Rec. 7. For assessing multichannel sound systems with or without accompanying pictures. measured in one-third octave bands using pink noise on the main axis (directional angle = 0°). Especially in cases when shortcomings will affect the characteristics of the stereophonic sound image. loudspeakers must be used if influences on all reproduction channels played simultaneously are to be assessed. Absolute sound level values are referenced to a measurement distance of 1 m to the acoustic centre. each loudspeaker must be acoustically matched in the relevant frequency ranges so that there are minimal inherent timbral differences among them. the frequency response curve over the range 40 Hz-16 kHz. loudspeaker reproduction should be used. high-quality power amplifiers and appropriate crossover networks. For assessing two-channel stereophonic sound systems. 7. namely.
no harmonic distortion component.37) of the original level.2. .2. should not exceed the limits set in Recommendation ITU-R BS.6 Dynamic range The maximum operating sound level which the loudspeaker can produce for a time period of at least 10 min without thermal or mechanical damage and without overload circuits being activated.2. 7. It usually corresponds to the geometrical mid-point of the surface radiating the highest frequencies of the loudspeaker.s.775. 7.4 Transient fidelity The decay time measured on an oscilloscope to a level of 1/e (approximately 0.2. 7. It should be indicated by the manufacturer.2. measured with one-third octave band noise. measured with a programme simulating noise signal (according to International Electrotechnical Commission (IEC) Publication 268-1c).2. In the case of systems with accompanying pictures. (slow). should be: Leff max > 108 dB measured by using a sound level meter set to flat response and r. That means the decay time of a sinusoidal tone burst may not exceed five times the period of the corresponding sine wave. Rec. referenced to a distance of 1 m from the acoustical centre (see Note 1) should be: Lnoise < 10 dBA NOTE 1 – The acoustical centre is the reference point for measuring purposes.m.5 Time delay Time delay differences between the channels for a stereophonic or multichannel system should not exceed 100 µs.1116-1 9 7. shall exceed the following values: –30 dB (3%) for f < 250 Hz – 40 dB (1%) for f ≥ 250 Hz 7. (on the main axis only) should be: ts < 5 / f where f: frequency.3 Non-linear distortion A constant voltage input signal producing an average sound pressure level (SPL) of 90 dB is supplied to the loud- speaker. over the frequency range 500 Hz to 10 kHz.2. should be within the limit: 6 dB ≤ C ≤ 12 dB The directivity index should increase smoothly with frequency. The equivalent acoustic noise level generated by a single reference monitor loudspeaker and associated amplifier. in the fundamental frequency range 40 Hz to 16 kHz.2. the overall time delay of the reference monitor loudspeaker in combination with the system(s) under test.2. NOTE 1 – This does not include the time delay from loudspeaker to listening position. Related to that SPL. ITU-R BS.2.2 Directivity index The directivity index C.
Minimum requirements for a reference listening room are described below. . should not exceed the limits set in Recommendation ITU-R BS.2.2.2 Geometric properties The following values describe suitable net dimensions for a reference listening room.1116-1 7.3. the listening room should fulfil at least the requirement on the background noise level.2 Reference listening room 8.708.3.2. the requirements on the sound field conditions and on the loudspeaker arrangements mentioned in the subsequent sections should be fulfilled at least.1 Frequency response The diffuse-field frequency response of studio monitor headphones is recommended in Recommendation ITU-R BS. – For multichannel stereophonic reproduction: 30-70 m2. equalized to diffuse-field response.2. 8.1 General The following requirements should be observed for subjective tests in the case of loudspeaker reproduction.775. 8 Listening conditions 8.3 Reference monitor headphones 7.1 Room size (floor area) – For monophonic or two-channel stereophonic reproduction: 20-60 m2. In the case of headphone reproduction only. 8. for sound reproduced by loudspeakers.3.2 Room shape The room should be symmetrical relative to the vertical plane on the mid-perpendicular of the stereo base. – the location of the reference listening point or area. The floor area should preferably be shaped as a rectangle or a trapezium. – the arrangement of the loudspeakers in the listening room. ITU-R BS.1 General The term “listening conditions” describes the complex acoustic requirements for a reference sound field affecting a listener in a listening room at the reference listening point. In the case of systems with accompanying pictures. 7. NOTE 1 – The smaller sizes of room will place constraints on the maximum number of listeners who can be accommodated at one time. some geometric and room acoustic requirements for a reference listening room are given to ensure the viability of the listening conditions described.2.2.2 Time delay Time delay differences between the channels for a stereophonic system should not exceed 20 µs.2 Electro-acoustical requirements 7. If the test room cannot fulfil these dimensions. Because the state of the art does not yet allow the description of the reference sound field completely and uniquely by acoustical parameters only. which are producing the resulting sound field characteristics at that point or area. 8.2. the overall time delay of the reference monitor headphones in combination with the system(s) under test. This includes: – the acoustical characteristics of the listening room. 8.1 General Reference monitor headphones means high-quality studio listening equipment.10 Rec. 7.2.3.
ITU-R BS. 8.1 s 63 100 200 4 000 8 000 f (Hz) 1116-01 FIGURE 1116-01 = 10. FIGURE 1 Tolerance limits for the reverberation time. 1.25 (V / V0)1/3 s where: V : volume of room V0 : reference volume of 100 m3. the conditions l / h < 3 and w / h < 3 should apply.1 w / h ≤ l / h ≤ 4.2.5 CM . NOTE 1 – There are difficulties in measuring small values of reverberation time at low frequencies.3 s + 0.3 Room proportions The following dimension ratios should be observed to ensure a reasonably uniform distribution of the low-frequency eigentones of the room: 1.3 Room acoustical properties 8. measured over the frequency range 200 Hz to 4 kHz should be: Tm = 0.05 s Tm – 0.1116-1 11 8. Additionally.05 s – 0.2. The tolerances to be applied to Tm over the frequency range 63 Hz (see Note 1) to 8 kHz are given in Fig. Rec.5 w / h – 4 where: l : length w : width h : height.1 s + 0. Tm. relative to the average value. Tm Tn + 0.3.2.1 Reverberation time The average value of reverberation.2.
etc.3. Under no circumstances should the background noise exceed NR 15.3. 8.1 Operational room response curve The operational room response curves are defined as the one-third octave frequency responses of the sound pressure levels produced by each monitor loudspeaker at the reference listening position. tonal colorations.3 Reverberation time (See § 8.2.1 Operational sound pressure level (reference listening level) The reference listening level is defined as a preferred listening level. 2.4 Listening level 8. auditory events and their reproducibility at other listening places or rooms. cyclical or tonal in nature.3). measured in the listening area at a height of 1.1 Loudspeaker reproduction 8.1116-1 8.3.3. 8.4.2 m above the floor should preferably not exceed NR 10 (see Figs.1.4. and are referenced to the listening arrangement being used (see § 8.3.2 Background noise The continuous background noise (produced by an air conditioning system.2. or the quality assessment of. These characteristics result from the interaction of the loudspeaker(s) and the listening room.2 Direct sound 8.1.3. measured under free field conditions. 8. 8.2.2. 8.3.) 8. such as flutter echoes.3 Reference sound field conditions 8.1 Early reflections Early reflections caused by the boundary surfaces of the listening room. should be attenuated in the range 1-8 kHz by at least 10 dB relative to the direct sound. using pink noise over the frequency range 50 Hz-16 kHz. which reach the listening area during a time interval up to 15 ms after the direct sound.1 Frequency response of monitor loudspeaker The frequency response of the loudspeaker(s).4. . ITU-R BS.3.1 General The characteristics of the sound field at the listening area are most important for the subjective perception of. The level alignment of each of the loudspeakers of a listening arrangement must be carried out using pink noise.3. produced with a given measuring signal at the reference listening point. The differences between the operational room response curves produced by each of the (stereo or multichannel) front loudspeakers at the reference listening point should not exceed the value of 2 dB within the whole frequency range. The measured operational room response curves shall fall within the tolerance limits given in Fig.2.5).3. it is necessary to avoid other significant anomalies in the sound field.4. It characterizes the acoustic gain of the reproduction channel in order to ensure the same sound pressure level in different listening rooms for the same excerpt.3. should fulfil the requirements shown in § 7.12 Rec. At the present time the following characteristics may be described.3.3.4 Steady state sound field 8.3.2 Late energy In addition to the specified requirements for early reflections and reverberation (see § 8. 8. The background noise should not be perceptibly impulsive. internal equipment or other external sources). 3 and 4).3 Reflected sound 8.
based on the former ISO NR curves. ISO Recommendation R1996 (1972) 70 60 Sound pressure level of band (dB rel. ITU-R BS.5 63 125 250 500 1 000 2 000 4 000 8 000 One third-octave band centre frequency (Hz) Noise rating curves NR 10 (recommended) and NR 15 (maximum) 1116-03 FIGURE 1116-03 = 12 CM . 20 µPa) 50 40 30 20 10 15 10 0 – 10 31. Rec.1116-1 13 FIGURE 2 Tolerance limits for operational room response curve 3 dB Lm 3 dB 2 dB/octave 1.5 dB/octave 50 250 2 000 16 000 f (Hz) Lm: average value of the sound pressure level 1116-02 FIGURE 1116-02 = 10 CM FIGURE 3 One-third octave band background noise level limits noise rating curves.
To determine equal loudness the subject should be positioned at the reference listening point. Normally listening tests will be conducted in the reference and other recommended listening positions.14 Rec.m. Thus.5 63 125 250 500 1 000 2 000 4 000 8 000 Octave band centre frequency (Hz) Noise rating curves NR 10 (recommended) and NR 15 (maximum) 1116-04 FIGURE 1116-04 = 12 CM For a measuring signal with an r. ITU-R BS. ISO Recommendation R1996 (1972) 70 60 Sound pressure level of band (dB rel.) 8.s. based on the former ISO NR curves. the gain of the amplifier shall be adjusted to give the reference sound pressure level (IEC/A-weighted.1 General The listening arrangement describes the positioning of loudspeakers and listening places (listening area) in the listening room. Lref = 85 – 10 log n ± 0. However it is also necessary to evaluate any effects due to significant off-centre listening. At the present time it is not known whether this will affect the audibility of some of the artefacts being assessed.4.1116-1 FIGURE 4 Octave band background noise level limits noise rating curves.645.e.25 dBA where n is the number of reproduction channels in the total set-up.5. 1992]) fed in turn to the input of each reproduction channel (i. 20 µPa) 50 40 30 20 15 10 10 0 – 10 31. NOTE 1 – This assumption of equal channel gains may not be appropriate for some source material. (It has been noted from previous test sequences that individual listeners may prefer different absolute listening levels. if the subjects do adjust the gain of the system. . The “worst case” listening positions are included for this reason. voltage equal to the “alignment signal level” (0 dBµ0s according to Recommen- dation ITU-R BS. it is not always possible to prevent subjects from requiring such a degree of flexibility. 8. slow).2 Headphone reproduction The level should be adjusted in such a way that a loudness equal to the reference sound field produced by loudspeakers is achieved. a power amplifier and its associated loudspeaker). – 18 dB below the clipping level of a digital tape recording.5 Listening arrangements 8. Whilst this is not a preferred option. according to [EBU. this fact should be noted in the test results.
8. 5) For reproduction of monophonic signals.1. The minimum listening distance should be 2 m and all listening positions should be within an angle of ± 30° from the loudspeaker axis. ITU-R BS.2 Distance to the walls For free standing loudspeakers.5.2 Monophonic reproduction (Fig. should be about 1. the distance of the acoustical centre of a loudspeaker from the surrounding reflecting surfaces should be at least 1 m.1 Height and orientation of monitor loudspeakers The height of all the monitor loudspeakers. FIGURE 5 Reference listening arrangement with loudspeaker M and permitted listening area for monophonic sound systems M D rmin = 2 m 30° 0. This represents the ear height of a seated listener.2 m above floor level. measured to the acoustical centre of the loudspeaker. Rec. The orientation of the loudspeakers should be such that their reference axes should pass through the reference position at a height of 1. a single loudspeaker has to be used.5.2 m. 8.1116-1 15 8.5.7 m Reference listening positions D: listening distance 1116-05 FIGURE 1116-05 = 17 CM .1.
Values of B up to 4 m may be acceptable in suitably designed rooms.1116-1 8.5. 6) FIGURE 6 Test listening arrangement with loudspeakers L and R for stereophonic sound systems with small impairments B L R B D= 30° ½B rB = B Reference listening position Worst case listening positions B: loudspeaker base width D: listening distance 1116-06 FIGURE 1116-06 = 19 CM 8.3.7 B (m).3 Two-channel stereophonic reproduction (Fig. ITU-R BS.3.5. B Preferred limits are B = 2-3 m.16 Rec.1 Base width.2 Listening distance. D (distance between the loudspeaker and the listener) Limits of listening distance are D = 2 to 1.5. . 8.
7) The listening arrangement should in principle correspond to the 3/2 multichannel sound layout.3. ITU-R BS. as specified in Recom- mendation ITU-R BS. 6. Fig.5.775. The recommended listening area should not exceed the radius of 0.4 Multichannel stereophonic reproduction (Fig.3 Listening positions The so-called reference listening point is defined by the listening angle of 60°.1116-1 17 8. FIGURE 7 Test listening arrangement with loudspeakers L/C/R and LS/RS for multichannel sound systems with small impairments B C L R D=B ½B 30° rB = B = D 110° LS RS Reference listening position Worst case listening positions B: loudspeaker base width D: listening distance 1116-07 FIGURE 1116-07 = 19 CM .7 m around the reference listening point. Additional “worst case” listening positions are also shown in Fig. 1 “Reference loudspeaker arrangement with loudspeakers L/C/R and LS/RS”. Rec.5. 8.
Scheffe. neither proscribes nor prescribes any particular statistical method. Values of B up to 5 m may be acceptable in suitably designed rooms. Only if important properties of the data show severe departures from the assumptions underlying the analysis of variance (ANOVA) should alternative analysis methods (e. 8. preferably in a graphical form. 9 Statistical analysis The fundamental aim of the statistical analysis of test results is to identify accurately the average performance of each of the systems under test and the reliability of any differences among those average performance figures. Such a presentation may be supported by more detailed quantitative information. non-parametric ones) be considered.g. 10 Presentation of the results of the statistical analyses 10.1 Base width Preferred limits are B = 2-3 m. Specifically.4. Additional “worst case” listening positions are also shown in Fig. comparisons of different grades can only be made on the basis of rank order. .4. etc.2 Listening distance and base angle The reference listening distance shall be B and thus the reference base angle is equal to 60°. The latter aspect requires estimation of the variability or variance of the results. the primary analysis.1116-1 8. however.5.5. it is recommended to apply an ANOVA model as the first stage. each step on the grading scale is approximately of equal size to all others. other methods (such as t-test. i. Subsequently. These certainly have an influence on what type of meaningful conclusions can be derived from a non-physical scale. Provided that the assumptions underlying parametric statistics are reasonably met. It should be noted that.4. unless the grading scale can be shown to be linear. although full detailed numerical analyses should be in appendices. Thus it is suggested that a supplementary data analysis (such as Wilcoxon.e. Neuman-Keuls.3 Listening positions The so-called reference listening point is defined by the listening angle of 60° as mentioned above. The basis for a decision may be strengthened if a particular hypothesis is found to hold also for a validation with an alternative statistical method. Initially any reader wants to see the overall experimental outcome.) is applied. It is also important to consider the psychometric aspects at some stage.) using variance estimates provided by the ANOVA can be used to study in more detail where the significant overall effects revealed by ANOVA (if any) are to be found. A specific hypothesis can often be validated by several different statistical methods.5.18 Rec. 8. If the tests have been conducted according to the procedures discussed in other sections of the present document. 7.1 General The presentation should be made so that a naive reader as well as an expert is able to evaluate the relevant information. then this approach provides the most sensitive and powerful one and is therefore recommended. then it is likely that the scale will be interval-like. etc. The achieved scale property. ITU-R BS.
1116-1 19 10. 11 Contents of test reports Test reports should convey. 10.3 Difference grades The difference between the grades given to the hidden reference and the object is the appropriate input for statistical analyses. Lawrence Erlbaum Associates. Significance levels should be stated. replicate the study in order to check empirically on the outcome. which normally are of prime interest. Alignment level in digital audio production equipment and in digital audio recorders. and the analyses and conclusions. Sufficient detail should be presented so that a knowledgeable person could. in principle. – the processing of data. Consequently the observations are not independent and statistical analysis of these absolute grades will not lead to meaningful information and should not therefore be done. – the experimental design. instructions. However. including the details of descriptive and analytic inferential statistics. This is due to the fact that when using the test method recommended here. may give a good initial overview of the data. Special attention should be given to the following: – the specification and selection of subjects and excerpts. Switzerland. European Broadcasting Union. training.  Bias in quantifying judgments. A graphical presentation clearly reveals the actual distances to transparency. It is. . as clearly as possible.05 is traditionally chosen. such as the underlying reasons for the study. experimental sequences.2 Absolute grades A presentation of the absolute mean grades.C. test procedures. the experimental design methods and execution.4 Significance level and confidence interval The test report should provide the reader with information about the inherently statistical nature of all subjective data. the value 0. possible to use either a one-tailed or a two-tailed test depending on the hypothesis being tested. including the room dimensions and acoustic characteristics. the rationale for the study. An informed reader ought to be able to understand and develop a critique for the major details of the test. ITU-R BS. REFERENCES POULTON. the methods used and conclusions drawn. Such details might include confidence intervals or error bars in graphs. EBU  Recommendation R-68. the transducer types and placements. electrical equipment specification. in principle. Rec. for the object and the hidden reference separately. as well as other details about statistical methods and outcomes that will facilitate understanding by the reader. a subject explicitly knows that one of the sources in the paired comparison is identical to the reference. Geneva. data generation. Hillsdale. 1992. One should however keep in mind that this is not an appropriate basis for any detailed statistical analysis. There is of course no “correct” significance level. United States of America. 10. E. – the physical details of the listening environment and equipment. – the detailed basis of all the conclusions that are drawn.
If this null hypothesis is rejected for a given subject. or easy. that each subject for which this is true has demonstrated that he or she was not. embedded within a context where most of the impairments are “large” (easy to detect). because. then the average of the difference grades would deviate from zero in a negative direction. Given this. items are almost inevitably found. If it turns out. since more of the grades would be negative than positive. Let us assume that we are subtracting the grade for hidden reference from the grade for the object. in the t-test assessments. that many “large” impairments were included in a test.0. some large-impairment. The opposite case. rather than only “small” ones. since the greatest weight for the magnitude of t would be given by the large-impairment items. overall. If the subject was able. say. . the special items introduced for their known impact would have more weight in the t-tests. all “easy” or large-impairment items should be routinely excluded from the t-test procedure for assessing listener expertise. The effect on the t-test of leaving the apparently transparent items in. The effect of leaving the large-impairment items in the t-test analysis would be to exaggerate or overestimate the apparent expertise of subjects. guessing. overall. The data thus obtained are subjected to a one-sided t-test. These might be all items which received low average grades across all subjects. overall. The data of the other subjects – those who were. rather. It is obvious that a few truly “small” (difficult to detect) impairments. This would be true. these subjects may be said to have shown sufficient expertise to justify including their data in the final analyses of the experimental results. then the method of post-screening applied naively as described above may lead to false or inappropriate conclusions. then one may conclude that the data for that subject originates from a distribution with a mean greater than zero in a negative direction. for whatever reason. even though these are usually far short of being a majority of items. experts who do correctly judge the small-impairment items may be indistinguishable in overall performance from non-experts who perform at “guessing” levels on those items. merely guessing. always.1116-1 APPENDIX 1 TO ANNEX 1 Statistical considerations for the post-screening of subjects 1 Evaluation of listener expertise The double-blind triple-stimulus hidden-reference method provides two grades on each trial and makes it possible. as intended. A “large” impairment here. then. Even in the best of tests of “small impairments”. to assess the likelihood that the mean of the distribution for each subject is zero. performance on the small-impairment items may be lost in the statistical noise. difference grades between – 2. to compare these two grades directly and to examine these comparisons across all trials for that individual. In that case. it is the apparently transparent (“too difficult”) items that might be omitted in the post- screening t-tests. it is recommended that. If the subject was not successful overall at correctly identifying the hidden reference versus the object. to detect which was the hidden reference and which the object correctly. and inclusion of such items in the t-test will obscure rather than facilitate. on an individual subject-by-subject basis. even by “non-expert” listeners. since there would be both positive and negative grades tending to balance each other out on the average. Then. then. will bear little weight in a t-test as described above. where there may be too many “truly transparent” items has been introduced in § 5 of this Recommendation. would be to underestimate the expertise of subjects. then the average of all the difference grades from that subject in the listening test would be at or close to zero. for the exclusive purpose of sufficiently rigorous post-screening t-tests. Thus. at a given level of confidence. of course. subtracting in the same direction.20 Rec. It may be concluded. by this statistical criterion – can be rejected from further analysis.0 and – 4. It should be remembered that the recommendations which are the subject of this text are concerned exclusively with small impairments. means one which is relatively easy to detect. For such items. For each trial one can take the algebraic difference between the two grades for a trial. ITU-R BS. the assessment of differential subject expertise. the majority of subjects will have correctly discerned the object from the hidden reference.
if there are negative suspicions about the “goodness” of the data. This. A listener who had sufficient expertise for a past test which included relatively “easily audible” artefacts may not be sufficiently expert in a test where these more audible artefacts are not present. Rec. In this case. the subject’s data may be adding “statistical noise” to the total data.0 for all trials. Hence. thus masking true differences perceived by other subjects. Significant and replicable results are quite usual from sensitive experiments even when they include deviant but expert subjects. rather than accepting the empirical evidence of actual outcomes. Such post hoc methods must NOT be used. the total data set from that subject might be difference grades of – 4. it would be very difficult to apply such post hoc criteria for acceptability of data. As an extreme example. But the data may show that he or she gave a grade of 1. However.1116-1 21 In general. As long as the total number of subjects in an experiment is adequate. although a listener’s t-score may indicate sufficient expertise for the experiment as a whole. perhaps. the only recourse is to repeat the entire experiment de novo. In other words. The unique advantage of appropriately applied post-screening t-tests is that the sufficiency of expertise for a given experiment is assessed by performance in that experiment. On the other hand.0 to all the objects on all trials. a given subject’s data may be accepted or rejected as appropriate for specific test outcomes. Furthermore. the listener may not have sufficient expertise to discriminate differences between the reference signal and a very high quality coded signal. the very odd pattern of responding from that one subject (all “– 4. then even a highly deviant expert subject’s data will have very little distorting influence on the total data set. Assuming that all the other subjects in that experiment showed a “more usual” distribution of grades across trials. using an entirely new set of subjects. . is a fine-tuning of the concept of “expertise”. In such cases. In a series of experiments involving the same subjects in different experiments. A word of caution should be stated here. and striving to correct any suspected flaws in the experimental procedures used previously. items which are consistently either “too difficult” or “too easy” are non-differential for distinguishing adequate experts from inadequate ones. a subject may correctly differentiate objects from hidden references on 100% of the trials in an experiment. APPENDIX 2 TO ANNEX 1 Evaluating a subject’s level of expertise Currently all of a subject’s data in a given test is used to evaluate his t-score. The data from all subjects with sufficiently high t-scores are then included in the ANOVA. in effect. there is no assurance that the data from a subject who properly passes a t-test post-screening is necessarily good data. rejection of data on grounds of poor expertise as objectively determined by rigorous post-screening is justified. ITU-R BS.0” difference grades) might lead one to argue for rejection of that data. 2 Further evaluation of listener expertise As the quality of perceptually based lossy codecs increases there will inevitably be fewer listeners with a sufficient degree of expertise to discern the remaining coding artefacts. in an obviously highly deviant single case as described here for illustration. This would be tantamount to deliberately shaping the data according to an experimenter’s preconception. some of these subjects may be adequately expert for a subset of the experiments but not for all of them as shown by post-screening. beyond what is possible with exclusive reliance on pre-screening. After an experiment has been completed. An insufficiently expert subject cannot contribute good data. it may be found that while all the subjects successfully pass pre-screening. except. then.
ITU-R BS.1116-1 FIGURE 8 Method for discarding data points prior to t-test µ + ∆s Point distribution frequency 0 –1 –2 –3 –4 µ Differential grade 1116-08 FIGURE 1116-08 = 10 CM In the present proposal we suggest that several iterations of t-tests be conducted on subsets of each subject’s data. From this.22 Rec. The process is repeated N times with the criteria. Therefore. Appropriate values of ∆ i s and N are currently being investigated using data from previous studies conducted at the CRC (Communications Research Center (Canada)).. First. those data points falling beyond µ + ∆ 1 s (the shaded area) are discarded and the remaining data points (the non-shaded area) are used in the subsequent t-test. then that subject’s data would be eliminated entirely from all subsequent ANOVAs. all data points for a subject which fall beyond a certain criterion (µ + ∆ 1 s) would be discarded and a new t-test would be done on the remaining data points. If. x − µ It is defined as z = where x is a data point. µ is the sample mean and s is the standard deviation for the s sample: ∑ x2 (∑ x) 2 N − s = N ( N − 1) . This process is then repeated with an even more stringent expertise criterion. NOTE 1 – The z-score represents the score normalized for a distribution having zero mean and a standard deviation of 1. the subject is still shown by the t-test to have sufficient expertise. the mean and standard deviation for the subject’s data is calculated. with each iteration. then his data would be included in the subsequent ANOVA. This would then be used to determine the corresponding z-scores (see Note 1) for that subject’s data. for the remaining data points. As shown in the figure. µ + ∆ i s with i = 0. For each iteration the criterion for evaluating a subject’s level of expertise would become more stringent. the criterion for sufficient expertise is increased and an ANOVA is conducted with the data from the remaining subjects. The proposed criteria for evaluating expertise is outlined below. 8 for a hypothetical data set... The process is shown in Fig. then all of that subject’s data would be included in the subsequent ANOVA. N. µ + ∆ 2 s. 1. If the subject failed to show sufficient expertise in the t-test. A subject’s level of expertise would be re-evaluated and if he demonstrated a sufficient level of expertise.
9 may be awarded according to the detected difference. 1 Familiarization or training phase The purpose of the training phase is to allow listeners to identify and become familiar with potential distortions and artefacts produced by the systems under test. it is important that you should not discuss this personal interpretation with the other subjects at any time. “B” or “C” at any time during the presentation.0 Very annoying 1. It is the differences between “A” and “B” and between “A” and “C” that are to be graded. During the afternoon tests you will be required to grade the presentations according to the scale of Table 2: TABLE 2 Impairment Grade Imperceptible 5. Audio sequences will be typically 10 to 25 s long and can be played repeatedly for as long as you want.0 The meaning of the scale is to be described to the subject. . Whilst you should be considering during the training phase how you.0 Slightly annoying 3. you should know “what to listen for”. will give two grades of 5. On the video monitor screen. If you find “B” or “C” better than the reference. You can switch freely between “A”. NOTE 1 – The goal of the recommended change is to force the subject to make a “best guess” as to which stimulus is the coded material.0 to 4. will interpret the audible impairments in terms of the grading scale. the reference version will be identified by the letter “A” and the processed version of the signal and the “hidden reference” by the letters “B” and “C”. due to their conservative approach. This should allow a fine and detailed comparison between “A”. but not annoying 4. as an individual.0 Annoying 2. then this implies that a “perceptible but not annoying” difference was found and a grade between 4. you will be asked to blind grade all the audio material you will audition this morning. You are free to use either loudspeakers. a perfect replica of the reference) at least one grade of 5. This afternoon.0 Perceptible. After training. You will hear both the reference (original) and processed versions of each item of audio material. The recommended change would resolve this issue. You have up to three hours to train on all the items which you will be rating formally in the blind grading phase this afternoon.e. headphones or both during training.0 (but only one (see Note 1)) is expected on each trial. ITU-R BS.1116-1 23 APPENDIX 3 TO ANNEX 1 Example of instructions to subjects The terminology used in these instructions does not adhere strictly to the glossary definitions.0 rather than committing themselves. “B” and “C”. Rec. This should stress that the grading scale is to be considered as a continuous equal interval scale with anchor points defined at specific values. During the training phase you will also become familiar with the test procedure. We feel that some subjects are actually able to detect very small artefacts but. Since each trial in the afternoon contains a hidden reference (i.
This may include listening to the test items again. lasting up to three hours. using some of the most impaired items. One of “B” or “C” is a processed version and the other is a hidden reference (identical to the reference).1116-1 2 Example for contents of a training phase The main training. Audio sequences can be played repeatedly until you are confident about your evaluations. On subsequent tests days. should be carried out with groups of about four subjects in the morning of the first day. § 1 of this Appendix)) is expected to be given on each trial. At your discretion. At least one grade of 5. “B” or “C” at any time. “A” is always the reference (original) version against which both “B” and “C” are to be compared and graded.0 Annoying 2. – a brief explanation of the systems under test and a spoken presentation of the impairment categories established by the pre-selection panel. You will be able to switch freely among “A”. 3 Blind grading phase The purpose of the blind test is to grade the various audio material you heard this morning during the training phase. one for “B” and one for “C”.0 (but only one (see Note 1. On each trial. you are asked to rate the perceived difference (if any) between “B” and “A” on the one hand and the difference between “C” and “A” on the other hand using the five-grade scale shown in Table 3.0 Very annoying 1. “B” and “C” on the video monitor screen. It should be emphasised that the grading scale is to be considered as a continuous equal interval scale with anchor points defined at specific values. – training in switching and grading. the subjects should be reminded of the points covered in the main training session. hence the term “blind” for this grading phase. The meaning of the scale is to be described to the subject. These will be labelled “A”. Table 3 would then be shown to the subject and a copy would be available throughout the blind grading sessions. You are not told which of “B” and “C” is the processed version and which is the hidden reference. a paper grading sheet may be used. prior to carrying out the formal tests. – replay of the selected test excerpts to enable the test subjects to become familiar with the sound presentation and to get to know the programme material to assess later on. In each trial. you can move on to the next trial when satisfied with the evaluation on a given trial. – explanation of the five-grade impairment scale.0 Perceptible.0 . ITU-R BS. – demonstration of the impairments. The training session should include the following points: – a brief introduction to the aims and objectives of the test.0 Slightly annoying 3.24 Rec. Rather than entering grades into a computer. you will audition three versions of a given audio material. Please enter your grades on the computer at the end of each trial. TABLE 3 Impairment Grade Imperceptible 5. – explanation of the attribute to be graded. but not annoying 4. The subjects should be sent a written instruction in advance. Two grades must therefore be given on each trial.
suitable for assessing the individual characteristics or parameters of sound quality of a given system under test. . R-DAT. It could be just the geographical place or the position of the subject in the listening room. Attribute A perceived characteristic of a hearing event. used as a comparison basis for an impairment test. Test excerpts are available normally as sound recordings (CD. Item An excerpt. Reference Test excerpt. Grade The numerical expression of the magnitude of an attribute according to a given scale. Rec. Hidden reference Reference not identified to the test subject. Listening panel The whole group of subjects that produce the data for a listening test. Object The system under test. which are processed by the system under test. Double blind test A blind test in which there is no possibility of uncontrolled interactions between experimenter and the listening test. ITU-R BS. speech or other sound event. processed by the system under test. It can be one of the factors in the test. Location The place where the listening test is carried out. or other recording or source formats). 9 which illustrates the interrelationship of some of these terms. represented by a number of excerpts. reproduced without the processing by a test object. See also Fig.1116-1 25 APPENDIX 4 TO ANNEX 1 Subjective assessment: Glossary The following terms used in this Recommendation are defined here for clarity. according to a given verbal or written definition. Blind test A test in which the only source of information for the subject about the trials is the stimuli. Excerpt A sample of a piece of music. Session The whole group of trials which are to be evaluated by a subject or a listening panel in a continuous period. Stimulus The combination of either the object or the hidden reference or the reference and part or all of an excerpt.
and the object. FIGURE 9 Illustration of the interrelationships of some of the terms used in the Glossary Listening test Location 1 Location 2 Location “j” Location X Session 1 Session 2 Session “k” Session X Trial 1 Trial 2 Trial “m” Trial X Trial X Grading Grading Excerpt Excerpt Excerpt Excerpt Excerpt Time Time Stimulus Stimulus Represents the choice being made by the subject between the reference. the hidden reference. Trial A subset of a session which begins with the presentation of a set of stimuli and ends with their grading. 1116-09 FIGURE 1116-09 = 17 CM .26 Rec. The two trials shown illustrate end points on a range of possible arrangements.1116-1 Subject A test person evaluating the stimuli in a listening test. ITU-R BS.

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