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Subjective Assessment of Audio Quality – the Means and Methods Within the EBU

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Subjective Assessment of Audio Quality – the Means and Methods Within the EBU Subjective assessment of audio quality – the means and methods within the EBU W. Hoeg (Deutsch Telekom Berkom) L. Christensen (Danmarks Radio) R. Walker (BBC) This article presents a number of useful means and methods for the subjective quality assessment of audio programme material in radio and television, developed and verified by EBU Project Group, P/LIST. The methods defined in several new 1. Introduction EBU Recommendations and Technical Documents are suitable for The existing EBU Recommendation, R22 [1], both operational and training states that “the amount of sound programme ma- purposes in broadcasting terial which is exchanged between EBU Members, organizations. and between EBU Members and other production organizations, continues to increase” and that “the only sufficient method of assessing the bal- ance of features which contribute to the quality of An essential prerequisite for ensuring a uniform the sound in a programme is by listening to it.” high quality of sound programmes is to standard- ize the means and methods required for their as- Therefore, “listening” is an integral part of all sessment. The subjective assessment of sound sound and television programme-making opera- quality has for a long time been carried out by tions. Despite the very significant advances of international organizations such as the (former) modern sound monitoring and measurement OIRT [2][3][4], the (former) CCIR (now ITU-R) technology, these essentially objective solutions [5] and the Member organizations of the EBU remain unable to tell us what the programme will itself. It became increasingly important that com- really sound like to the listener at home. The mon rules for subjective sound assessment should human ear alone is able to judge the aesthetic or be specified and, consequently, the EBU set up artistic quality of programme material and, in- Project Group P/LIST to develop tools for the sub- deed, certain aspects of the technical quality as jective assessment of sound programme quality. Original language: English Manuscript received 1/12/97. well. These tools are described in several EBU Recom- 40 EBU Technical Review Winter 1997 Hoeg et al. mendations, Technical documents and other printed material. 0.3 0.2 2. Listening conditions 0.1 0.05 2.1. General 0 Tm –0.05 Typically, the monitoring of programme material –0.1 in sound production and broadcasting is done by listening in a certain room using loudspeaker pre- Difference in reverb. time (T – Tm) sentation. (Listening by headphones is also used in certain cases, but is not covered in this article.) 100 1000 10000 63 200 4000 8000 It is self-evident that both the acoustics environ- Frequency (Hz) ment and the electro-acoustic properties of the loudspeakers must be controlled, in order to allow consistent subjective assessments to be made. 3 The main components of the reproduced sound field are the direct sound, the early reflections and 0 Lm the later reflections which form the reverberant field. All these components are time- and fre- –3 quency-dependent. Relative level (dB) 1 dB per octave The following is a brief summary of the parame- –6 ters and other requirements for loudspeaker pre- sentation, as specified in EBU document Tech. 3276 [6]. They also largely meet the requirements given in ITU-R Recommendation BS.1116 [7]. 100 1000 10000 50 2000 16000 2.2. Requirements of the reference Frequency (Hz) sound field. Tm should lie in the range: 0.2 < Tm < 0.4 s Figure 1 (upper) Direct sound Tolerance limits for As a function of frequency, the reverberation time the reverberation The quality of the direct sound is mainly deter- (T) should conform to the tolerances shown in time. mined by the relevant loudspeaker parameters, as Fig. 1. measured in anechoic conditions (see Section 2.3.). Operational room response curve Figure 2 (lower) Tolerance limits of Early reflections The tolerance limits for the operational response the operational room curves, measured at any point in the listening response curve. The levels of reflections earlier than 15 ms rela- room, are given in Fig. 2. Lm is the mean value of 1 tive to the direct sound should be at least 10 dB the /3-octave bands from 200 Hz to 4 kHz. The below the level of the direct sound for all frequen- tolerances should be met for each channel sepa- cies in the range 1 kHz to 8 kHz. rately. For stereophonic reproduction, the close matching of the room response of each channel is Reverberation field important. The reverberation field should be sufficiently dif- Listening level fuse over the listening area to avoid perceptible acoustical effects such as flutter echoes. For pink noise at the “alignment signal level”, the gain of each loudspeaker channel is adjusted so that the sound pressure level at the reference lis- The nominal reverberation time (Tm ) for the 1 tening point is: /3-octave bands from 200 Hz to 4 kHz is found as follows: LLISTref + 85 * 10 log(n)ądB(A) 1 Tm + 0.25ā(RoomāvolumeāńāRef.āvolumeā(100))3 where: n = number of reproduction channels in the total configuration. EBU Technical Review Winter 1997 41 Hoeg et al. The measurement signal is available from the production according to the layout given in Fig. 3. EBU tape of R-DAT Levels [8]. The base width, b, should be within the range 2 to 4 m. Background noise Separate bass loudspeakers The sound pressure level (SPL) of the continuous background noise should not exceed the ISO If separate bass loudspeakers are used, the opti- Noise Rating Curve, NR 15, and should prefer- mum cross-over frequency between the bass and ably not exceed NR 10. It should not be percepti- the main loudspeakers depends on many factors, bly impulsive, cyclical or tonal in nature. including the positions of the loudspeakers in the room, the room acoustics and the desired overall 2.3. Geometrical conditions for the frequency response. To prevent the separate bass listening arrangement source locations from becoming perceptible, lower cross-over frequencies (between 80 and Positioning of sound sources and listeners 160 Hz) will be required for bass loudspeaker positions which are located further from the main The height of the acoustical centre of the loud- loudspeakers (for more details, see [6]). speaker monitor should be at least 1.2 m above floor level and the inclination angle of its refer- Room dimensions ence axis in relation to the horizontal plane should not exceed 10°. The monitor’s reference axis The minimum floor area should be: should intersect the reference listening point at the height of the ears of a seated person. – 40 m2 for a reference listening room; If the loudspeaker monitor is not installed into the – 30 m2 for a high-quality sound control room. wall, the distance of its acoustical centre from the surrounding walls should be at least 1 m. All lis- The volume should not exceed 300 m3. tening positions should be situated at least 1.5 m from the side walls and the back wall of the room. The following limits for the length-to-height and the width-to-height ratios should be observed: Figure 3 Stereo listening Typical layout of a 1.1w / h v l / h v 4.5w / (h – 4) stereo listening Two loudspeaker monitors should be placed in the arrangement. listening room for two-channel stereophonic re- l <3h w <3h where: l = larger dimension of floor plan, 2.0m < b < 4.0m LRirrespective of orientation; w = shorter dimension of floor plan, b irrespective of orientation; h = height. Ratios of l, w and h which are within "5% of inte- h ger values should be avoided. Careful design and good workmanship in the construction of listening rooms can greatly en- q hance the acoustic environment. Additional de- Listening area sign considerations are given in [6]. (radius = rL) 2.4. Requirements of the monitoring loudspeakers Frequency response curve The frequency response curve is measured in 1 /3-octave bands with a pink noise test signal. The measurements are taken on the main axis and the Reference listening curve should fall within a tolerance band of 4 dB position over the frequency range from 40 to 16 kHz. 42 EBU Technical Review Winter 1997 Hoeg et al. Directivity index, D 250 Hz to 16 kHz: 4 v D v12 dB. Harmonic distortion (sinusoidal test signals) 40 Hz < f < 250 Hz –30 dB (3%) 3 250 Hz < f < 16 kHz –40 dB (1%). 0 Decay time –3 Relative level (dB) Relative Mean curve The decay time, ts, using a sinusoidal tone burst shall not exceed the following limit: –6 ts v 2.5 / f where f is the frequency. Time delay of loudspeaker system 100 1000 10000 50 2000 16000 The time delay of the loudspeaker system should not cause the relative delay of the sound and Frequency (Hz) vision components at the listening position to ex- ceed that defined in EBU Recommendation R37 [9]. The measurement results have been collected as Figure 4 an internal report of the EBU. They mainly meet Measured room Operational SPL the requirements and show that the listening con- response curves in ditions specified in [6] are quite realistic and could different listening The maximum operational sound pressure level be reached by several existing listening rooms rooms: mean values which the loudspeaker monitor can produce for a (see Fig. 4). and standard period of at least 10 minutes without thermal or deviations in relation mechanical damage and without overload circuits to frequency. being activated is: 3. Subjective assessment methods Leff-max w 108 dB. 3.1. General Self-generated noise level The method described below has been developed to assess the quality of “classical music” pro- The maximum allowable self-generated noise grammes: symphonic music, orchestral music, level is: choral music, opera, chamber music and solo per- formances.
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