1、ITU-T RECMN*P-BLi 93 Li862591 0585742 075 INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU TELEPHONE TRANSMISSION QUALITY SUBJECTIVE OPINION TESTS P.84 (03/93) SUBJECTIVE LISTENING TEST METHOD FOR EVALUATING DIGITAL CIRCUIT MULTIPLICATION AND PACKETIZED VOI
2、CE SYSTEMS ITU-T Recommendation P.84 (Previously “CCIlT Recommendation”) ITU-T RECMNxP-84 93 W 4862571 0585793 TOI FOREWORD The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecom- munication Union. The ITLJ-T is responsible for studying technical,
3、operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication standardization Conference (WTSC), which meets every four years, established the topics for study by the ITU-T Study Groups which, in t
4、heir turn, produce Recommendations on these topics. ITU-T Recommendation P.84 was revised by the IT-T Study Group XII (1988-1993) and was approved by the WTSC (Helsinki, March 1-12, 1993). NOTES 1 As a consequence of a reform process within the International Telecommunication Union (ITLJ), the CCIT
5、ceased to exist as of 28 February 1993. In its place, the ITU Telecommunication Standardization Sector (ITU-T) was created as of 1 March 1993. Similarly, in this reform process, the CCIR and the IFRB have been replaced by the Radiocommunication Sector. In order not to delay publication of this Recom
6、mendation, no change has been made in the text to references containing the acronyms “CCIT, CCIR or IFRB” or their associated entities such as Plenary Assembly, Secretariat, etc. Future editions of this Recommendation will contain the proper terminology related to the new IT structure. 2 telecommuni
7、cation administration and a recognized operating agency. In this Recommendation, the expression “Administration” is used for conciseness to indicate both a O ITU 1994 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical,
8、 including photocopying and microfilm, without permission in writing from the ITU. ITU-T RECMN+P=84 93 = 48b259L 0585744 948 CONTENTS 3 1 Introduction 1.1 Purpose . 1.2 Test philosophy . 2 Source recordings . 2.1 Apparatus and environment 2.2 Speech material . 2.3 Procedure 2.4 Calibration signals a
9、nd speech levels . Simulating system load 3.1 Requirements for a generic voice load simulator . 3.2 Determining load capacity of tested systems 3.3 Controlling load applied to tested systems . 4 Processing of the speech 5 Test design . 5.1 Test No . 1 - Effect of applied load . 5.2 Test No . 2 - Eff
10、ect of digital errors in the DCME control channel 6 Listening test procedure . 7 Analysis of results Annex A - Description of Digital Circuit Multiplication Equipment A.l Definitions A.2 Digital Speech Interpolation (DSI) . A.3 Speech Detection A.4 DCME load . AS Overload strategies . A.6 Silence re
11、construction methods A.7 Circuit versus packet mode . A.8 Packet reconstruction Annex B - Speech material used to construct speech sequences . Annex C - Instructions on the use of a limited number of sentences . Page 1 1 1 2 2 2 3 4 4 4 5 5 6 6 8 9 9 10 10 10 13 13 13 14 16 16 17 17 19 Recommendatio
12、n P.84 (03/93) i ITU-T RECMNaP.84 93 4862571 0585745 884 Recommendation P.84 SUBJECTIVE LISTENING TEST METHOD FOR EVALUATING DIGITAL CIRCUIT MULTIPLICATION AND PACKETIZED VOICE SYSTEMS) (Melbourne, 1988, amended at Helsinki, 1993) 1 Introduction 1.1 Purpose The purpose of this Recommendation is to d
13、escribe a subjective listening test method which can be used to evaluate the speech quality of Digital Circuit Multiplication Equipment (DCME) or packetized voice systems. This Recommendation is intended for use with DCME systems, such as those described in Recommendation G.763, which use digital sp
14、eech interpolation (DSI) techniques. For subjective test evaluations of DCME systems not using digital speech interpolation techniques, described in Recommendation G.726 40, 32, 24, 16 kbit/s Adaptive Differential Pulse Code Modulation (ADPCM), use Recommendation P.83. Many of the degradations found
15、 in DCME or packetized voice systems have not been tested before and their effects on other systems in the network are unknown. Therefore the only definitive method is the conversation test where the effects of non-linearity, delay, echo, etc. and their interactions can be verified. For DCME systems
16、, degradations can include not only the effects of variable bit-rate coding, DSI gain (channel allocation), clipping, freezeout and noise contrast, but also those due to non-linearities in the speech detection system, such that the system may function differently for different speech input levels or
17、 activity factors. For packetized voice systems the subjective effect, for example, of “lost packets” is unknown. Listening tests play an important preliminary role in the assessment, and can supply useful information service to narrow the range of conditions needing a complete conversation test. Th
18、is listening test method will not provide results for generating network application rules based on factors analogous to the quantizing distortion unit (qdu). Future improvements of the test will allow such results to be obtained. Evaluation of DCME in tandem with other DCME has not been considered
19、at this stage, nor have the effects of systems using encoding at different rates. This Recommendation will subsequently be updated when information of these specific points becomes available. This Recommendation confines itself solely to listening tests; a separate recommendation, on conversation te
20、sts, will be formulated when sufficient information on evaluation techniques is available. Alternatively, this Recommendation may be revised to include conversation test methods. 1.2 Test philosophy In order for a test to satisfactorily evaluate DCME performance the test methodology should meet cert
21、ain conditions. These are as follows: i) ii) the method should use principles, procedures, and instrumentation that are acceptable to CCITT; the method should be adaptable to different languages and should yield results that are comparable to those from other tests performed using this Recommendatio
22、n; 1 The specifications in this Recommendation are subject to future enhancements and therefore should be regarded as provisional. Recommendation P.84 (03/93) 1 ITU-T RECMN*P.84 93 M 4Bb259L 058574b 710 M iii) the method should permit DCME performance to be compared subjectively (or objectively) to
23、reference conditions. Examples of suitable reference conditions are hypothetical reference connections (HRCs), white noise and speech correlated noise. The HRCs should model the facilities the DCME is designed to replace, when these facilities are known. The results of the comparisons should permit
24、making “equivalence statements” about the DCME, e.g. a DCME system is subjectively equivalent to x asynchronously tandemed 64 kbih PCM systems. Ideally, the method should yield results from which a network application rule can be derived; iv) the DCME should be tested with a realistic load simulator
25、 and circuit-under-test signal conditions applied. Most of the transitory impairments arise when the DCME is operating in the range of applied load which forces the use of DSI. Therefore, to subjectively measure the effects of these impairments it is necessary to vary the applied load on the DCME up
26、 to and marginally beyond the maximum design load. The clipping produced by the speech detector is affected by the type of signal being transmitted on the circuit under test. Therefore, only a realistic speech signal which also contains appropriate additive noise should be used on the circuit under
27、test; v) the methodology should, ideally, yield results which can be used to produce new opinion models or modify existing models. 2 Source recordings 2.1 Apparatus and environment SeeB.l.liP.80 throughB.1.3iP.80. 2.2 Speech material The format of the speech material must be suitable for the opinion
28、 scale being used in the test. This will normally be the listening-quality scale, but optionally the listening-effort scale will be used (see clause 5). When the listening-quality scale is used, the following requirements apply: i) The speech material should consist of short passages (called segment
29、s), chosen at random (from current non-technical literature or newspapers for example), easy to understand, and more or less self-contained in meaning. Each segment when spoken naturally should have a duration not less than 9 seconds and not more than 11 seconds. ii) iii) Each segment should consist
30、 of at least three “sentences” in a broad sense, that is, parts that can naturally be separated by pauses in speaking, but connected in meaning to what precedes and follows within the segment. iv) Within each segment there should be at least one pause which naturally, in view of the meaning and stru
31、cture of the text, would last for 1 to 2 seconds. The other pauses must be of natural length, since unnaturally long or short pauses may well be interpreted as reducing the quality of the speech. The simplest way of ensuring this is to construct the script of each text with either a special mark or
32、the beginning of a new paragraph at the point where the 1 to 2 second pause is desired. The talkers recording the segments can then be instructed to read consecutively, making sure that they pause for a second or two at the marked point, and pausing naturally at other points. When the listening-effo
33、rt scale is used, the following requirements apply: v) The speech material should consist of single meaningful sentences, easy to understand, chosen at random (from current non-technical literature or newspapers for example) and assembled into groups .(called segments). The number of sentences in ev
34、ery segment should be the same (three is the recommended number). vi There should be no obvious connection of meaning between one sentence and another in the same segment. This precaution is to reduce the context information within the segment to a minimum, so as not to inflate the opinion scores ar
35、tificially. 2 Recommendation P.84 (03/93) ITU-T RECMN*P=84 93 486257L 0585747 657 I II II I I II II I End d I II II I I II II I I II II I n-1 I II II I segment 5sgap I Sentence1 I I Sentence2 I I Sentence3 I 5sgaP vii) The pauses between the sentences must be at least one second in length. This ensu
36、res that the listeners perceive the sentences as isolated from each other in meaning, and puts the DCME system to the test in respect of the unbridged gaps in speech. Moreover, each segment should have a duration not less than 9 seconds and not more than 11 seconds including the pauses. start of seg
37、ment n+l This structure can be attained by either: - giving timing cues to talkers at the recording stage see Section 2.5.8.1 d) of the Handbook on Telephonometry; or - editing the recordings afterwards. Either of two approaches may be used for the listening-quality scale: i) to have as many differe
38、nt segments as there are conditions (an example of suitable material from which segments may be constructed is contained in Annex B); or ii) to have a more limited number, e.g. 10 segments per talker, where combinations of two segments can be used (this is shown in detail in Annex C). In this case,
39、additional precautions would have to be taken in the analysis of variance of results from the tests. The first approach is essential when the listening-effort scale is used, because listening-effort scores are affected when the listener has heard the same sentences before. Enough segments must be av
40、ailable to cater for all the test conditions, plus a sufficient number for use in a practice session. 2.3 Procedure A silent period containing only circuit noise of approximately one second should proceed each segment and the segment should end with a similar silent period containing only the circui
41、t noise. To facilitate the processing of the recorded speech through the DCME, i.e. to allow for the starting and stopping of the recorders between segments and to allow time for adjusting the DCME for the next test condition, segments should be separated by a 5 second gap on the tape. Therefore, th
42、e recorded source segments will have the pattern on the tape shown in Figure 1 (this is an example for the listening quality scale; for the listening effort scale all pauses must be at least 1 second). Note that if the speech is stored digitally (e.g. on a disk based system) the segments will not re
43、quire these 5 second gaps. Natural 1s Normal 1-2 s 1s silence pause pause silence 1 1 1 1 Segment n Sselengthll s FIGURE 1m.84 Example of a speech segment in the format required for the listening quality scale, containing three sentences T1203910-92/d0 1 Recommendation P.84 (03/93) 3 ITU-T RECflN*P.
44、84 73 4862573 0585748 573 The recording procedure detailed in Section 2.5.8.1 (Listening Tests) of the Handbook on Telephonornetry should be followed. Only the part dealing with recording through an IRS is required for this Recommendation. NOTE - When this recording is made through a physical IRS, t
45、he sidetone path of the IRS should be set to i2 dB STMR. This helps to stabilize the speech level of the talker. Segments should be played back to listeners complete with the silent period. After the segment has ended, a sufficient length of complete silence should be provided to permit the listener
46、 to vote. Talkers should pronounce the segment of sentences fluently but not dramatically and have no speech deficiencies such as “stutter”. (See also B. 1.6F.80.) 2.4 Calibration signals and speech levels See B. 1.7P.80 and B. 1.8F.80. 3 Simulating system load 3.1 Requirements for a generic voice l
47、oad simulator Digital Circuit Multiplication Equipment (DCME), by definition, is used to gain an advantage in the number of circuits multiplexed onto a digital transmission facility. With this advantage, however, comes potential degradation of transmission quality when carried loads exceed that for
48、which the DCME was engineered. Thus, a rigourous performance evaluation of DCME includes studying the behaviour of the DCME under conditions of no load, engineered load, and overload. Because the transmission performance of DCME under load depends critically upon the load characteristics, it is nece
49、ssary to know and control simulated loads in order to properly assess DCME performance. This subclause describes the generic requirements for a voice load simulator for the purpose of facilitating DCME performance evaluations under conditions that are meaningful. Use of voice load simulators with the generic requirements described here will also enable the comparison of results from different studies of various DCME. Either the load simulator or the DCME system itself must be programmed so as to record, for each individual segment played through it, the proportion of time during t
