1、- ITU-T *RECMN*G-111 93 m 4862591 0587459 295 m INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU G.lll (03193) TRANSMISSION SYSTEMS AND MEDIA GENERAL RECOMMENDATIONS ON THE TRANSMISSION QUALITY FOR AN ENTIRE INTERNATIONAL TELEPHONE CONNECTION LOUDNESS RATIN
2、GS (LRs) IN AN INTERNATIONAL CONNECTION ITU-T Recommendation G.lll (Previously “CCITT Recommendation“) ITU-T *RECflN*G.LLL 93 m 4862591 05894b0 TO7 m FOREWORD The IT Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecom- munication Union. The IT-T is resp
3、onsible for studying technical, 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 th
4、e IT-T Study Groups which, in their turn, produce Recommendations on these topics. ITU-T Recommendation G.lll 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 Telecommu
5、nication Union (ITU), the CCITT 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 Similarly, in this reform process, the CCIR and the IFRB have been replaced by the Radiocommunication Sector.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
6、 delay publication of this Recommendation, 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
7、new ITLJ structure. 2 telecommunication 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 pari of this publication may be reproduced or utilized in any form or by any
8、 means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the . CONTENTS Nominal LRs of the national systems . 1 . 1 Definition of nominal LRs of the national systems 1.2 Recommended values . Nominal overall loss of the international chain LRs and dir
9、ectional effects in a complete connection 3.1 Nominal LRs for each transmission direction 3.2 Traffic-weighted mean values of OLRs 3.3 Difference in transmission loss between the two directions of transmission Variation in time and effect of circuit noise . 4.1 Variations in time . 4.2 Effect of cir
10、cuit noise Practical limits of the OLR between two operators or one operator and one subscriber . Incorporation of PCM digital processes in international connections 6.1 Connections with a digital 4-wire chain extending to the local exchanges 6.2 Mixed analogue/digital connections . Annex A . Explan
11、ations related to Recommendations G . 1 1 I. G.12 1. G . 122. G . 13 1. G . 134: properties and uses of loudness ratings General explanations of loudness rating terms as used in the G-Series Recommendations . Psycho-acoustic model for loudness ratings . Measurement of loudness ratings of telephone s
12、ets Annex B - Recommended values and limits of the loudness ratings for circuits in international connections Annex C - Translation of LR values into CRE values References . A . 1 A.2 A.3 A.4 Application of loudness ratings in the Series-G Recommendations . Recommendation G.lll (03/93) Page 1 1 1 1
13、2 2 2 2 3 3 3 3 4 4 5 5 5 6 7 8 15 16 17 i ITU-T +RECMN+GmLLL 73 48b259L 0589462 88T PREAMBLE Clauses i to 5 of this Recommendation apply in general to all-analogue, mixed analoguedigital and all-digital international telephone connections. However, where Recommendations are made on specific aspects
14、 in clause 6 for mixed analoguddigital or all-digital connections, clause 6 will govern. In the international transmission plan, the loudness rating (LR) between two subscribers is not strictly limited; its maximum value results from various Recommendations. The values given below, either in terms o
15、f LR should be used to verify that international telephone connections provide an adequate loudness of received speech. Administrations employing CREs should preferably translate the LRs of this Recommendation into their national CREs by the methods given in Annex C or, as a second choice, apply the
16、 values given in Volume III of the Red Book (IT, Geneva, 1985). NOTES 1 2 The main terms used in this Recommendation are defined and/or explained in Annex A. For many telephone sets using carbon microphones, the SLR and STMR values can only be determined with limited accuracy. Recommendation G.lll (
17、03D3) ITU-T *RECMN*G.333 93 D 4862593 0589463 71b = Recommendation G.111 LOUDNESS RATINGS (LRs) IN AN INTERNATIONAL CONNECTION (Geneva, 1964; amended at Mar del Plata, 1968; Geneva, 1972.1976 and 1980; Malaga-Torremolinos, 1984; Melbourne, 1988, and Helsinki, 1993) 1 Nominal LRs of the national syst
18、ems 1.1 Definition of nominal LRs of the national systems Send and receive loudness ratings, SLRs and RLRs respectively, may in principle be determined at any interface in the telephone network. In this Recommendation and in Recommendation G.121, the SLRs and RLRs of the national systems are referre
19、d to the Virtual International Connecting Points (VICPs) as defined in 2.12/G.101; see Figure 1. The virtual international connecting point is a point in the international circuit at the International Switching Centre (ISC). The relative levels at the VICP are: - Sending: OdBr; - Receiving: O dBr fo
20、r digital circuits; -0.5 dBr for analogue or mixed analogue/digital circuits. The nominal circuit loss of international circuits at the reference frequency (1020 Hz) is therefore O dB for digital circuits and 0.5 dB for analogue and mixed analoguddigital circuits. NOTES i in digital international ci
21、rcuits, the VICP will be at the digital bit stream. In analogue exchanges, the VICP often will be a hypothetical, not accessible point; see 2.12/G.i01 and 2.13/G.101. 2 Earlier, the Virtual Analogue Switching Points (VASPs) having the relative levels -3SdBr and -4 dBr were used as the reference poin
22、ts. 1.2 Recommended values Recommendation G.121 gives objectives for the nominal SLR and RLR of national systems. 2 Nominal overall loss of the international chain Each international analogue and mixed analogue/digital circuit should in principle have a 0.5 dB loss at 1020 Hz. However, some circuits
23、 can be operated with higher losses (see 2.UG.131) and certain analogue and mixed analoguddigital circuits may be operated at zero loss (see 4.2/G.101). Digital circuits have O dB transmission loss (see clause 6 below). As far as transmission is concerned. there is no strict limit on the number of i
24、nternational circuits which may be interconnected in tandem, provided each analogue and mixed analoguddigital circuit has 0.5 dB nominal loss, and provided there is a 4-wire interconnection. Naturally, the fewer the number of interconnected circuits the better the transmission performance is likely
25、to be. NOTE - Information on the actual number of circuits which are found in international connections is given in 3/G. 101. Recommendation 6.111 (03/93) 1 ITU-T *RECMN*G.LLL 93 = 48b259L 05894b4 652 = - 4 I v I t v v. I v v I n b SLR Intemabbnal chuit the nominal CLR, circuit loudness rating, of t
26、he international chain (see! Annex A); the nominal RLR, receive loudness rating, of the national receiving system (see 4/G. 121 and Annex A). - - 2 Recommendation 6.111 (OW3) ITU-T *RECMN*G-333 93 W 4862593 05894b5 599 W 3.2 Traffic-weighted mean values of OLRs For connections under practical condit
27、ions a suitable value of OLR seems to be 10 dB in most cases. The long-term objective for the traffic-weighted mean value should lie in the range of 8 to 12 dB. An objective for the mean value is necessary to ensure that satisfactory transmission is given to most subscribers. NOTES 1 For connections
28、 totally free from echo and sidetone problems, investigations have shown the optimum OLR to be somewhat lower than 10 dB, about 5 dB, but the optimum is rather flat so that moderate deviations from the given value have little subjective effect. (However, the “preferred OLR” in a particular applicati
29、on will to some extent depend on what subscribers have become used to. Thus, in some analogue PBXs, internal calls have had a very low OLR. Replacing such a PBX by a digital PBX having a higher OLR might cause some subscriber comments on “low speech levels”. Some Administrations have solved this pro
30、blem by means of a manual volume control in the receive part of the telephone set, the total range of variation being in the order of 10 to 12 dB. Note that in mobile telephony a common practice is to include a volume control which affects both the receive and send sides but in opposite directions.)
31、 The long-term values cannot be attained at this time and an appropriate short-term objective for OLR is a range of 8 to 21 dB. The 0.5 dB transmission loss of each analogue and mixed analoguddigital circuit in the international chain (see clause 2 above) has been allowed for by noting that the aver
32、age number of international circuits encountered in international connections is 1. I. (See 3/G. 1 O1 .) 2 3 As a result, the ranges mentioned above do not include allowances for connections between countries which - - Clause 1/G.121 gives values for national systems based on the overall objectives
33、of this Recommendation. The ranges stated for OLR are for planning and do not include measuring and manufacturing tolerances. Besides loudness, other important factors have to be considered in transmission planning. Sidetone, echo and stability problems may cause degradation of the overall speech qu
34、ality in a connection. Thus, it is important to adopt an adequate impedance strategy in the national transmission plan to avoid harmful mismatches in the network; see Supplement No. 31/G.121. involve more than one 0.5 dB international circuit; involve a single international circuit which has a highe
35、r loss than 0.5 dB as permitted by 2.1/G. 131. 4 5 6 3.3 Difference in transmission loss between the two directions of transmission The difference in nominal loss between the two directions of transmission on an international connection should not exceed 8 dB, preferably not 6 dB. The asymmetry intr
36、oduced by each of the two national systems is limited by the provisions of 2.2/G. 121. The international circuits could, in practical circumstances outlined in the general remarks in 4/G. 101, introduce additional asymmetry. This additional asymmetry will be acceptably small. 4 Variation in time and
37、 effect of circuit noise 4.1 Variations in time The LR values calculated for national systems (see 4/G.121) do not cover variations in time of the loss of various parts of the national system. Clause 3/G. 15 1 gives the objectives recommended by the CCITT for transmission loss variations on internat
38、ional circuits and national extension circuits as compared with the nominal values. 4.2 Effect of circuit noise See Recommendation G. 113. 5 Practical limits of the OLR between two operators or one operator and one subscriber The same loudness rating limits as between two subscribers should apply. R
39、ecommendation G.111 (03/93) 3 ITU-T *RECMN*G-LLL 93 q86259L 0587466 425 6 Incorporation of PCM digital processes in international connections 6.1 Connections with a digitai 4-wire chain extending to the local exchanges As the national network develops, an international telephone connection might hav
40、e the configuration indicated in Figure 2, in which the analoguddigital interface occurs at the local exchange. In such a connection, the nominal transmission loss introduced by the 4-wire chain of national and international digital circuits is O dB. Consequently, the 4-wire chain generally does not
41、 contribute to the control of stability and echo. However, part of the loss required to control stability and echo is at the local exchange, as indicated by the R and T pads, the remainder being provided by the balance return loss at the 2-wird4-wire terminating unit (see also Recommendation G.122).
42、 .- . R T 3 X ab DRS LE 1 Tp 1 circuit (8) 4M.101 Analogue transmissiai Digitai tra?Smssion Analogue pads (represent nomird bss between loudness rating is an objective measure of the loudness loss, i.e. a weighted, electro-acoustic loss between certain interfaces in the telephone network. (The natur
43、e of the weighting will be dealt with later.) If the circuit between the interfaces is subdivided into sections, the sum of the individual section LRs is equal to the total LR. How to determine and to apply LRs in the G-Series Recommendations is described in subclauses A.3 and A.4. The methods are s
44、ufficiently accurate for all practical purposes. (Fundamentally, loudness ratings are based on subjective methods as described in Recommendations P.76 and P.78. However, subjectively measured values, in general, vary too much with time and test teams to be really useful for transmission planning.) I
45、n loudness rating contexts, the subscribers are represented from a measuring point of view by an artificial mouth and an artificial ear respectively, both being accurately specified. A.1.2 subscribers ear via a connection. overall loudness rating (OLR): The loudness loss between the speaking subscri
46、bers mouth and the listening A.1.3 send loudness rating (SLR): The loudness loss between the speaking subscribers mouth and an electric interface in the network. The loudness loss is here defined as the weighted (dB) average of driving sound pressure to measured voltage. A.1.4 receive loudness ratin
47、g (RLR): The loudness loss between an electric interface in the network and the listening subscribers ear. The loudness loss is here defined as the weighted (dB) average of driving e.m.f. to measured sound pressure. Recommendation 6.111 (0393) 5 ITU-T *RECNN*G.LLL 93 4862591 05894b8 2TB A.1.5 circui
48、t loudness rating (CLR): The loudness loss between two electrical interfaces in a connection or circuit. each interface terminated by its nominal impedance which may be complex. The loudness loss is here approximately equivalent to the weighted (dB) average of the composite electric loss. NOTE - Jun
49、ction loudness rating (JLR) is a special case of CLR, the terminations being 600 ohms resistive. A.1.6 Sidetone loudness losses A.1.6.1 talker% sidetone, sidetone masking rating (STMR): The loudness loss between a subscribers mouth and his (earphone) ear via the electric sidetone path (see Recommendation P.10 for a full definition). A.1.6.2 listeners sidetone rating (LSTR): The loudness loss between a Hoth-type room noise source and the subscribers (earphone) ear via the electric sidetone path (see Recommendation P.10 for a full definition). A.1.7 Echo loudness los
