1、INTERNATIONAL TELECOM MUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU Series P Supplement 1 O (1 1 /88) SERIES P: TELEPHONE TRANSMISSION QUALITY, TELEPHONE INSTALLATIONS, LOCAL LINE NETWORKS Considerations relating to transmission character ist ics for analog ue handset telep
2、hones ITU-T P-series Recommendations Supplement 1 O (Previously CCITT Recommendations) ITU-T P-SERIES RECOMMENDATIONS TELEPHONE TRANSMISSION QUALITY, TELEPHONE INSTALLATIONS, LOCAL LINE NETWORKS Vocabulary and effects of transmission parameters on customer opinion of transmission quality Subscribers
3、 lines and sets Transmission standards Objective measuring apparatus Objective electro-acoustical measurements Measurements related to speech loudness Methods for objective and subjective assessment of quality Audiovisual quality in multimedia services Series Series Series Series Series Series Serie
4、s Series P. 10 P.30 P.300 P.40 P.50 P.500 P.60 P.70 P.80 P.800 P.900 For further details, please refer to ITU-T List of Recommendations. FOREWORD ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of telecommuni- cations. The ITU Telecommunication Stand
5、ardization Sector (ITU-T) is a permanent organ of the ITU. The ITU-T is responsible 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
6、(WTSC), which meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. 1. In some areas of
7、 information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with IS0 and IEC. NOTE In this Recommendation the term recognized operating agency (ROA) includes any individual, company, corporation or governmental organization that operates a
8、public correspondence service. The terms Administration, ROA and public correspondence are defined in the Constitution of the ITU (Geneva, 1992). INTELLECTUAL PROPERTY RIGHTS The ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of
9、a claimed Intellectual Property Right. The ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation,
10、 the ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. O ITU 1
11、999 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. Supplement 10 - Series P (11/88) i CONTENTS 1 Introduction 2 Receiving frequency
12、 response 3 Receiving sensitivity 4 Sending frequency response . 5 Sending sensitivity . 6 Regulation 8 Sidetone balance impedance . 7 Impedance presented to the line . 9 Interworking with the existing network References . Page 1 11 Supplement 10 . Series P (11/88) Supplement 10 to P-series Recommen
13、dations CONSIDERATIONS RELATING TO TRANSMISSION CHARACTERISTICS FOR ANALOGUE HANDSET TELEPHONES (Malaga-Torremolinos, 1984; amended Melbourne, 1988) 1 Introduction This Supplement based on reference 9 summarizes available information on how some characteristics for handset telephones can be optimize
14、d. It contains information about sending and receiving sensitivities, frequency responses, sidetone characteristics, influence of impedance and handset dimensions. It must be remembered that there are different ways to make an optimization. For instance the number of degrees of freedom are essential
15、. As there are different opinions in different countries (for instance, the different assumptions made) the results of the optimization will be different. This Supplement touches some of these aspects. 2 Receiving frequency response Most Administrations seem to prefer a fairly flat frequency respons
16、e between 300 Hz and 3400 Hz. This probably derives from the early days of telephone networks, when it was determined that possible pre-emphasis at higher frequencies should be located at the sending end to obtain the best possible overall signal-to-noise performance. If we consider free-field, two-
17、ear listening as a reference (face to face conversation) and assume a frequency-independent (flat) response, we should in principle simulate these conditions also at one-ear telephone listening. Then, at the earphone listening, we should have a frequency response of the earphone as in Figure 1 to si
18、mulate the diffraction effect we have at free-field two-ear listening i. However, most Administrations seem to prefer a flat response and to put the corresponding correction at the sending end. It may also be easier to construct a receiver with high efficiency if the goal is a flat response. Referen
19、ce 2 has suggested a response as in Figure 2 optimized for a mean local line. Where mains noise may cause problems, a response with greater loss at lower frequencies, e.g. at 200 Hz and lower frequencies, may be appropriate. 0.05 0.1 a2 0.5 1 2 5 10 kHz Frequency CCITT - 82750 FIGURE 1 The diffracti
20、on effect around the head at 1 m distance in free field i Supplement 10 - Series P (11/88) 1 FIGURE 2 Receiving frequency response according to 2 3 Receiving sensitivity Receiving sensitivity today often is represented by values between an RLR of 4 dl3 and -12 dl3 respectively. A further increase of
21、 the sensitivity by the use of amplifiers might technically be possible. However, the probability for the audibility of crosstalk will increase with increased sensitivity. Therefore, the information gathered in Recommendation P.16 must be considered and it is doubtful if it can be recommended to inc
22、rease the sensitivity further beyond an RLR of -12 dl3. Increasing the receiving sensitivity also decreases the margins against the effects of speech-off noise on the connection, e.g. unwanted modulation products from PCM systems. The stability against singing will also be affected. 4 Sending freque
23、ncy response Having chosen the receiving response to be flat, the sending frequency response can be optimized to give the proper overall characteristic. Reference 3 suggests an optimization achieved by asking the listeners for the “preferred” response. The result is shown in Figure 3. Reference 4 su
24、ggests a 2 to 3 dl3 increase per octave with increasing frequency. This result was obtained in tests regarding “naturalness”. Reference 2 suggests a steeper curve (Figure 4) as a result of an optimization where maximum loudness, minimum listening effort and lowest output level are combined. The degr
25、ee of freedom used by 2 is of course less than in 3 and 4. Here we may have a difference in opinion concerning which assumptions we must include in the optimization. If the signal-to-noise ratio is a problem, some decibels could be gained (without overloading) in the way shown by 2. If there are no
26、signal-to-noise ratio problems, an optimization for best naturalness as in 3 and 4 can be used. Thus, the result will depend on the assumptions. Different opinions may also exist about the local cable length for which the frequency response should be optimized and if the high frequency loss at long
27、lines should be compensated. Reference 2 suggests optimization of the mean local line which will be optimum to the highest number of subscribers (because of the statistical distribution of cable lengths). 2 Supplement 10 - Series P (11/88) 0.02 0.05 0.1 0.2 0.5 1 2 5 IO 20 kHz Frequency CCITT - anm
28、FIGURE 3 Sending frequency response according to 3 FIGURE 4 Sending frequency response according to 2 The curves according to Figure 4 and 4 give with a flat receiving frequency response an overall characteristic close to what is obtained by the diffraction effect at free-field listening. However, t
29、his is probably not the whole explanation to the preferred curves. Even if the receiving responses were flat during sealed measuring conditions, hardly anyone keeps the earphone tight to the ear during conversation. Therefore, the actual responses during conversation probably give some additional lo
30、w frequency cut-off that certainly has an influence on the results (see also reference 5). Supplement 10 - Series P (11/88) 3 5 Sending sensitivity When we want to choose the sending sensitivity we have one degree of freedom less that at the receiving end. We must consider both the probability of cr
31、osstalk and the probability of overloading other parts of the telephone system. Actual output levels from the telephone must be considered. As shown in 6 different output levels for the same SRE-value have been found in different countries. However, the different results show one important feature i
32、n common: output levels during normal conversation are generally lower than during reference equivalent measurements. Hopefully we will get better agreement on this point in the future if we use the measuring distance defined in Recommendation P.76, Annex A for loudness rating measurements. 6 Regula
33、tion A possibility to increase the sending sensitivity on long lines exists if we use sending regulation dependent on line length. The probability for overloading and the probability for far end crosstalk will not increase if the mean power is kept to the same value as today. See also 2. The probabi
34、lity of near end crosstalk in the local cable will of course increase and has to be considered. If regulation is introduced both at sending and receiving, more subscribers may experience an overall loudness rating close to a preferred optimum, i.e. less calls will be rated poor and unsatisfactory. A
35、nother reason to introduce regulation is to obtain a better sidetone performance on short and long lines at the same time. 7 Impedance presented to the line Some considerations concerning this topic are as follows: - a conjugate match with the line maximizes the power transferred but creates sideton
36、e problems on short lines and also stability/echo problems on long-distance calls; - an image match to the line reduces the range of impedance presented to the exchange and eases the sidetone problem except for short subscriber-lines connected to resistive junction plant (e.g. PCM circuits); - an im
37、pedance approximating the reference resistance (e.g. 600 ohms) eases standardization problems particularly in respect of alternative uses of the local line for non-speech services, but the optimum in respect of sidetone cannot be attained over the whole range of local line lengths. References 2, 7 a
38、nd 1 i touch upon this subject. 8 Sidetone balance impedance The degree of sidetone suppression is governed by the following parameters: - microphone sensitivity; - earphone sensitivity; - sidetone balancing arrangement within the telephone instrument circuit; - the impedance of the line to which th
39、e telephone is connected. The microphone and earphone sensitivities and the instrument circuit are in part controlled by the required sending and receiving sensitivities. The impedance of the line to which the telephone is connected is not usually within the control of the telephone instrument desig
40、ner. The only parameter freely available to the telephone designer to control the sidetone level is ZSO, the sidetone balance impedance 7, SI, the impedance which when connected to the telephone completely suppresses sidetone (see also ref. 12). If a transformer hybrid is used in the telephone then
41、the internal balance network impedance is equal to the sidetone-balance impedance ZSO modified by the turns ratio of the transformer. However, the concept ZSO is not affected if the circuit uses any other form of balancing arrangement instead of a transformer. 4 Supplement 10 - Series P (11/88) 9 Th
42、e design of new handset telephones to be introduced into the telephone network must take account of the need to give satisfactory transmission on connections to existing local telephone circuits either directly or via the long-distance network. Reference 7 contains information touching upon this asp
43、ect. Reference lo is an example of a specification used in North America. Guidance for desirable sending and receiving levels are given as well as characteristics to be minimally acceptable for connection to the public switched network. It should be noted that this specification uses IEEE terminolog
44、y, which is different from that found in CCITT Recommendations. Interworking with the existing network References CCITT Recommendation Description of the ARAEN, Green Book, Vol. V, Rec. P.41, Fig. 4, ITU, Geneva, 1972. CCITT Contribution COM XII-No. 32 (U.K. Post Office), Study Period 1973-1976. CCI
45、TT Contribution COM XII-No. 22 (Australia), Study Period 1973-1976. GLEISS (N.): Sound transmission quality, Tele. No. 1, 1972, pp. 44-53. CCITT Contribution COM XII-No. 229 (Sweden), Study Period 1985-1988. CCITT Recommendation Subjective effects of direct crosstalk; thresholds of audibility and in
46、telligibility, Yellow Book, Vol. V, Rec. P.16, ITU, Geneva, 1981. CCITT Manual Transmission planning of switched telephone networks, Chapter V, Annex 1, ITU, Geneva, 1976. RICHARDS (D. L.): Telecommunications by speech, Chapter 5, Butterworths, London, 1973. CCITT Contribution COM XII-No. 105 (LME),
47、 Study Period 1973-1976. EIA Specification RS 470. CCITT Contribution COM XII-No. 144 (British Telecom), Study Period 198 1-1984. CCITT Handbook on Telephonometry, Geneva, 1987. Supplement 10 - Series P (11/88) 5 ITU-T RECOMMENDATIONS SERIES Series A Series B Series C Series D Series E Series F Seri
48、es G Series H Series I Series J Series K Series L Series M Series N Series O Series P Series Q Series R Series S Series T Series U Series V Series X Series Y Series Z Organization of the work of the ITU-T Means of expression: definitions, symbols, classification General teleconlmunication statistics
49、 General tariff principles Overall network operation, telephone service, service operation and human factors Non-telephone telecommunication services Transmission systems and media, digital systems and networks Audiovisual and multimedia systems Integrated services digital network Transmission of television, sound programme and other multimedia signals Protection against interference Construction, installation and protection of cables and other elements of outside plant TMN and network maintenance: international transmission systems,
