ITU-T P 48-1988 SPECIFICATION FOR AN INTERMEDIATE REFERENCE SYSTEM《中间参考系统的技术规范》.pdf

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1、)45G134 0 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%0(/.%G0G042!.3-)33)/.G0G015!,)4942!.3-)33)/.G0G034!.$!2$330%#) amended at Geneva, 1980,Malaga-Torremolinos, 1984, Melbourne, 1988)SummaryThis Recommendation intends to specify the intermediate reference system (IRS) to be used for defininglo

2、udness ratings. The description should be sufficient to enable equipment having the required characteristics to bereproduced in different laboratories and maintained to standardized performance.1 Design objectivesThe chief requirements to be satisfied for an intermediate reference system to be used

3、for tests carried out onhandset telephones1)are as follows:a) the circuit must be stable and specifiable in its electrical and electro-acoustic performance. The calibrationof the equipment should be traceable to national standards;b) the circuit components that are seen and touched by the subjects s

4、hould be similar in appearance and “feel”to normal types of subscribers equipment;c) the sending and receiving parts should have frequency bandwidths and response shapes standardized torepresent commercial telephone circuits;d) the system should include a junction which should provide facilities for

5、 the insertion of loss, and othercircuit elements such as filters or equalizers;e) the system should be capable of being set up and maintained with relatively simple test equipment.Note The requirements of a) to d) have been met in the initial design of the IRS by basing the sending andreceiving fre

6、quency responses on the mean characteristics of a large number of commercial telephone circuits andconfining the bandwidths to the nominal range 300-3400 Hz.Since the detailed design of an IRS may vary between different Administrations, the following specificationdefines only those essential charact

7、eristics required to ensure standardization of the performance of the IRS.The principles of the IRS are described and its nominal sensitivities are given in 2, 3, 4 and 5 below;requirements concerning stability, tolerances, noise limits, crosstalk and distortion are dealt with in 6 to 9 below.Some i

8、nformation concerning secondary characteristics is given in 10 below.Certain information concerning installation and maintenance are given in 1.2 Use of the IRSThe basic elements of the IRS comprise:a) the sending part,b) the receiving part,c) the junction._1)For other types of telephone, e.g. heads

9、et or loudspeaking telephone, a different IRS will be required. The IRS is specified forthe range 100-5000 Hz. The nominal range 300-3400 Hz specified is intended to be consistent with the nominal 4 kHz spacingof FDM systems, and should not be interpreted as restricting improvements in transmission

10、quality which might be obtained byextending the transmitted frequency bandwidth.2 Volume V - Rec. P.48When one example each of a), b) and c) are assembled, calibrated and interconnected, a reference(unidirectional) speech path is formed, as shown in Figure 1/P.48. For performing loudness rating dete

11、rminations,suitable switching facilities are also required to allow the reference sending and receiving parts to be interchanged withtheir commercial counterparts.3 Physical characteristics of handsetsThe sending and receiving parts of an IRS shall each include a handset symmetrical about its longit

12、udinal placeand the profile produced by a section through this plane should, for the sake of standardization, conform to thedimensions indicated in Figure 1/P.35. In practice, any convenient form may be considered use being made, forexample, of handsets of the same type as those used by an Administr

13、ation in its own network. The general shape of thecomplete handset shall be such that, in normal use, the position of the earcap on the ear shall be as definite as possible,and not subject to excessive variation.The microphone capsule, when placed in the handset, shall be capable of calibration in a

14、ccordance with themethod described in Recommendation P.64. The earcap shall be such that it can be sealed on the circular knife-edge ofthe IEC/CCITT artificial ear for calibration in accordance with IEC 318, and the contour of the earcap shall be suitablefor defining the ear reference point as descr

15、ibed in Annex A to Recommendation P.64.Transducers shall be stable and linear, and their physical design shall be such that they can be fitted in thehandset chosen. A handset shall always contain both microphone and earphone capsules, irrespective of whether eitheris inactive during tests. The weigh

16、t of a handset, so equipped, shall not exceed 350 g.4 Subdivision of the complete IRS and impedances at the interfacesFigure 1/P.48 shows the composition of the complete IRS, subdivided as specified in 2 above. The principalfeatures of the separate parts are considered below.4.1 Sending partThe send

17、ing part of the IRS is defined as the portion A-JS extending from the handset microphone A to theinterface with the junction at JS. The sending part shall include such amplification and equalization as necessary toensure that the requirements of 5.1 and 7 below are satisfied.The return loss of the i

18、mpedance at JS, towards A, against 600 / 0 ohms, when the sending part is correctly setup and calibrated, shall be not less than 20 dB over a frequency range 200-4000 Hz, and not less than 15 dB over afrequency range 125-6300 Hz.4.2 Receiving partThe receiving part of the IRS is defined as the porti

19、on JR-B extending from the interface with the junction at JRto the handset earphone at B. The receiving part shall include such amplification and equalization as necessary toensure that the requirements of 5.2 and 7 below are satisfied.The return loss of the impedance at JR, towards B, against 600 /

20、 0 ohms, when the receiving part is correctlyset up and calibrated, shall be not less than 20 dB over a frequency range 200-4000 Hz, and not less than 15 dB over afrequency range 125-6300 Hz.Volume V - Rec. P.48 34.3 JunctionFor loudness balance and sidetone tests, the junction of the IRS shall comp

21、rise means of introducing knownvalues of attenuation between the sending and receiving parts, and shall consist of a calibrated 600 ohm attenuatorhaving a maximum value of not less than 100 dB(e.g. 10 10 dB + 10 1 dB + 10 0.1 dB)and having a tolerance, when permanently fitted and wired in position i

22、n the equipment, of not more than 1% of thedial reading or 0.1 dB, whichever is numerically greater. Provision shall be made for the inclusion of additional circuitelements (e.g. attenuation/frequency distortion) in the junction. The circuit configuration of such additional elementsshall be compatib

23、le both with that of the attenuator and the junction interfaces. The return loss of the junction against600 / 0 ohms, both with and without any additional circuit elements, shall be not less than 20 dB over a frequencyrange 200-4000 Hz, and not less than 15 dB over a frequency range 125-6300 Hz. For

24、 these tests, the port other thanthat being measured shall be closed with 600 / 0 ohms.5 Nominal sensitivities of sending and receiving partsThe absolute values given below are provisional and may require changes to some extent as a result of thestudy of Question 19/XII 2.5.1 Sending partThe sending

25、 sensitivity, SmJis given in Table 1/P.48, column (2) (see 3).5.2 Receiving partThe receiving sensitivity, SJe, on a CCITT/IEC measured artificial ear (see Recommendation P.64) is given inTable 1/P.48, column (3) (see 3).4 Volume V - Rec. P.48TABLE 1/P.48Nominal sending sensitivities and receiving s

26、ensitivities of the IRS(These values were adopted provisionally)Frequency(Hz)SmJSJedB V/Pa dB Pa/V(1) (2) (3)10012516020025030031540050060063080010001250160020002500300031503500400050006300800045.836.125.619.214.311.310.88.46.96.36.14.93.72.30.60.31.81.51.87.337.252.273.690.027.518.810.82.72.76.47.2

27、9.911.311.811.912.312.612.513.013.113.112.512.63.931.654.967.590.0Volume V - Rec. P.48 56 StabilityThe stability should be maintained, under reasonable ranges of ambient temperature and humidity, at leastduring the period between routine recalibrations. (See also 1.)7 Shapes and tolerances on sensit

28、ivities of sending and receiving partsThe shape of the sensitivity/frequency characteristics of the sending and receiving parts of the IRS shall liewithin the limits of masks formed by Table 2/P.48 and plotted in Figures 2/P.48 and 3/P.48. The sending and receivingloudness ratings shall both be set

29、to 0 0.2 dB when calculated in accordance with the principles laid down inRecommendation P.79.Note One excursion above or one excursion below the limits is permitted provided that:a) the excursion is no greater than 2 dB above the upper or below the lower limit;b) the width of the excursion as it br

30、eaks the appropriate limit is no greater than 1/10th of the frequency at themaximum or minimum of the excursion.TABLE 2/P.48Coordinates of sending and receiving sensitivity limit curvesLimite curve Frequency(Hz)Sending sensitivity(dB with respectto an arbitrary level)Frequency(Hz)Receiving sensitivi

31、ty(dB with respectto an arbitrary level)Upper limit10020040034003600600041166+6+460100200300500340036004500240+9+14+16+1340Lower limitUnder 20020040030003400Over 3400 211114 Under 20020030050032003400Over 3400 20+4+9+10+4 6 Volume V - Rec. P.48 Volume V - Rec. P.48 78 Noise limitsIt is important tha

32、t the noise level in the system be well controlled. See 4.9 Nonlinear distortionIn order to ensure that nonlinear distortion will be negligible with the vocal levels normally used for loudnessrating, requirements in respect of distortion shall be met.10 Complete specificationsCertain secondary chara

33、cteristics of an IRS may be included in Administrations specifications. Particularly,special care must be given to adjustable components, stability and tolerances, crosstalk, installation and maintenanceoperations, etc. Reference 1 gives some guidance on these points.References1 Precautions to be ta

34、ken for correct installation and maintenance of an IRS, Orange Book, Vol. V,Supplement No. 1, ITU, Geneva, 1977.2 CCITT Question 19/XII, Contribution COM XII-No. 1, Study Period 1985-1988, ITU, Geneva, 1985.3 Precautions to be taken for correct installation and maintenance of an IRS, Orange Book, Vol. V,Supplement No. 1, 9.2, ITU, Geneva, 1977.4 Ibid., 5.