GB2121549A

GB2121549A - Apparatus for determining the loudness rating of a telephone system

Info

Publication number: GB2121549A
Application number: GB08215938A
Authority: GB
Inventors: George John Preston Barnes; Michael Stanley Hurley; David Ebenezer James Dawson; Qiang He Jian
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-06-01
Filing date: 1982-06-01
Publication date: 1983-12-21
Also published as: GB2121549B

Abstract

A noise generator (1) produces a wide-band signal which, in effect, simulates speech. This is passed through the portions of the system to be rated, and the signal after each such passage is analysed by a bank of 1/3 octave filters (3), each of whose outputs is passed via an analogue-to-digital converter (4) to a computer which produces a loudness rating. By controlling the relays (r1, r2, r3, r4, r5) in a relay set the computer controls a sequence of different loudness tests. <IMAGE>

Description

SPECIFICATION Apparatus for determining the loudness rating of a telephone system This invention relates to an apparatus for the determination of the loudness ratings of a telephone system.

In any telephone system, however carefully designed, some attenuation to the speech being transmitted takes place. Hence the loudness loss in telephone connections in a basic parameter used widely in network planning. Reference equivalents have been the conventional units used internationallyto quantify loudness loss, but the loudness rating method has many advantages and is tending to replace the more conventional methods. To some extent such methods have involved subjective testing, with all of the disadvantages thereof.

An object of the invention is to provide an apparatus for loudness rating determination which will give a sound objective result.

According to the invention, there is provided an apparatus for the determination of the loudness ratings for a telephone system, in which a wide-band signal whose characteristics simulate those of speech is transmitted via the portions of the system to be tested, in which the signal after it has passed via the portions of the system to be tested is subjected to a set of third-octave analysis in a number of bands spaced over the system's audio band-width, in which the results of the analysis are each converted by an analogue-to-digital converter into multi-bit combinations each appropriate to the amplitude of a portion of the audio band-width, and in which the multi-bit combinations produced by the analogue-to-digital conveter are applied to a digital computer so programmed as to derive threfrom a loudness rating for the portion of the system being tested.

In one embodiment of the invention the thirdoctave analysis uses a bank of third-octave filters whose outputs pass to the converter. Another possibility is to do the third-octave analysis using a fast Fourier transform technique.

Such an apparatus enables accurate calibrations for both acoustic and electrical measurements, and is capable of measuring the sensitivity/frequency characteristics of an unknown telephone system. It can calculate the various loudness ratings which need to be determinded to assess the characteristic of a telephone system. Further, the use of wide-band noise, the speech-simulating signal referred to above, as a test signal is suitable for use where the system uses carbon microphones.

An embodiment of the invention will now be described with reference to the accompanying block schematic, in which the computer is an Intel ISBC 88/40 single-board computer with an integral analogue input. Detailed information about this computer will be found in the Intel ISBC 88/40 Measurement and Control Computer Hardware Reference Manual.

It will be appreciated, however, that other computers can also be used in apparatus embodying the invention.

Since a number of different loudness ratings need to be determined when studying the characteristics of a telephone system, the operations involved in each such determination will be described separately. In the drawing the contacts of the relays in the relay set are all shown in their conditions when the relays are inactive. The computer controls the relay set so as to establish the connections appropriate to each stage of the operation.

Calibration Initially the microphone portion of the artificial ear is calibrated, after this, the calibration is effected in two stages, the first of which is to determine sound pressure level and the spectrum of the artificial mouth, and in this condition only the relay r2 is operated. Hence the noise generator 1 is disconnected from the apparatus output, and the output from the artificial ear AE is applied via a measuring amplifier 2, contact rl-l, and a set of 1/3 octave filters 3 to an analogue-to-digital converter 4. Thus when the outputs from the filters are scanned to provide the converer inputs an assessment is made of the quiescent condition of the system.

The electrical portion of the calibration has both relays rl and r2 operated, in which case a 1 kH3 generator 5 is connected via /5-1, r3, r1-2, an amplifier 6 and 41-1 to the filters 3. This generator produces a signal of amplitude 1 volt, and the result of the monitoring of this signal gives an electrical reference value.

Send loudness rating In this case there are two stages, the first of which is a mouth reference one in which no relays are operated. This in effect enables the apparatus to determine the noise level (if any) of the chain of devices from the artificial ear AE via the amplifier 2, filters 3 and converter 4 to the computer.

For the actual rating determination we have relays rl, r3 and r4 operated. Thus the output from the noise generator 1,which may be a simulation of a speech signal or wide band noise e.g. having a spectrum falling at 3dB/octave and known as pink noise, passes via the amplifier 7, contact r2-1, artificial mouth AM, the mouthpiece in the handset 8 to a telephone set 9. From here it passes via the cable 10 to a feeding bridge 11 and therefrom to a 1:1 transformer 12. The signal then developed in the secondary of the transformer 12 passes via contacts r4, r3, and rl-2 to the amplifier 6 and therefrom to the converter 4. Here again the computer initiates a scan of the outputs of the filter 3, and each such output is digitized by the converter 4.

Receive loudness rating Here there is a preliminary calibration operation, in which relays rl, r2 and r3 are operated. Hence the output of the noise generator 1 passes via the amplifier 7, r2-1, a 6000hm impedance 13, r2-2, r3 and r1 -2 to the amplifier 6. From there it passes, in the usual way via ri -1 to the computer. This gives a "base level" from which the loudness rating will be determined.

For the determination of the rating, we have relays r2 and r4 operated. Hence the noise generator output passes via amplifier 7, r2-1, impedance 13, r2-2, r4, to the winding of transformer 12 which is now acting as a primary winding. From the other winding of 12, the signal goes via feeding bridge 11, cable 10, telephone set 8. artificial ear AE, amplifier 2 and rl-l from which it reaches the computer. Thus the receive loudness rating is determined.

Junction loudness rating In this a junction 15 is connected between the lower-most windings of transformer 12 and 16, both of which are 1:1 transferance. Calibration is as for the Receive Loudness Rating given above.

The rating determination has relays ri, 42, r4 and fi operated. Thus the noise signal passes via amplifier 7, r2-1, impedance 13, r-2, and r4 to the transformer 12. From there it goes via the junction under test to the transformer 16 from which it goes via r5-2, r3, r1-2, amplifier 6 to contact rl-l, and therefrom to the computer.

Overall loudness ratio and side-town masked rating (STIR) Finally the condition with all relays released enable overall loudness ratings and sidetone masking loudness (STMR) ratings to be determined for the system.

Claims

1. An apparatus for the determination of the loudness ratings for a telephone system, in which a wide-band signal whose characteristics simulate those of speech is transmitted via the portions of the system to be tested, in which the signal after it has passed via the portions of the system to be tested is subjected to a set of third-octave analysis in a number of bands spaced over the system's audio band-width, in which the results of the analysis are converted by an analogue-to-digital converter into a multi-bit combinations each appropriate to the amplitude of its portion of the audio band-width, and in which the multi-bit combinations produced by the analogue-to-digital converter are applied to a digital computers programmed as to derive therefrom a loudness rating for the portion of the system being tested.

2. An apparatus as claimed in claim 1, in which the third-octave analysis is effected by applying the signal to a bank of a third-octave filters whose pass-bands are spaced over the system's audio band-width.

3. An apparatus as claimed in claim 1, in which the third-octave analysis is effected by the use of a fast Fourier transform analysis.

4. An apparatus as claimed in claim 1, in which the loudness rating determination is effected for junctions in a telephone system.

5. An apparatus for the determination of the loudness ratings for a telephone system, in which a wide-band signal whose characteristics simulate those of speech is transmitted via the portions of the system to be tested, which portions include at least one telephone transducer, in which the signal after it has passed via the portions of the system to be tested is applied to a set of third-octave filters whose pass-bands are spaced over the system's audio band-width, in which the outputs from the filters are each converted by an analogue-to-digital converter into a multi-bit combination appropriate to the amplitude of its portions of the audio band-width, and in which the multi-bit combinations produced by the analogue-to-digital converter are applied to a digital computer so programmed as to derive therefrom a loudness rating for the portion of the system being tested.

6. An apparatus as claimed in claim 5, in which several separate portions of the system are to be determined, in which a relay set is provided the contact of whose relays control the interconnection of the units of the apparatus, and in which the computer is programmed to set up the apparatus successively for each test operation to be performed so that a complete set of loudness ratings can be produced for a telephone system.

7. An apparatus for the determination of the loudness ratings for a telephone system, substantially as described with reference to the accompanying drawings.

8. A method for determining the loudness ratings for a telephone system, in which a wide-band signal whose characteristics are appropriate to the intended pass band of the telephone system is transmitted via the portions of the system to be tested, in which the signal after it has passed via the portions of the system to be tested is subjected to an analysis in a number of frequency bands spaced over the system's audio band width, in which the results of the analysis are converted into a number of multi-bit combinations each appropriate to the amplitude of the signal passed in a respective one of said frequency bands, and in which the multi-bit combinations thus generated as a result of said conversions are applied to a digital computer which is so programmed as to derive from those combinations a loudness rating for the portions of the system being tested.

9. A method as claimed in claim 8, in which the analysis is effected by applying the wide band signal to a bank of third octave filters whose pass bands are spaced over the system's audio band width.