NL9302095A - System for generating a diffuse noise field and a measuring system for testing objects in terms of their behaviour in a diffuse noise field - Google Patents

Abstract

System for generating a diffuse noise field within an area 4 by means of a number of transducers 15, particularly relating to the measurement of the behaviour of objects in "ambient noise", such as "listener sidetone" measurement on telephone terminals 17. The transducers are connected via a distribution device 16 to one common noise generator 3, the distribution device connecting the transducers in a time-division manner to the common noise generator. <IMAGE>

Description

System for generating a differential noise field and measuring system for testing objects for their behavior in a diffuse noise field

A. BACKGROUND OF THE INVENTION

The invention relates to a system for generating a diffuse noise field within a space by means of a number of transducers arranged in or around that space. The invention also comprises a measuring system for testing objects for their behavior in a diffuse noise field.

Systems for generating a diffuse noise field are used, among other things, when testing equipment for behavior under 'random' environmental conditions. For example, devices intended to record sounds, such as telephones, cassette recorders, hearing aids, dictation devices, etc., can be tested for their operation in environmental noise. Likewise, however, other devices can be tested for their operation under ambient light, ambient vibrations, magnetic or electric fields, etc. In all such cases, it is generally important that the field representing the environment is diffuse so that the arrangement of the equipment under test within the test room does not affect the test result.

An obvious possibility to generate a diffuse noise field is to connect the different transducers to each their own noise generator. Each noise generator, independently of the others, generates a noise signal excited by the transducer in space. Because the noise generator signals are not correlated with each other, a diffuse field is created in space, without any pattern of standing waves. A drawback of this solution is that each transducer has its own noise generator, making the test system quite expensive and bulky, especially with a somewhat larger test room (more transducers).

B. SUMMARY OF THE INVENTION

The present invention aims to provide a system for generating a diffuse noise field, whereby fewer noise generators will suffice. It is characteristic of the invention that the transducers are connected via a distribution device to at least one common noise generator, the distribution device connecting those transducers in a time-divided manner to that at least one common noise generator. In practice, it usually seems sufficient to use only one common noise generator, which is thus 'time multiplexed' to each of the transducers (for example, 8 transducers, one in each corner of a test room). The distribution member may connect the transducers to the common noise generator on a regular or irregular basis in time; there will usually be a preference to do this regularly.

A preferred embodiment of the invention is characterized in that the at least one common noise generator comprises a signal buffer with a noise signal previously recorded for a certain period of time, and that the distribution device successively reads out different locations of that signal buffer and distributes the signal elements (samples) thus read out over the different transducers. This is explained in more detail in the exemplary embodiment below.

The applicant has in particular intended the system according to the invention as a subsystem of a measuring system for testing objects, in particular telephone sets, on their behavior in a diffuse, in this case acoustic, noise field; however, it was quite conceivable that the system could also be used in other areas.

C. REFERENCES None

D. IMPLEMENTATION EXAMPLES

General

The applicant has implemented a measuring system, called TOTEM, with which the electro-acoustic properties of (ISDN and analog) telephone sets can be measured. The measuring system is used to determine whether new products meet the legal requirements. This prevents these products from being presented unnecessarily for inspection and thus speeds up their introduction.

The TOTEM measuring system measures the electro-acoustic requirements fully automatically and generates a measurement report. The measurement system indicates for each measurement whether the device meets the requirement. The total package of requirements, which consists of 20 separate measurements, is approx.

Measured for 12 minutes.

In addition to determining whether a device meets the legal requirements, the measuring system is also suitable for making a quick statement about the objective quality of telephones. With increasing competition, it is very important to supply high-quality products. The measuring system is an important tool in the selection of the range.

The package of requirements includes requirements with regard to: 1. Frequency characteristic (transmit and receive side) 2. Loudness SLR and RLR (transmit and receive side) 3. Overflow (Sidetone) 4. Echo and stability 5. Distortion (transmit and receive side) 6. Overflow distortion 7. Linearity (transmit and receive side) 8. Sensitivity of "out of band" signals 9. Noise (transmit and receive side) 10. Sampling frequency failure 11. Hearing protection 12. Delay

Many measurements are measured on both the transmitting side and the receiving side. In the case of a transmission side measurement, an acoustic signal must be applied via a artificial mouth on the horn and the line signal delivered by the device must be measured. With a receive side measurement, a measuring signal is sent to the device and the acoustic signal that the device emits is measured via a microphone (ear coupler).

The system according to the invention can be switched on when investigating the requirements mentioned under 3 (appliance overflow). The device overflow or sidetone consists of a talker and a listener sidetone. The talker sidetone is the measure of how loud one hears oneself through the horn. For this, a signal is placed on the artificial mouth and the signal strength is measured via the artificial ear. This is measured for 14 frequencies and the sidetone is then the weighted number determined by formula 2.1 from CCIIT Recommendation P.79.

The listener sidetone is a measure of how loud one perceives ambient noise through the horn. This is achieved by placing the horn in a uniform sound field and measuring the magnitude of the signal strength emitted by the horn on the coupler. The same weighting is again applied here.

Figure description

Figure 1 shows the global scheme of the TOTEM measurement system.

Figure 2 schematically shows a signal buffer forming part of a noise generator.

The heart of the measuring system is a PC (Personal Computer) 1. This is equipped with a Siemens ISDN card 2 and an AT&T DSP32C card 3. The device 17 to be measured is placed in a low-echo box 4 in which a B&K testhead has been placed. This test head is equipped with a artificial mouth (speaker) 5 and an artificial ear (microphone) 6. For measuring the acoustic signals via the artificial ear microphone, a measuring amplifier 7 is used. This amplifier 7 provides the microphone power and amplify the microphone signal. An amplifier is used to control the artificial mouth.

The ISDN card 2 is responsible for establishing the connection with the ISDN device 17. The DSP card 3 has a connection via an interface 9 to the ISDN card and to D / A resp. A / D converters 10,11,12. The DSP card 3 can generate signals completely independently and at the same time read and process the returning signal via its input line. After the measurement, the PC 1 retrieves the results from the DSP card 3, processes them and displays them.

A third EC card is a National Instruments IEEE card, which provides control of the measuring amplifier 7 and the interface circuit 9. The interface circuit 9 not only makes signal adjustments but also switches the correct signal paths.

As indicated above, the listener sidetone is a measure of how loud one perceives ambient noise through the horn. This is achieved by placing the receiver of the telephone set 17 to be examined in a diffuse sound field and measuring the magnitude of the signal strength emitted by the receiver on the coupler 6. The noise field required for this measurement is generated in space 4 in the following manner.

In system initialization, a buffer of the DSP card 3 is "filled" with a (digitized) noise signal with a so-called 'hoth' spectrum. The (128000) samples of that noise signal are used as the basis for eight, in fact independent, noise signals during the measurement to the listener sidetone. By making use of the loudspeakers 15 and amplifiers 14 in an iterative process, which is not further described, when filling the DSP buffer, the influence of the frequency characteristic of those loudspeakers 15 and amplifiers 14 can be neutralized in advance. During the measurement itself, eight different samples are always sent from the buffer of the DSP card 3 to the eight different loudspeakers 15, whereby a diffuse noise field is generated in space 4, starting from the same common DSP card 3, which functions as a noise generator. .

During the measurement, the DSP software places a "pointer" in the sample buffer and reads the signal value at that location. Subsequently, that signal value is presented via a (serial) output of the DSP card 3, an interface 9 and a D / A converter 12 to a multiplexer 16, with one input and eight outputs. This multiplexer 16 then transmits the presented signal element via an amplifier 14 connected to the output to one of the loudspeakers 15 in the test room 4. After thus reading a memory location of the sample buffer of the DSP card 3, the pointer is moved to a next memory location, not in the immediate vicinity of the previous memory location in order to avoid correlation, and in the same way the memory content is read at that new address and, via the multiplexer 16 now switched to a next output, is sent to a next loudspeaker 15. In this way, all eight loudspeakers 15 are "operated" with different signal values each time, so that a diffuse noise field is created in the test room 4 (after all, the signal values are not correlated). Figure 2 schematically shows the DSP buffer 18, in which the said 128000 signal elements 19 are located. The consecutive locations of the pointer to drive the eight loudspeakers are represented by pl ... p8, p'l, p'2, etc. If the pointer is at the end of the buffer, it will start again at the beginning, whereby also the signal elements - see figure 2 - which have been skipped by the pointer in an earlier cycle, are dealt with.

It should be noted that the outputs of the multiplexer 16 are equipped with "Sample & Hold" circuits, so that each speaker maintains its last signal value as long as it is not its turn. Incidentally, the multiplex frequency (160 kHz) is high relative to the noise frequency (which is limited to 10 kHz).

Finally, it should be noted that in said embodiment described above, in fact, in addition to the multiplexer 16, the said distribution element is also constituted by the buffer read-out mechanism (pointer control) of the DSE card 3.

Claims (6)

System for generating a diffuse noise field within a space (4) by means of a number of transducers (15) arranged in or around that space, characterized in that the transducers are connected via a distribution member (16) to at least one common noise generator (3), wherein the distribution means connects said transducers to said at least one common noise generator in a time-divided manner. System according to claim 1, characterized in that the distribution member connects the transducers regularly divided over time to the at least one common noise generator. System according to claim 1, characterized in that the distribution member connects the transducers irregularly in time to the at least one common noise generator. System according to claim 1, characterized in that the noise field is an acoustic noise field and the transducers are acoustic transducers, such as loudspeakers. System according to claim 1, characterized in that the at least one common noise generator comprises a signal buffer (18) with a noise signal previously recorded for a certain period of time, and that the distribution device successively has different locations (ρΐ,.,., Ρδ) from that signal buffer and distributes the signal elements (19) thus read across the various transducers (15). Measuring system for testing objects (17) for their behavior in a diffuse noise field, characterized by a system for generating a diffuse noise field according to claim 1.