EP2070388B1 - Method and measuring device for monitoring loudspeaker systems - Google Patents

Description

The invention relates to a method for monitoring loudspeaker lines by impedance measurement, in which the input of a loudspeaker line at least an alternating voltage signal of predetermined frequency is supplied, voltage and current are measured at the input of the line and determined from these measurements, the input impedance to the line and with a reference value is compared.
Likewise, the invention relates to an apparatus for carrying out the method according to the invention.
In any case, in the context of the present invention, PA systems are to be understood as a system in which at least one loudspeaker line, to which a plurality of loudspeakers is generally connected, is fed by one or more power amplifiers to a larger area or a building complex, such as a Sports stadium, an airport, a department store, etc. to sound. Since in addition to usual announcements, copywriting, background music, etc. in emergencies or disasters even vital calls on the speakers must be heard at each point of the system, it is necessary and in many cases prescribed by appropriate standards that the public address system regularly on their reliability is checked.
For such a review, various methods have become known, but so far the complexity of the task did not do justice. This complexity lies in the fact that a loudspeaker line has a large number of loudspeakers, which are usually connected to the line via a corresponding number of transformers, as is the case with the 100 volt system. The types of faults that can result in a non-sonicated room are manifold: line shorts, line breaks, voice coil cracks, clamped voice coils, faulty transformers, etc. The measurement must therefore detect all these faults, taking into account the temperature dependence of the impedances , In addition, the measurement should not interfere with the normal operation of the system and connected to a speaker line measuring signals should not or barely audible.
An object of the invention is to provide a method which takes into account the above-mentioned problems and which enables a particularly rapid and reliable detection of errors.

This object is achieved with a method of the aforementioned type, in which according to the invention a plurality of analog measured values of voltage and current are converted into digital signals and subjected to a Fourier transformation in order to determine an impedance to be compared with a reference value. transformed values of impedance and averaged over the resulting mean value with a reference impedance value, wherein when the resulting mean value deviates from the reference impedance value by a definable tolerance value, an error signal is output becomes.

In a variant, it is provided that in a large signal measurement, a single alternating voltage signal predetermined frequency of a speaker line is supplied via a power amplifier with a level which is below the nominal operating level, and the AC signal has such a low frequency that one for the human ear not or only Barely perceptible sound level is generated. The impedance values obtained in this way have the advantage that they were measured under actual operating conditions.

In terms of an adaptation of the measurement signal to the actually occurring level and frequency distributions, it makes sense if the useful signal before each measurement is subjected to a spectral analysis to determine the useful signal level at a plurality of frequencies, this plurality of frequencies also contains the only low frequency in that a weighted mean value level is formed from the plurality of received signal signal levels obtained, and for each measurement the level of the single low-frequency measuring signal supplied to the loudspeaker line is set at a predefinable fixed distance to the weighted average level.

In order to eliminate the influence of the hysteresis phenomena in loudspeaker transformers, provision may be made for a degaussing signal of low frequency to be supplied to a loudspeaker line via a power amplifier having a level which rises to a maximum level and then falls again before a measurement.

On the other hand, impedance values can be determined over a wide frequency range, if at least one alternating voltage signal of predetermined frequency / frequencies is supplied to a loudspeaker line having a level which is selected such that the generated sound levels are not or only barely perceptible to the human ear ,

• Fig. 1 in einem vereinfachtem Blockschaltbild den prinzipiellen Aufbau einer Messeinrichtung nach der Erfindung und
• Fig. 2 in einem Ablaufdiagramm die wesentlichen Verfahrensschritte des erfindungsgemäßen Verfahrens.

The determination of reference values can advantageously take place by supplying signals of different frequencies and / or levels in a learning phase of the loudspeaker line prior to the operation of the system, converting the measured values of current and voltage into digital signals and then subjecting them to a Fourier transformation and then Reference values for different frequencies and / or levels can be determined and stored from the current and voltage values. In this case, it is expedient for a multiplicity of individual measured values for current and voltage to be determined for each frequency / level and mean values of the individual measured values to be set for the following determination of the reference impedances.
To avoid that slow and quite "normal" impedance changes, eg. B. due to a change in temperature, lead to error indications, it can be provided that the tolerance value is changed during operation to the effect that slow changes of actual impedances a further tolerance range of the reference impedances is assigned, as rapid changes.
In practice, it has been found to be advantageous if the absolute value of the impedance is determined and compared with a reference value, although in certain cases an active power-related measurement of the real part of the impedance can also be advantageous.
The object of the invention is also based on a device for monitoring loudspeaker lines by impedance measurement with at least one signal generator and at least one amplifier and controlled switches for supplying at least one alternating voltage signal of predetermined frequency to the input of at least one speaker line, with measuring devices for measuring voltage and current the latter is set up to carry out the method according to the invention and controls the at least one signal generator, to which the measured values of the measuring devices can be fed, and to determine the input impedance using the above-mentioned features of the inventive method is set up at the line and for comparison with a reference value and for outputting an error signal if the determined impedance value is within determinable tolerances of an R differs from reference value.
The invention together with further advantages is explained in more detail below by way of example embodiments, which are illustrated in the drawing. In this show

• Fig. 1 in a simplified block diagram of the basic structure of a measuring device according to the invention and
• Fig. 2 in a flow chart, the essential steps of the method according to the invention.

In the illustration after Fig. 1 are outlined three speaker lines of a public address system, but it should be clear that depending on the type of system also a different number of speaker lines may be present. A plurality of loudspeakers are connected to each line, with larger systems preferably employing a high-impedance, so-called "100 volt" variant, in which each loudspeaker lies above a transformer on the line.

To an input of the speaker line can be switched via a switch SCH power amplifier, in the present case, for simplicity, only a power amplifier LEV is shown, which can be controlled by different audio sources AQ1, AQ2, AQ3 to voice, music and warning signals to those of the Speakers supplied places to bring. In addition, for the purposes of the measurement operations described below, a signal generator SIG controlled by or part of a digital signal processor DSP may provide signals to the input of the power amplifier LEV, such as the output of the power amplifier LEV. B. sinusoidal signals at 15 Hz or 23.4375 Hz.

For measuring the current I and the voltage U at the output of the amplifier LEV or at the input of the speaker line current and voltage converters are provided, the measured values, optionally after external A / D conversion, the mentioned signal processor DSP are supplied.

It may be noted at this point that even more speaker lines LL2 and LL3 can be connected to power amplifiers and checked by the measuring method according to the invention, which is technically implemented by appropriate switches and a cyclical sequence of measurements on the individual lines LL1, LL2, LL3 , It should also be pointed out that each loudspeaker line can also be closed in the manner of a loop, and in this case the infeed thus does not take place at an "end" of the loudspeaker line. Such a ring structure has the advantage that an interruption of a conductor or both conductors at one point does not lead to failure of the speakers.

While the signal generator SIG is used for large signal measurement with the aid of the power amplifier, another signal generator GEN is provided for a small signal measurement. This signal generator can also be controlled by the signal processor DSP or be part of the same and supplies signals to the input of the loudspeaker line LL1, whose level is selected so that no or barely audible sound pressure levels are generated. In the example described, three frequencies are used for the small signal measurement, namely 70 Hz, 250 Hz and 1 kHz.

The following are with reference to Fig. 2 the method according to the invention as well as further details of the associated device described in one embodiment, which are in many ways possible for the skilled person within the scope of the invention, for example, as regards the number of frequencies and levels used and the order of different measurement steps.

When a sound system is completed, d. H. All lines are laid and the required speakers are connected, the system was thus configured and intact lines, speakers, transformers, etc. can be assumed, are first determined in a learning phase, the nominal (reference) values of the impedances at different frequencies, in the present case, at least at the frequencies 23, 4375 Hz, 70 Hz, 250 Hz and 1 kHz. Furthermore, the impedances are expediently also determined at different levels, since, for example, high levels result in different impedance values due to heating, in particular of voice coils, and due to nonlinearities in transformers and / or loudspeakers.

In order to determine the reference values Z _Ref , the same method can be used, which is used to determine the actual impedances. In the learning phase, the measured values of current and voltage are converted into digital signals at the said different frequencies and levels and subjected to rapid Fourier transformation (FFT) in the digital signal processor DSP. For example, an FFT length (or sample number) of 2048 is used. At each frequency or level, 50 readings are taken and then an average is formed from these values which, at least in the measurements in the learning phase, can be an arithmetic mean, but weighting at different frequencies or levels is also possible. The determined reference values Z _R23 , Z _R70 , Z _R250 , Z _R1000 , generally Z _Ref , are then stored in order to be available for the following measurements.

As will be explained below, it is also useful to demagnetise the device by a swelling and then decreasing sinusoidal signal of z. B. 15 Hz made to delete remanence in the individual transformers.

In the operating phase of the public address system, a distinction can be made during or before the measurement as to whether a useful signal, i. H. a public address signal, such as. As music or speech, is present to perform either a large signal measurement or a small signal measurement.

In the case of a useful signal, a small-signal measurement is not expedient or would be very complicated because of the difficult-to-control level differences between a conventional measuring signal of low level (for example -6 dBu) and the useful signal level (for example + 42 dBu). Therefore, in this case, a measurement is made with a higher signal level, but with the already mentioned low frequency fu of 23.4375 Hz, which is practically inaudible in normal operation. For this purpose, the output signal of the signal generator SIG is supplied to the input of the power amplifier, which then feeds the loudspeaker line to be measured. The signal level is chosen to be lower than the useful signal level so as not to overdrive the amplifier when the useful signal is already high, e.g. B. 13 dB below the Nutzsignalpegel.

In order to determine an impedance Z to be compared with a reference value Z _Ref , several, e.g. For example, 50 analog measurements of voltage and current are converted to digital signals, subjected to Fourier transform, and correspondingly many impedance values are determined from the transformed values of current and voltage. An average value is formed from these impedance values and this is compared with a reference impedance value. If the resulting mean value deviates from the reference impedance value by a definable tolerance value, an error signal is emitted.

The large signal measurement offers the advantage that background noise does not affect the measurement results, since interference signals striking the speaker diaphragms only produce signals whose level is far below the operating level. Also, the influence of the line length on the measurement result at the applied low measurement frequency is low.

The magnetic hysteresis of the iron core of transformers, which the 100 volt line to the usually low-resistance, z. B. adapting 8 ohm speakers, at low frequencies can lead to significant changes in the measured impedance, so it is expedient to remove in the cores of the transformer existing remanences by a demagnetizing step. For this purpose, a demagnetization signal s _E low frequency f _E , z. B. 15 Hz, a speaker line via a power amplifier with a rising to a maximum level and then dropped again level. The demagnetization signal - like all other signals used here expediently a sinusoidal signal - rises from a zero level to a level which corresponds at least to the highest occurring operating level and then falls back to a zero level, to eliminate with certainty all remanence. In practice, a time of 1/3 second is sufficient for this process.

Another useful method step is that before a large signal measurement, a practical level for the measurement signal is determined in the following manner: One subjects the useful signal, which as already mentioned voice and / or music signals, but also other signals, such as alarm signals, may contain before the actual impedance measurement of a spectral analysis, again z. B. by a fast Fourier analysis to form from the determined level values at a plurality of frequencies, for example at 2048 frequency points, which should contain at least the later used low measuring frequency fu, a weighted average level. The level of the subsequently applied measurement signal is then automatically kept at a predeterminable distance from the determined level, proven at least 13 dB below it.

A small signal measurement can be carried out if there is no useful signal, or if the speaker line to be measured is not connected to any amplifier. For the purpose of small signal measurement, the audio generator GEN is used, which - controlled by the processor DSP - generates signals of a specifiable frequency, for example 70 Hz, 250 Hz and 1000 Hz. These signals are applied in succession to the loudspeaker line to be measured, as well as in the large-signal measurement current and voltage continuously measured, the measured values are sampled and Fourier-transformed. For this purpose, the digital signal processor DSP is also used.

The signal levels are chosen so that sound levels are generated at the respective frequencies, which are not audible to people in the sonicated areas or at least not perceived as disturbing. The level selection is dependent on the frequency corresponding to the auditory curve, for a 100 volt system in practice voltage level in of the order of 300 to 400 mV at 70 Hz are typical. Demagnetization of the transmitter cores is generally not required for higher frequency signals.

$$\mathit{Varianz}:{\mathrm{var}}_x\left[n\right]=k{\left(x\left[n\right]-\bar{x}\left[n\right]\right)}^2+\left(1-k\right){\mathrm{var}}_x\left[n-1\right];$$

$${\mathrm{var}}_x\left[1\right]=x\left[1\right]/4\leftarrow \underset{\_}{\mathit{Schätzwert}}$$

For each frequency, a large number of measured values are determined in quick succession, for example 50 measured values per frequency again, and then averaged. In one embodiment, this measurement is done every 100 seconds. It is particularly expedient if a statistic is determined via the average values obtained as above and a moving average is formed therefrom. The number of values averaged over (by moving average) is determined by the variance and the desired tolerance of the line. The variance is the mean squared deviation of the measured values from their mean value. The higher the variance, ie the higher the measured values fluctuate around their mean value, and the lower the desired tolerance, the more values must be averaged. In a current measurement method, exponential moving averages are used, ie the current mean value with a certain factor <1, z. For example: 0.1 multiplied and added to the current average multiplied by 1 minus this factor: $$\mathit{Average}:\tilde{x}\left[n\right]=k\cdot x\left[n\right]+\left(1-k\right)\tilde{x}\left[n-1\right];\mathrm{}\tilde{x}\left[1\right]=x\left[1\right]$$

$$\mathit{variance}:{\mathrm{var}}_x\left[n\right]=k{\left(x\left[n\right]-\tilde{x}\left[n\right]\right)}^2+\left(1-k\right){\mathrm{var}}_x\left[n-1\right];$$

$${\mathrm{var}}_x\left[1\right]=x\left[1\right]/4\leftarrow \underset{\_}{\mathit{estimated\; value}}$$

This moving average can then, for. B. every 100 s, with the reference (reference) value to be compared.

It is clear to the person skilled in the art that the choice of the frequencies used, as well as their number, can be adapted to the respective conditions within the scope of the invention. Likewise, it will be advantageous if periodically, z. B. with the mentioned 100 second period, both large signal and small signal measurements would be performed.

Claims (10)

1. Method for monitoring loudspeaker lines by measuring impedance, in which at least one a.c. signal of a predetermined frequency is fed to the input of a loudspeaker line, the voltage and current at the input of the line are measured and the input impedance on the line is determined from these measured values and compared with a reference value, characterized in that, for determining an impedance (Z) that is to be compared with a reference value (ZRef) in a measuring operation, a plurality of analog voltage and current measured values are converted into digital signals and are subjected to a Fourier transformation, a corresponding plurality of impedance values are determined from the Fourier-transformed current and voltage values, these impedance values are averaged and the resultant average value is compared with a reference impedance value, an error signal being output if the resultant average value deviates from the reference impedance value by a definable tolerance value.
2. Method according to Claim 1, characterized in that, in a large-signal measurement, a single a.c. signal (su) of a predetermined frequency is fed to a loudspeaker line via a power booster at a level that lies below the normal operating level, and the a.c. signal has such a low frequency (fu) that a sound level that is imperceptible or only scarcely perceptible to the human ear is produced.
3. Method according to Claim 1 or 2, characterized in that, if there is a useful signal, the useful signal is subjected to a spectral analysis before each measurement, in order to determine the useful signal level at a plurality of frequencies (m), this plurality of frequencies also including the single low frequency (fu), a weighted average value level is formed from the plurality of useful signal levels obtained and, for each measurement, the level of the single measuring signal of low frequency that is fed to the loudspeaker line is set at a predeterminable fixed interval from the weighted average value level.
4. Method according to one of Claims 1 to 3, characterized in that, before a measurement, a demagnetizing signal (SE) of a low frequency (fE) is fed to a loudspeaker line via a power booster at a level that increases up to a maximum level and then falls back again.
5. Method according to one of Claims 1 to 3, characterized in that, in a small-signal measurement, at least one a.c. signal (si) of a predetermined frequency/frequencies (fi) is/are fed to a loudspeaker line at a level/levels chosen such that the sound levels produced are imperceptible or only scarcely perceptible to the human ear.
6. Method according to one of Claims 1 to 5, characterized in that, before the actual operation of the system, in a learning phase the loudspeaker line is fed signals of different frequencies and/or levels, the measured current and voltage values thereby obtained are converted into digital signals and are then subjected to a Fourier transformation, and then reference values (ZRef) are determined from the current and voltage values for various frequencies and/or levels and are stored.
7. Method according to Claim 6, characterized in that, for each frequency/level, a plurality of individual current and voltage measured values are determined and average values of the individual measured values are used as a basis for the following determination of the reference impedances.
8. Method according to one of Claims 1 to 5, characterized in that the tolerance value is changed during operation to the effect that slow changes of actual impedances are assigned a wider tolerance range of the reference impedances than rapid changes.
9. Method according to one of Claims 1 to 8, characterized in that the absolute value of the impedance is determined and compared with a reference value.
10. Device for monitoring loudspeaker lines by measuring impedance, with at least one signal generator and at least one booster and also controlled switches for feeding at least one a.c. signal of a predetermined frequency to the input of at least one loudspeaker line, with measuring devices for measuring voltage and current at the input of the line and also with at least one microprocessor, which is designed for carrying out the method according to one of Claims 1 to 9 and, by using the features of this method, controls the at least one signal generator, to which the measured values of the measuring devices can be fed and which is designed for determining the input impedance on the line and for comparing it with a reference value and also for emitting an error signal if the impedance value determined deviates from a reference value within definable tolerances.

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