Membrane advance correction
The membrane advance correction (also TPS equalization circuit , transducer preset system ) according to Pfleiderer is a patented circuit for correcting certain playback errors in loudspeakers . The name membrane advance correction results from the fact that an attempt is made to compensate for the error before it occurs - i.e. before the sound conversion .
Basic idea
The basic idea of the diaphragm advance correction is to correct some reproduction errors of the overall system by generating a correction signal with the opposite sign from the input signal and measured parameters of the system and at a suitable point, e.g. B. preamplifier, is added to the actual audio signal. The loudspeaker is fed with a pre-distorted signal. Ultimately, what matters is not whether the drive center of the loudspeaker membrane moves proportionally to the audio signal, but how well the sound pressure and sound velocity correspond to the audio signal.
From a technical point of view, this is a control and not a regulation, since there is no feedback from the system output (sound) to the system input (electrical signal).
The necessary control parameters are determined during calibration.
Mathematical basics
In the membrane advance correction, the dynamic chassis is viewed as an exactly linear system . This is determined by its pole-zero configuration. It is known from control engineering that a pre-filter can compensate for these poles and zero positions by setting a zero position on each original pole or a pole position on the original zero. This compensates for the linear characteristics of the chassis. It is now necessary to add the desired pole zeros, each chassis must have a bandpass character so that the overall gain remains finite. Through this electronic pre-filtering, the chassis can be given almost any properties within the framework of linearity, as is known from the theory of filter design. There are e.g. B. Assertions that square-wave signals should be reproduced as unadulterated as possible by the chassis, this can be easily achieved by constant group delay when designing the filter. It is also important that amplitude errors can be corrected using a prefilter, e.g. B. a too early drop towards the low frequencies.
Limitations
The membrane advance correction cannot compensate for errors of any size, i.e. it cannot turn a poor, narrow-band loudspeaker into a hi-fi system. A good basic system is therefore required for a membrane advance correction.
The method has limitations. On the one hand, depending on the chassis and the desired transfer function, a high order of the pre-filter may be necessary. It can also happen that the overall gain shows very high values in places, which leads to very high power consumption in the chassis (the power increases with the square of the gain). Large deflections are also associated with this. This leads to the principle problem that non-linear artifacts cannot be addressed at all. Unsuitable combinations of chassis and pre-filter can produce extreme non-linear distortion. The pre-filter doesn't make a bad chassis a better one - but it sounds better because the pre-filtering is adapted to the chassis after the calibration. Nothing changes in the fact that high sound pressure levels or even medium levels outdoors or in large rooms cannot be reproduced with low distortion with single chassis. Rather, several chassis will be required for different frequency ranges (usually 3 to 4). Each individual could be equalized using a pre-filter, but their combination in the radiation field cannot, since different coefficients would be necessary depending on the point of view. Finally, it should be noted that, as shown above, the listening room with its reflections has an extreme influence on the amplitude response and group delay. In the diffuse field it is therefore difficult to measure the presence or absence of the corrective measure, while in the free field it is easy to measure.
With a correspondingly powerful bass chassis and corresponding amplifier power, pre-filtering is an effective method of realizing a deep bass with a minimal housing size.
Web links
credentials
- ↑ Peter M. Pfleiderer: HiFi in a nutshell: playback technology for unadulterated hearing . Books on Demand , 2014, ISBN 978-3-7357-9736-0 . ( limited preview in Google Book search)
- ↑ Patent EPEP0145997B1 : Device for compensating playback errors of an electroacoustic transducer . Published on November 22nd, 1984, inventor: Peter Michael Pfleiderer.
- ^ Siegfried Linkwitz: Active Filters. Retrieved October 20, 2018 .