Anti-Mode Technology

Room acoustics

Hi-fi enthusiasts often seek flawless frequency responses from their amplifiers and speakers. When a high-end equipment is installed in to the listening environment, such as typical living room, these curves are degraded. The distortion is caused by walls, floor and roofs of the listening room. The early reflections will inflict both constructive and destructive interferences, thus introducing up to +/- 6dB peaking or attenuation per single reflection. The worst of all are the modal resonances also known as the "standing waves", which will impair the bass response with even as high as 30 dB peaking and narrow resonances which decay slowly. Every room has commonly at least a couple of high-Q resonances.

Modal technology concentrates in modelling the mechanism behind these standing wave resonances with room geometry. Simple models can be derived from room dimensions, the  larger the room, the lower the frequencies. In reality there exists multiple resonance frequencies that can be modelled as standing waves, the room modes. As these resonances have very low frequencies, they are difficult to be tamed with absorbing materials which in general offer poor efficiency for the lowest ranges.

Compensating for room modes

Anti-ModeTM technology is designated to eliminate the devastating room resonances, but it in fact also compensates for the response of the whole system.

The basis of the technology relies in measuring the transfer function from the sound source to the listener with a microphone. As the counter-model of the room is achieved by a sophisticated algorithm, both amplitude and phase characteristics of the system will be improved. The algorithm is capable of making crucial decisions of which phenomena can and should be compensated for. For example the steep dips in the magnitude response are always extremely local by nature and hence should never be corrected by narrow peaking filters or such. As a result of the robust counter model designing algorithm of Anti-ModeTM devices, the result is achieved with a fully automatic process without a user involvement.

The calibration program self-generates sweep-like measurement signals and picks up the data with the included microphone. The process will autonomously determine the parameters of the room modes, such as center frequency, Q-value / width, gain, frequency antisymmetry. The filter design functions will further convert this data to complex zero-pole presentations into the Z-domain in order to derive optimal counter-model for the acoustic system. This will eventually lead to the design of custom-fitted Anti-Modal Filters (AMFs). Up to 28-36 distinct AMFs can be designed, although far less are usually ever needed for one position. The extra AMFs give the algorithm more freedom when extending the effective area of the compensation for more globally improved results.

Furthermore, the filter design functions compute digital IIR coefficients for the AMFs to be used in real-time operation after the calibration. The computations use accurate 32/40 -bit integer arithmetic, which will virtually set unlimited space of parameters for the system.

In the operational mode the pass-trough delay of the signal is approximately 3ms (for the Anti-Mode 8033-series). This is mainly due by analog-to-digital and digital-to-analog converters and the delay is small enough to be inaudible. However, it can be compensated by basically every A/V receiver on the market by adding distance of the subwoofer by 1 meter (3 feet), or using the automatic speaker setup of the receiver.

Other existing approaches

Some products in the market utilize fixed-band graphical equalizers when trying to struggle against room distortion. This kind of solution is very inaccurate and is potentially incapable against narrow resonances that can occur at arbitrary frequencies.

They usually also involve the user for measurements and fine tuning. Some equipment also only tries to compensate for the amplitude response ignoring phase completely. In such cases the compensation will not lead to improved transition response.

Another problem in the existing parametric equalizers is the limitations in their spectral resolution. Missing a center frequency by even as small a deviation as 1 Hz, the modal characteristic can be made even worse than the original. In Anti-ModeTM, the spectral resolution is very accurate and the computation power of 24 AMFs is enough to make even the worst room compatible with subwoofers.