The actual sound reproduced by the subwoofer, including the low-frequency fundamentals, is subtracted from the original source signal, and the resulting difference signal is sent to the main speakers.
No HPF (high-pass filter) is inserted in the main-speaker signal path. Instead, the difference-signal method described above reduces the large excursion burden associated with low-frequency reproduction. Depending on the system configuration, it can also avoid changes in amplifier loading associated with an HPF.
As a result, the main speakers are relieved of large low-frequency excursions, making it easier for them to reproduce low-level signals, including harmonics, as well as more subtle sonic information.
Furthermore, if the subwoofer exhibits a response delay, the component corresponding to that delay is also included in the difference signal and sent to the main speakers. This is a form of signal processing that is difficult to achieve with a conventional HPF-based approach.
MFFB® Motional Feed-Forward Bass
A subwoofer that puts the main speakers center stage.
MFFB does more than simply add bass. It creates a more favorable physical and electrical reproduction environment in which the main speakers can express the music more freely.
The trapezoid on the left represents the reproduction range of the subwoofer, while the trapezoid on the right represents the reproduction range of the main speaker.
Some low-level bass signals that the subwoofer cannot reproduce sufficiently can still be reproduced by the main speaker. In conventional HPF-based systems, this region is also attenuated.
With MFFB®, components that are not reproduced by the subwoofer are sent to the main speaker as a difference signal, allowing the main speaker to supplement them within its reproducible range.
Example of Frequency and Phase Responses Subwoofer + Main Speaker
Frequency response: subwoofer (purple) / main speaker (blue) Phase response: subwoofer (khaki) / main speaker (green)
In conventional systems, the LPF and HPF cause significant phase changes during the transition between the reproduction bands.
The MFFB® system does not use an HPF on the main speaker. The main speaker reproduces the difference signal and helps compensate for phase shifts within its capabilities.
Systems that supplement the bass response of main speakers by adding a single subwoofer are widely used. Configurations once referred to as “3D systems” were based on the same concept, while today the more familiar term is “2.1-channel.” However, when a subwoofer is added to an existing speaker system, the combination and adjustment can sometimes result in sluggish bass or poor integration with the main speakers.
In the MFFB® system, the actual sound reproduced by the subwoofer is captured by a microphone and subtracted from the original signal. More specifically, the microphone signal is phase-inverted, level-matched, and added to the original signal, producing a difference signal that represents components not fully reproduced by the subwoofer, among others. This difference signal is then sent to the main speakers.
The difference signal contains not only the mid- and high-frequency content, but also very low-level bass that the subwoofer could not fully reproduce, as well as differences caused by response and phase delays. Within their own reproduction capability, the main speakers reproduce the mid- and high-frequency content in real time while also helping to compensate for missing or delayed low-frequency components.
A low-pass filter (LPF) is used for the subwoofer, while no high-pass filter (HPF) is used to cut the low frequencies sent to the main speakers. By subtracting what the subwoofer has actually reproduced, MFFB® avoids excessive overlap in the bass region while allowing the subwoofer and main speakers to complement each other.
MFFB® also aims to reduce the large cone excursions associated with low-frequency reproduction by the main speakers. This helps lessen the influence that increased mechanical stiffness in the diaphragm and suspension system—such as localized increases in spring stiffness—may have on low-level signal reproduction. Electrically, because no HPF is added to remove low frequencies from the main-speaker signal path, the phase shift associated with such a filter can also be avoided. Compared with configurations that insert a filter at the amplifier output, the associated changes in amplifier loading can likewise be avoided.
By supplementing the low frequencies while reducing these mechanical and electrical effects, MFFB® is designed to reproduce mid- and high-frequency harmonics and low-level details more faithfully, allowing the main speakers to reveal more of their inherent expressive capability.
That is the concept behind MFFB®.
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〈参考資料〉
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デジタル処理により倍音を付加する機器
Roland (ローランド) / MA-15D モニタースピーカー
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