Product How-To: Optimizing passive radiator acoustics using the QF3DFX processor

Quickfilter Technologies' QF3DFX ProfoundSound® audio processor and QFPro® software, provide the ability to accomplish all of the above. The QF3DFX-DK development kit is capable of interfacing with analog or digital systems. It can be used standalone independent of a host PC with the ability to boot from 1 of 4 configurations. It can also be utilized as an interface to an embedded QF3DFX target for configuration via QFPro.

Most importantly, QFPro is a real-time development environment thereby optimizing a tuner's ability to audibly hear changes as they are made including the ability to turn on and off a feature or bypassing the QF3DFX for further instantaneous confirmation (either audible or measured). QFPro also includes a monitor feature which allows the tuner to view signal levels and gain parameters at strategic points within the QF3DFX device.

Using Quickfilter's Virtual Bass function, the designer can easily enhance the energy delivered by the active speakers at low frequencies without adding distortion. Because this is done digitally, it is much more accurate than physically creating virtual bass. Harmonic generation is easily controlled and distortion minimized. The Virtual Bass function allows the designer to set the frequency at which Virtual Bass will be reproduced and also enables the user to add gain, effectively counteracting the 18dB/octave roll-off of the passive radiator. See Figure 1.

The Parametric Equalizer real-time attributes significantly aid the tuner is getting as much energy into the active speakers without audible distortion while also avoiding the resonate frequency of the passive radiator. By simply moving the center frequency or bandwidth dots, the frequency, filter Q and boost of a filter is modified.

Upon release, the Parametric Equalizer is updated in real-time and the resulting change is audible. This technique allows optimized placement of various filter types, especially in the low-frequency region, for maximum passive radiator performance. See Figure 2.

The previous features are used to tune for optimum bass with maximum volume (air pressure). A technique is needed to maintain air pressure as the active speaker volume is decreased.

Complementing the Virtual Bass Function and Parametric Equalizer are the Dynamic Bass and Treble functions. These functions boost both the Bass and Treble frequencies when volume is lowered. This makes it easier to maintain the internal air pressure (translated to bass performance) as the volume is decreased. Frequency and gain can be optimized to the characteristics of the active speakers. See Figure 3.

Due to the small physical size of today's audio components, the active speakers are physically mounted very close together. This diminishes the stereo effect, and the separate content in each channel becomes washed out.

Quickfilter's WideAudio® 3D virtual surround technology enhances the stereo effect, and makes the audio appear to fill the room as opposed to originating at a point source. The technology is unique in that audio that is meant to be centered, i.e. a vocalist center stage, remains centered while audio in the side channels is moved out. In addition, depth is perceived such that overall sound staging is enhanced. See Figure 4.

Much of today's audio is compressed in order to optimize the limited memory space available in today's audio devices. This compression sacrifices much of the higher frequencies contained in the original source. For example, MP3 compression typically eliminates all frequencies above about 13 KHz.

Quickfilter's High Frequency Restoration function restores the high frequencies lost during this compression. This aids in providing more energy to the active speakers which aids in optimizing the tuning of the passive radiator. Control includes frequency gain and slope matching parameters. See Figure 5.

Many listening environments are non-optimal and many times the background noise is above the dynamic range of the source. This makes it more difficult for the end user to hear the source content. Dynamic range compression is used to solve this problem, and this can be accomplished in the ProfoundSound® audio processor via Automatic Gain Control and Soft Clipper blocks. Dynamic range compression is accomplished by adjusting the thresholds above or below which gain is increased or decreased. See Figures 6 & 7.

Beside the need to optimize the performance of the passive radiator, the design should also consider additional features that enhance the end-user experience. One such feature is the inclusion of a Graphic Equalizer or GEQ. The use of a GEQ allows the designer to create profiles which the end-user can select and thus have some control over his audio listening preferences.

Profiles such as "More Bass", "More Treble", "News", "Jazz", "Rock", etc., allow the end-user to adjust the audio output to his listening preference. The ProfoundSound® audio processor contains a 5 band GEQ with which 8 profiles may be constructed, allowing the end-user great flexibility in adapting the equipment to his preferences. See Figure 8.

Sometimes, a GEQ does not provide enough control for the end-user, especially if he uses the equipment for multiple purposes (listening to music, playing video games, as a karaoke machine, etc.). For this situation, sometimes multiple optimizations of the audio equipment are necessary. The baseline configuration of the ProfoundSound® audio processor consumes only 4.9KB of storage memory.

Additional configurations can be based on differences from the baseline configuration such that each additional configuration will consume much less memory than the original baseline. These multiple configurations can be loaded under microprocessor control in a matter of milliseconds such that switching between configurations can be done with button presses and without audio popping or noise. This allows the end-user to quickly move between various configurations that are optimized for his use.

Summary
The inclusion of passive radiators in small speaker systems has become a popular way of enhancing the bass response of these systems. However, the tuning requirements of these passive radiators is very demanding and not generally well supported by typical audio system-on-a-chip integrated circuits that are presently on the market.

Quickfilter Technologies' QF3DFX ProfoundSound® audio processor addresses these challenges and provides an easy, accurate, and cost effective means for optimizing the response of passive radiators. The additional features included in this device benefit the overall system design and enhance the end-user experience. See Figures 9, 10 and 11.

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