The following pictures demonstrate the capability of VIR filter in wide frequency range applications (10 Hz – 48 KHz @ 96 KHz Sampling). It is obvious that VIR Filter works very well in a Hi-Res audio environment.

In the beginning of the digital audio era, simpler digital filters such as Biquad filters were used to form graphic or parametric equalizer, even though it had many serious shortcomings in performance.

Later, with the improvement of processors such as DSP and CPU, IIR (Infinite Impulse Response) and FIR (Finite Impulse Response) filters became popular. These had better performance in audio applications. However, IIR has difficulties to realize complex filter curves and its computation footprints get large. FIR filter does better and its applications are increasing.

However, FIR filters had poor resolution at low frequencies due to their linear frequency scale nature. This is a critical problem for consumer audio products which can only afford short tap filters.

To cope with the problem, WFIR (Warped FIR) filters were introduced. These can be configured to provide better low frequency resolution by warping the frequency domain calculations. However, a WFIR filter consumes four times the per-tap computation power of an FIR filter. This reduces the available number of taps to one-fourth, reducing high frequency resolution.

The combination of WFIR (for low frequencies) and FIR (for higher frequencies) is a coarse attempt to more evenly cover the entire frequency range, but the system — the software to generate the filter coefficients and the hardware to implement it — becomes complex, and is still only a crude approximation of the optimal evenly spaced frequency response.

The newest solution is Eilex’ proprietary VIR (Variable-resolution Impulse Response) filter. It solves all of these problems. It requires much less computation while significantly increasing the filter resolution. It evenly covers the entire frequency range, with the ability to arbitrarily and/or automatically allocate the correction points.

VIR filter effortlessly equalizes the subwoofer-range deep bass. That is not the case for FIR filter due to its insufficient low frequency resolution. Using FIR filter in the subwoofer range is only possible by reducing the sampling rate to increase the filter resolution once, then up-sample to match the DAC frequency. VIR filter can run at the system sampling frequency (for example 192KHz). All available sections are allocated to the low-end (below 100Hz) to maximize the resolution and no sections are wasted in the higher frequencies (above 100Hz). VIR filter can form a steep cut-off filter even in the 10–20Hz range to eliminate unwanted rumbles.

The resolution of VIR filter can be chosen from the range of 10–200 sections depending on the application. For example, high-end audio uses 100, 150 or 200 sections, a TV uses 20–30 sections and personal portable audio uses 40 sections.

VIR filter is a minimum phase filter by nature and its sections have controlled phase characteristics. VIR filter exhibits minimum ringing and excellent transient response in the time domain. It is the ideal filter for audio applications.