Our ‘Audio FIR Filtering’ guide features as the ‘Tech Topic’ in the November issue of ‘Live Sound International’ magazine. A huge thanks to the folks at LSI for their support.

The guide has been split in two with the second half to appear in the next issue. It discusses digital signal processing basics and fundamental FIR filter concepts & applications in loudspeakers.

Download a digital copy of the November issue – we’re on page 52 to 55 – or  read the guide on our website (a PDF download is also available).

Sign up for the ‘Live Sound International’ print and digital editions at https://www.livesoundint.com/

FIR Designer’s new IIR filter reference mode and editor enable fast conversion of IIR settings from one brand to another and ensure IIR response matching when working with multiple brands of amplifiers & processors.

In the past it’s been difficult to use common, matched IIR DSP settings across multiple brands, as some standard IIR filters produce different results on various amplifier/processor brands (learn more). Brands also have different filter parameter limits.

Linea Research, Powersoft & “General” IIR Filter modes were introduced to our software earlier this year, providing accurate, built-in emulation of each brand’s unique IIR filter behavior. This latest release adds Q-SYS and Symetrix IIR filter modes, as well as introduces a completely new IIR reference mode & editor functionality.

It’s now possible to set any one of these brands as an IIR ‘reference’, and adjust any other brand’s IIR filters to match the reference response on a plot that updates in real time. Also, a measurement (in a supported file format) of an IIR response from a processor can be loaded into the Target tab, and any of the IIR filters modes (including the reference) can be set to match the target. These new features enable the creation of presets or tunings with identical IIR performance when used across different brands.

IIR filter parameters and raw coefficients can be exported to text files, & the parameters copied to the desired control software. This process is automated (via Direct-To-Processor functionality) for Powersoft & Lake Processing. (Note: the Lake FIR 3-way module can accept raw IIR filter coefficients generated by any of the IIR modes).

When using FIR Designer to convert IIR filters for brands not listed, we recommend trying all five modes within the FIR Designer IIR tab, & comparing the plots displayed with the IIR plots generated by your own control software to determine which mode produces the closest match. The General mode is the default option and offers IIR filter types common to all three modes as well as Chebyshev & Elliptic high-pass and low-pass filters and Neville Thiele notched crossovers.

In the future, we hope to add IIR modes for additional brands, streamlining the conversion process even further. (One-click conversion is the goal!)

The new IIR reference editor & mode is available in FIR Designer M 1.3 & FIR Designer 2.11, and is only accessible to software licensees. (It’s not available in demo versions of the software.) Check out the video to see how it works.

FIR Designer M 1.3 Release Notes

FIR Designer 2.11 Release Notes

Rewindowing provides user-adjustable shortening of a FIR filter – from sources other than FIR Designer – so that it can be loaded into FIR-capable amplifiers & processors with specific tap length limits (and with a minimal change in filter response). Rewindowing is available within FIR Designer’s DTP (direct-to-processor) functions, which currently support Lake and Powersoft.

For example, a FIR filter with a tap length of 512 can be rewindowed to 384 so it can be loaded into a processor with a 384 tap-length limit.

Side-loaded FIR filters can be in almost any file format and are sample-rate converted to the output channel sample rate.

The rewindowed FIR filter can be sent – along with IIR, Gain, Polarity & Delay settings – via DTP to the target (Lake or Powersoft) device.

Rewindowing is only accessible to software licensees (since DTP isn’t visible in demo versions); this video shows how it works (using Lake as an example). Its based on FIR Designer but FIR Designer M & FIR Creator EX include identical functionality.

Existing FIR Designer 1 license-holders are eligible for a USD $100 coupon towards the first-year purchase of FIR Designer 2+ just contact us.

Our most full-featured & complex product – FIR Designer M – is now also the easiest in our FIR Filter product range to learn to use, thanks to its integrated Help. Since the launch of FIR Designer in 2016, our focus has been on using all available development time on new features & functionality. Providing how-to-use documentation wasn’t a priority since most users were able to quickly master the software’s intuitive workflow, and could take all the time needed to experiment with a demo version prior to purchase. However, as we’ve released new, increasingly-complex features, there has been a marked rise in queries about usability, as well as multiple repeat visits by users to the instructional videos.

So, with the launch of FIR Designer M last November, we decided to divert some of our development time to creating an integrated web-based Help. Website statistics indicate high usage and it’s clear that the Help function is shortening the learning curve, as well as highlighting lesser known features and filter types. We’ve learned some lessons about the value of in-app documentation, and will look at expanding Help functionality in other products in the future.

Get a free demo of any Eclipse Audio software on our website.

Some IIR filters may produce different results when used in various amplifier/processor brands. Common causes are differences in the way each brand’s software interprets some filter parameters, like Q and bandwidth (BW), and in the conversion between Q & BW. The differences typically appear in parametric (peaking) filters and shelf filters.

The plots below show various implementations of a 10 kHz 10 dB parametric filter. The upper row is for a sampling rate of 48 kHz and the lower for 96 kHz. There is no simple translation between the BW and Q in each implementation, so to help with the comparison, all have been manually & approximately matched by adjusting the BW or Q to give ~ +5dB at 5 kHz. Looking at the both the magnitude and phase, it’s clear that the implementations are different, particularly near the Nyquist frequency of 24kHz for the 48 kHz sampling rate. The 96 kHz implementations are more similar because Nyquist has been pushed out to 48 kHz. However even when overlaid there are subtle differences.

In the plots, the implementations in the right two columns (labelled Linea Research & Powersoft) each have unique equations to create 2nd order analog filters, which are then mapped to 2nd order digital designs using the Bilinear Transform with frequency warping. The plot on the left uses a method proposed by Sophocles Orfanidis which attempts to keep the digital filter shape close to the analog design near the Nyquist frequency. (This method is very similar to the implementation used by Lake Processing). For more details see the 1996 AES Paper “Digital Parametric Equalizer Design with Prescribed Nyquist-Frequency Gain.”

FIR Designer (& FIR Designer M) can be used to emulate IIR filters as part of a complete loudspeaker preset. Recent updates added three ‘IIR Filter Mode’ options (General, Linea Research & Powersoft) that address these alternative IIR implementations. The Linea Research & Powersoft modes accurately emulate IIR filters for these brands (for loading into System Engineer, and Armonia, respectively). The General mode is the default option and offers IIR filters common to all three modes as well as Chebyshev & Elliptic high-pass and low-pass filters and Neville Thiele notched crossovers.

When using FIR Designer to emulate IIR filters for other brands, we recommend trying all three modes within the FIR Designer IIR tab, & comparing the plots displayed with the IIR plots generated by your own control software to determine which mode produces the closest match. Note: the Lake FIR 3-way module can accept raw IIR filter coefficients generated by any of the three modes.

For all three modes, IIR filter parameters and raw coefficients can be exported to text files, & the parameters copied to the desired control software. This process is automated (via Direct-To-Processor functionality) for Powersoft & Lake Processing (and we’re hoping to have other brands implemented in the future).

Useful links

• Orfanidis DSP text book
• Audio EQ Cookbook – Formulae and Equations
• Robert Bristow-Johnson The Equivalent of Various Methods of Computing Biquad Coefficients for Audio Parametric Equalizers

Now available in FIR Designer 2.9 & FIR Designer M 1.1: Eclipse Audio has recently collaborated with Linea Research to offer a “Linea Research” IIR filter mode which provides accurate emulation of Linear Research filters, including the unique Hardman crossovers filters, when creating mixed IIR+FIR loudspeaker presets. FIR filters can be exported as SFI files and loaded into System Engineer, for use in Linear Research processors and amplifiers. https://www.linea-research.co.uk/

Also available is a “Powersoft” IIR filter mode whose Q parameter (for all-pass and peaking filters) now exactly matches Powersoft Armonia. Using the Powersoft Direct-To-Processor functionality, mixed IIR+FIR processing chains can be exported to XML and loaded into Armonia for use in Powersoft amplifiers. https://www.powersoft-audio.com/en/

Both the Linea Research and Powersoft IIR filter modes enforce parameter limits – gain, frequency, bandwidth & Q – specific to each brand.

In FIR Designer and FIR Designer M, all IIR filters from previous versions are grouped under a “General” IIR filter mode and include bandpass, low and high shelf, low-pass and high-pass Bessel, Butterworth, Chebyshev I & II, Elliptic, Linkwitz/Riley & Neville Thiele notched crossovers.

For all filter modes, IIR filter parameters and raw coefficients can be exported to text files.

When switching between IIR filter modes, FIR Designer and FIR Designer M make a best-effort automatic conversion of all filters, although some manual adjustment of bandwidth or Q may be necessary. Where filter types are unavailable, a similar filter type is chosen.

FIR Designer & FIR Designer M can export FIR filters in a variety of formats for loading into most FIR-capable products, including Lab Gruppen (LAKE), QSC, MiniDSP, Electro-Voice, Symetrix, Dynacord, BSS, DFM, Biamp & Apex.

Download a free demo (Windows or Mac) of either FIR Designer or FIR Designer M.

Please note, the FIR Designer 2.9 feature updates apply to all existing subscription licenses and any new perpetual licenses purchased. Existing perpetual license-holders – 2.8 and below – will receive a bug-fix update only. For more details see the FIR Designer Release Notes.

FIR Designer Release Notes    FIR Designer M Release notes

Build a complete loudspeaker preset for multiple channels simultaneously with FIR Designer M. It utilizes the same easy-to-follow workflow of FIR Designer but provides up to 6-way, simultaneous multi-channel filter design. Create finite impulse response (FIR) filters and complete mixed FIR + IIR filter loudspeaker presets for single and multi-way loudspeakers, using multi-way loudspeaker driver measurements. Simulate the combined multi-way frequency response and time/frequency (wavelet) behaviour.

FIR Designer M also includes a new integrated Help menu to further enhance the software’s usability.

Learn more about FIR Designer M or download a free demo (Windows or Mac).

Existing FIR Designer 1 license-holders are eligible for a USD $100 coupon towards the first-year purchase of FIR Designer M. Existing FIR Designer 2 license-holders are eligible to convert their license to a FIRd M license at no additional cost (just contact us to set it up).