Halogen 5.0’s FIR Import DSP block is available to all four current HAL models.* The FIR block permits importing finite impulse response (FIR) filter coefficients. To obtain the FIR coefficient’s file, either:
1) Download the applicable FIR file for the loudspeaker from the manufacture’s website – e.g., Fulcrum Acoustics, or
2) Generate the FIR coefficients from measured loudspeaker response data using the software packages that create FIR filters manually:
FIR-Capture from WaveCapture (not yet released)
The Halogen workflow for the FIR Import block is:
1) Obtain the FIR filter coefficient file(s) in one of the above ways.
2) Drag and drop the FIR block(s) in your Halogen processing (audio flow) along with the PEQ (IIR** filters), crossover filters, and cut filters. Cut filters offer the full suite of filter types and slopes.
3) While still offline, click each FIR dialog’s Import button and select the .csv or .txt file that contains the coefficients. Click Open.
Note that Fulcrum’s Halogen files already contain the Imported FIR data and other filter blocks required, so that you can skip step 3, the Import step.
Halogen always imports FIR files while offline. Thus, to avoid or at least minimize being kicked off live HAL devices, and then re-Applying new Halogen settings repeatedly, do as much Importing offline as possible. Then go live with your HAL. Halogen lets you import tap sizes up to 1,024 without needing to predetermine the number of taps you need. Just import any file with up to 1,042 taps. The file format must be either comma separated, or a single column of just the coefficients.
The advantage of the offline import requirement is that you don’t have to specify the number of taps for the FIR block ahead of time. Halogen figures out the number of taps automatically for you.
When you do Import while Connected to a live HAL, Halogen forces you offline (auto disconnect) because a DSP recompile is required to import the new filter set.
When creating your own FIR filters with any software package, make sure you use 1,024 taps or fewer. Also be sure your sample rate is always 48 kHz. Filters imported at other sample rates will not be pretty in HAL’s 48 kHz world. These are real world limitations on the use of FIR filters. See the further reading links below.
You should have a general idea about the basic shape and response of the FIR filter being imported, Halogen’s FIR block allows you to view the magnitude, group delay and impulse response after the import for comfort’s sake.
The entire EQ package
The software programs below allow the measured frequency response to be inverted, adjusted/smoothed, and then spline fit into some number of PEQ filters. Then one can export or save the PEQ data and import or paste the PEQ settings into a DSP. From a workflow perspective, this is considerably faster than manually adjusting PEQ settings, then re-measuring, adjusting, measuring, adjusting, measuring… Halogen is not yet supported in these packages, but this is on our radar.
In summary, equalization with IIR filters (e.g., PEQ) targets smoothing the bumps and dips in a loudspeaker response since both are minimum phase phenomena. Thus, the magnitude and phase anomalies of loudspeakers are equalized smooth (corrected) by the inverse magnitude and accompanying phase of the PEQ’s IIR filters. While the FIR filter is a more recent, though DSP power-intensive solution is offering independent control of magnitude and phase, better suited for low- or high-cut filters needed in band-limiting drivers or systems. Keep in mind that you still can’t get something for nothing because FIR filters add considerably more latency than their IIR counterpart, especially as the frequency of concern gets low. Together, IIR and FIR are clearly a winning combination to tackle both loudspeaker and room interaction responses.
Further or initial FIR reading:
FIR-ward Thinking – Part 1 by Pat Brown
FIR-ward Thinking – Part 2 by Pat Brown
FIR-ward Thinking – Part 3 by Pat Brown
Here’s an overview of a modern equalization process from SynAudCon’s January 2015 OptEQ Workshop.
* FIR is not supported in the discontinued HAL1 (no “x” suffix) which requires Halogen 4.1.1 or earlier. Reminder: you can run multiple versions of Halogen (4.1.1. & 5.0.x) on the same computer. See this link.
** Infinite impulse response