Reply to thread

Re: FIR filters

Sean,

The effects of FIR range from the mundane to the esoteric. I'll do my best to cover the bases here.

Like many things there is an element of truth in both of these, but neither is totally accurate:

• As for the first statement above, possibly the most esoteric feature of a "correctly" processed FIR box is what Bennett discusses. This is a cool consequence of physics that gets deeper into the world of what constitutes a signal.
• In regards to a box sounding better in the horizontal plane, it is not that simple. Any time you have displaced sources in a given axis they will add in some locations, and subtract in others. FIR does not change this. An FIR can offer the ability execute a transition from two transducers to one more quickly, which can help alleviate this. Unfortunately the way that this was done historically, with brick wall HP and LP that can have some undesirable sonic artifacts (a long, long post). Today more reasonable approach would be a high order L-R class crossover that is preceded by an allpass class filter that pre-corrects the phase rotation introduced by the L-R filter.
• Beyond these two effects, an FIR based filter typically offers much better frequency resolution to correct small behaviors than the typical IIR biquad filter we have used since the dawn of pro audio.
• An FIR gives you complete freedom to consider frequency and phase behavior arbitrarily, limited only by the overall delay you are willing to incur.
• Because FIR give you arbitrary control of the magnitude response, you can focus on aspects of product design with little thought to magnitude effects. An example might be using numerical simulation of the K-H integral to very precisely define wave propagation in a horn, without having to account for the horn's influence on the magnitude response.
• To the product designer the FIR offers a real possibility of cost savings. As an example, you might be able to implement a passive crossover using a minimal number of reactive components (and no resistive components). Then you can drive the loudspeaker with a single amplifier channel and use the FIR to implement a loudspeaker whose performance is nearly the same as a classic biamped design, but with a lower overall bill of materials (BOM) cost.
• Along this line the improvements in computing cost are making the use of DSP, be it FIR or IIR the most cost competitive way to implement filtering. Passives will increasingly become the millstone hanging around the designer's BOM.