FIR Designer question

[EA Support]

EA Support

6 August 2024 at 7:59 pm #885

Apologies if you have some literature that covers this on your website, maybe you could point it out to me if you do.

I have started using your FIR Designer software. I have noticed that all our older products were limited to a delay of 4 ms on our FIR filters, due to a lot of our products being used for live and DJ monitors etc. So, it seems the historical decisions was a hard stop at 4 ms.

My existing knowledge of FIR filter design suggests this would limit the tap length to around 384 taps (our DSP is 48 kHz), where delay equals No. of Taps ÷ (2 * sampling frequency)

Then I would work out the lower frequency limit using 1 ÷ (2 * delay) giving me answer of about 125 Hz. However, I have noticed that the FIR designer will still let you set a high pass filter below this frequency, so I was wondering how that works?

Keen to learn more. I know you have a lot of really useful application notes and articles on your website and admittedly I haven’t had a chance to read it all.

Regards,

I suggest you first read the whitepaper at

FIR Filter Guide

and in particular, Sections 4 and 5.

Filter delay being half the tap length is only relevant for linear-phase FIR filters, and if your products use primarily linear-phase filters, then yes, 4ms delay suggests 384 tap filters at 48 kHz.

FIR filters can actually be minimum-phase (like IIR filters) with essentially no delay, linear-phase, maximum-phase (where the delay is effectively the length of the filter) or mixed/arbitrary phase. In FIR Designer, the filter delay and length can be explicitly set on the Export tab, in FIR Setup sub-tab.

Yes, a longer FIR filter can affect lower frequencies but it’s not a direct relationship. How low in frequency is influenced by how aggressive the filter is or how high-Q the EQ adjustments are (in both magnitude and phase).

In the FIR Designer workflow, you design the ideal filter response that you want, then the Export tab shows the effect of the user-specified filter tap length and delay. The Export tab also shows the difference between the ideal filter and the windowed/truncated filter in the “Total Error” plot.

To better understand the workflow, look at the ‘FIR Designer 3 overview’ video on

FIR Filter Design Tutorials

and the tutorial

FIR Filter Design for Loudspeaker Equalization

Regards,
EA Support

• This topic was modified 4 months ago by EA Support.
• This topic was modified 4 months ago by EA Support.