Feature Request: Add "18dB" to Butterworth/Linkwiz-Riley

bang-olufsen / create

Software for the HifiBerry/BeoCreate Re-Create project for upcycling vintage loudspeakers

https://www.hifiberry.com/beocreate

MIT License

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Feature Request: Add "18dB" to Butterworth/Linkwiz-Riley #83

Open paulliewallie opened 4 years ago

paulliewallie commented 4 years ago

Please can you add "18dB" crossover slopes to butterworth/Linkwiz-Riley filters in the DSP. The slopes are currently 12dB or 24dB.

I am using the DSP as a LPF to a subwoofer. In testing, i am using EQ and crossovers to a bi-amped Bose Panaray system for the disco.

tuomashamalainen commented 4 years ago

Thanks for your request, I will look into adding this but can't promise an ETA at the moment.

In the meantime, you should be able to accomplish this by using a 2nd-order filter with Q 1 and a one-pole filter, which you can add with the "custom coefficients" option. Use this or another similar tool to calcualte a one-pole filter (sampling rate is 48000 Hz) and copy in the coefficients. Note: the website flips a and b, so a0 from the website should go to b0 and b1 should go to a1 in our filter design interface.

Let me know if I can further assist you.

Best, Tuomas from Bang & Olufsen

tuomashamalainen commented 4 years ago

If you're reading this through email, note that I updated my previous comment where I mistakenly stated that cascading a 2nd-order Butterworth filter with one-pole filter would create the desired result. You need to use Q=1 in the 2nd-order filter together with the one-pole filter. Apologies for that.

paulliewallie commented 4 years ago

Thank you for sharing this knowledge.

paulliewallie commented 4 years ago

Adding custom 2nd order filter was easy but custom coefficients doesn't let me change any of its numbers, have i discovered a bug or am i doing something wrong?

tuomashamalainen commented 4 years ago

This is a bug, but I've already fixed it for the next update because I remember running into the same issue. Sorry about that.

paulliewallie commented 4 years ago

Now the bug seems to have gone, i have added the custom coefficients. It works but it overlaps my eq points so how do i make the filter smaller so that it only effects the low pass filter?

tuomashamalainen commented 4 years ago

Good to hear that the text fields are now obeying you. I'm afraid I don't quite understand your issue – could you post a screenshot that might explain it better?

paulliewallie commented 4 years ago

paulliewallie commented 4 years ago

Here are the screenshots. I have one screenshot with the filter enabled and the other disabled.

paulliewallie commented 4 years ago

I think i've found my problem, i have not selected "one pole LP" in "type" on the biquad calculator website that you sent.

tuomashamalainen commented 4 years ago

Thanks for the screenshots, and good that you got the issue resolved. What I was going to say was that this needs to be solved in whatever tool you use to generate the filter (just like you did). When you input a filter from coefficients, Beocreate system has no control over the shape, position and strength of the filter, it will be just as the coefficients describe it.

Best, Tuomas from Bang & Olufsen

paulliewallie commented 4 years ago

It went wrong again and then i noticed that copy/paste didn't include the minus symbol! Now it works:

Thank you again!