LPF doesn't work as supposed
Posted: Sat Jun 06, 2020 7:18 am
1st order LPF implementation schematic can be found from here does not give correct response when exported to VST (VST Analyzer software). What I get are these responses:
(lower response after normalizing a0=1)
(NOTE: red curve is for high shelf filter )
Filter is designed using Octave:
and it gives this response in Octave (and in all other software I've tested so far):
(green = target response, blue thin -- = BLT version, Earlevel plot https://i.postimg.cc/j27DLqVg/earlevel-LPF1.png)
In comparison, 1st order high shelf filter implementation there in plot and schematic gives same response on both software.
Any suggestions on possible culprits for the issue?
- LPF1_issue.png (40.71 KiB) Viewed 26181 times
(lower response after normalizing a0=1)
(NOTE: red curve is for high shelf filter )
Filter is designed using Octave:
Code: Select all
%% Butterworth LPF1 approximation for Linkwitz crossfeed use
%% fc @ 700Hz
function [b, a] = LR_crossfeed_LPF1(x)
% x = fs
switch x
case { 44100, 88200, 176400, 352800 }
b(1) = -1.27962418247E-17*x^3+8.46647384248E-12*x^2-1.743160919708E-06*x+0.131910297106252;
b(2) = b(1);
a(1) = -1.63998176028E-17*x^3+1.089084487604E-11*x^2-2.2605860763131E-06*x+2.17451639264416;
a(2) = -1.97934198689E-17*x^3+1.3055959391E-11*x^2-2.669817551786E-06*x-1.80141773027551;
case { 48000, 96000, 192000, 384000 }
b(1) = -9.12314114159E-18*x^3+6.56981379479E-12*x^2-1.472180284138E-06*x+0.121238164126997;
b(2) = b(1);
a(1) = -1.17911077076E-17*x^3+8.51964831143E-12*x^2-1.9232664955434E-06*x+2.16129807049821;
a(2) = -1.40130321601E-17*x^3+1.00625912303E-11*x^2-2.240688152629E-06*x-1.81838475852698;
otherwise % other fs ... do nothing
b = [1 0 ];
a = [1 0 ];
endswitch
endfunction
and it gives this response in Octave (and in all other software I've tested so far):
- lpf1.png (16.02 KiB) Viewed 26164 times
(green = target response, blue thin -- = BLT version, Earlevel plot https://i.postimg.cc/j27DLqVg/earlevel-LPF1.png)
In comparison, 1st order high shelf filter implementation there in plot and schematic gives same response on both software.
Any suggestions on possible culprits for the issue?
