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Old 28th August 2008, 07:15 PM   #1
manu27 is offline manu27  France
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Default Digital active filtering using PC

As my presonus firepod has 8 analog output channels, and sounds very very fine, I would like to use the PC to digitally filter High / mid / low / sub speaker frequencies. Has anybody some experience in that field ?
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Old 28th August 2008, 11:02 PM   #2
dmills is offline dmills  United Kingdom
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Easily done, use biquads or (probably better in some ways) convolution with a suitable impulse response which will let you easily also provide delay compensation and the like.

The biquad approach needs doubles in the feedback path and will benefit from some sort of denormal removal code if running on a pentium (FP denormals are really expensive), plenty of example code in the musicdsp archives.

For the convolution approach produce the impulse by doing a complex to real inverse FFT (libfftw is good), or by doing overlap/add directly in the frequency domain.

Regards, Dan.
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Old 27th August 2010, 03:37 AM   #3
tony11 is offline tony11  Canada
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I don't really understand your post (though I'm new to this- butnot new to audio). What I am interested in myself is software that can be run on a small computer that will provide digital crossover filters for a three-way speaker system. Using this would probably require three sound cards to output the signals to each driver and of course the software would have to be able to rout the signal to each separate sound card or to different channels of a single sound card if it had enough discrete channels. Ican not write software myself. I have built high quality speakers but used passive crossovers from Madisound.-----tony111
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Old 27th August 2010, 02:19 PM   #4
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This has actually been covered fairly extensively here. The biggest thread is entitled something like 'A Howto for PC Xover', although since it runs to the thousands of posts it may not be the most concise place to look for info. Some search terms you might want to use to identify appropriate threads would be 'Allocator' (software from Thuneau that does crossovers), 'convolver' (fir engine for Windows), BruteFIR (fir engine for Linux) and 'vst console' (VST Host that many folks use under Windows)

There really are 2 different aspects to this problem
a) designing the xover filters
b) running the xover in real-time.

On part A, it's a bit of a hodge-podge. HOLMImpulse will do the driver response measuring, but as yet I haven't found a really good graphical 'biquad workbench' type tool that lets you apply the filters and see the response. Thuneau's 'Allocator' does this fairly well, but it's part of the overall runtime engine not a separate piece. The Hypex 'filter designer' is pretty decent, but it's oriented at their DSP modules and so you have to do some translation of the resulting filters.
For FIR filters, there's is basically 'Acourate' or else 'hand coded Matlab/Octave scripts'.

For running the filters, there are a lot of options but you have to define your expectations and requirements. e.g. what platform? 2 channel music only or multichannel/HT as well? FIR or IIR? Is this a 'normal' PC or do you want something to run headless and behave more like an audio component?

For the original poster, the Firepod is a studio-style interface with ASIO drivers similar to the Saffire Pro 24 that I'm currently using. I've got a basic setup in place using 'Reaper' REAPER | Audio Production Without Limits . This is a DAW product, but includes what seems to be a quite good IIR/Biquad VST effect, and is fully capable of doing a 3 or 4 way xover system with EQ. I'm only using it as a 2-way at the moment, but that will grow. Not the most intuitive system if you haven't been around studio software before, but it's a free download to get started. (nagware, and only $60 for personal use which is a screaming bargain)

Downloading the trial/demo of Allocator (Transient Perfect loudspeaker DSP tools . Windows DSP speaker crossovers.) would also be a decent starting point. I don't use this, but did eval it pretty thoroughly at one point and it's good stuff, and since the 'Lite' version is only ~$60 or so, quite reasonable. It's probably the easiest place to get started if you don't have experience with studio software.
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