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Post by twoifbysea on Jun 2, 2017 22:20:51 GMT 1
Rob,
I'm trying to put together a standard EQ with HP, Low Shelf, Low Mid Peak, High Mid Peak, High Shelf & LP and I'm using your RH-BiQuadSDK3. Due to my ignorance I'm getting odd behavior with the Q settings. Could you please offer some advice with the Q range settings for a clean non modulated response? Thank you.
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Post by Rob on Jun 3, 2017 18:31:41 GMT 1
Hi, It appears my module has a problem with the Q settings. Am looking into it, but it has been two years since I last looked at it and i'm no DSP guru ( good chance I can't fix it alone ).  And, what do you mean with a clean non modulated response? Cheers, Rob |
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Post by twoifbysea on Jun 4, 2017 3:35:03 GMT 1
Thanks for the response Rob. The Q setting seemed unpredictable to me and were causing high resonance at settings were I wouldn't expect them. That's what I meant, forget that said "modulated".
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Post by Rob on Jun 4, 2017 13:22:50 GMT 1
I see, strange. Had hoped the code would be stable as is.  |
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Post by twoifbysea on Jun 6, 2017 4:26:59 GMT 1
So it was my error, the module sounds great! After setting the max Q value at 2.2 on the High and Low Shelf all is well. Like I said in my first post it was simple ignorance. Thank you for your work!
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Post by Rob on Jun 7, 2017 18:40:28 GMT 1
Good to hear, thanks for letting me know.
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Post by twoifbysea on Jun 15, 2018 0:02:30 GMT 1
Rob, I was trying out some steeper Q values with your BiQuads and I noticed that there is a small cut between each frequency. It's more evident at higher Q's and smooths out more at lower Q's. Also, higher Q's boost the overall signal. Best, Dave   |
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Andrew
Developer
Posts: 110
Posts: 110 
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Post by Andrew on Jun 15, 2018 19:56:02 GMT 1
It actually looks like little boosts not cuts. @rob - maybe it's one of the approximations that is a tad-off around 1 or 0? I don't know much about biquads but if I where to guess - I'd look to see if there was an exp or pow in the code and check that first. twoifbysea - are you exactly sure your knobs are 0Volts? The readouts aren't very accurate, even 0.00001 Volt offset may produce what you're seeing. And what is the maximum deviation in dB? That analyzer doesn't show enough resolution, if the error is max +/- 0.25dB, you can safely ignore it - nobody is gonna hear that. Also note that increasing gain computation resolution will increase CPU in tandem. Random note1: Unless you have non-linear processing(distortion) before the out gain calculation, the input gain is redundant i.e. a waste of CPU. Random note2: Q in filters are kind of arbitrary in a way i.e. the numbers don't really make sense. Example: Biquad Lowpass Q = 0.7071 is exactly equivalent to a State variable filter with Q = 0.5! |
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Post by twoifbysea on Jun 15, 2018 21:42:15 GMT 1
I udpated the screenshots above with a higher resolution analyzer. Raising the Q does boost the overall level, but you can see that there are cuts occuring at all of the center frequencies. Are you exactly sure your knobs are 0 volts? The readouts aren't very accurate, even 0.00001 Volt offset may produce what you're seeing. Yes, when set to 0 volts, they output exactly 0 volts on the volt meter. And what is the maximum deviation in dB? I'm not sure how to measure that? Random note1: Unless you have non-linear processing(distortion) before the out gain calculation, the input gain is redundant i.e. a waste of CPU. There is distortion before the output. For proper gain staging, it makes sense to have the ability to adjust the input. But I get what you’re saying. Best, Dave |
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Post by Rob on Jun 16, 2018 16:28:54 GMT 1
Hi Dave,
Which filtertype are you using?
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Post by twoifbysea on Jun 16, 2018 19:45:21 GMT 1
Rob,
Those are all peak with a single cascade. There is also a butterworth HP & LP on the ends, but those are fine.
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Andrew
Developer
Posts: 110
Posts: 110
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Post by Andrew on Jun 17, 2018 10:34:54 GMT 1
I checked with the Impulse Response module, Peak = 1000Hz, Q = 3, Gain = 0. And yeah, Dave is right - it's not a flat wire.
That being said - comparing it to the EQ module I'm working on atm, mine passes the 'flat wire' test above but fails the complimentary symmetry test(RH Biquad passes that btw). The complimentary symmetry test is when you connect two of the same filters in series but with polar gains - like filter 1 = -6dB, filter 2 = +6dB; it should return a wire. P.S - not all filters will return a flat wire in the symmetry test, especially analog ones.
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Post by Rob on Jun 17, 2018 11:45:23 GMT 1
My Biquad filter has another problem.. When I build a simple EQ or just filter plugin with peak and try it in e.g. Renoise, I get with several types a DC spike below 50Hz. And stays whatever I do. Tried both versions I have.. :/ The module should be rebuilt using the new SDK version, and use C++ classes for once.  |
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Andrew
Developer
Posts: 110
Posts: 110
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Post by Andrew on Jun 17, 2018 13:48:13 GMT 1
Are you sure it's not the DAW sending a small DC signal to your plug? I can't see the actual level. P.S - some DAWs do that by default to prevent denormal numbers, and it seriously pisses me off 'cause it can interfere with sleep mode; preventing denormals should be the domain of the plugin! I checked in SE and no DC was present btw. As for c++ classes, I think any decent c++ programmer will have a royal fit if they saw my source code  |
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