[sdiy] Anyone do a voltage controlled "Tilt" Filter?

[Tom Wiltshire]

Richie’s simple 1st order version is pretty much what I did for a voltage-controlled Big Muff Pi tone control:

https://electricdruid.net/wp-content/uploads/2015/06/VCBigMuffTonecontrol.jpg

This was used in the Frequency Central "Meth Amp” eurorack module, which is mostly a eurorack-supply, voltage-controlled version of the Big Muff fuzz box…so we changed everything, pretty much!

==================
       Electric Druid
Synth & Stompbox DIY
==================



> On 31 May 2021, at 10:56, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> 
> Crossfading between the outputs of a 1st order low-pass filter and 1st order high-pass filter (both with fc=1kHz) gives the responses shown in TILT1.PNG attached.  Whilst it does approximate a "tilt" response, it's probably not quite what you want.  There are no phase cancellation or notches provided neither of the two filters inverts the signal, and they add to a perfectly flat "wire" when they're mixed in equal amounts, but the centre of the response drifts to the left and the right as the mixing ratio is changed. It doesn't always cross 0dB at the same place.
> 
> The correct tilt response is shown in TILT2.PNG.  Notice how the frequency response stays centred around the middle of the audio band and always crosses 0dB at 1kHz in this example.  In order to get this response the positions of the poles and zeros in the filters have to move.  (This is unlike the previous example where both filters had fixed 1kHz cutoff frequencies.)  When the pole and the zero are both at 1kHz they cancel each other out and the resulting response is flat.  Move the pole down below 1kHz, and the zero up by the corresponding amount and you get a tilted response with LF boost followed by HF cut.  Conversely, if the zero comes first you get a response with LF cut followed by HF boost.  That is how I generated those response curves.  Mathematically.  With no thought to what hardware is needed to implement the transfer functions!
> 
> So, you can get a kind of tilt response just by mixing the outputs from 1-pole LPF and HPF but it will move from side-to-side which might or might not be acceptable for your application.  To get a perfect balanced response will take something a bit more clever.
> 
> Hope this helps,
> 
> -Richie,
> 
> 
>> Neil Harper wrote:
>> 
>> my last guess was wondering if an equivalent circuit could be made by combining a lowpassed and hipassed audio signal and then crossfading them. I can imagine it would somewhat similar, but I'm not sure if you would get the same totally flat frequency response in the middle setting.
> 
> 
>> ColinMuirDorward wrote:
>> 
>> As for x-fading between high and low shelves, you might have phase problems and get a bit of notch filtering, no?
> 
> 
> -- 
> This email has been checked for viruses by AVG.
> https://www.avg.com
> <tilt1.png><tilt2.png>_______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org