It is currently Mon Dec 17, 2018 12:48 pm


Filter Design

Where to share your BC Modular creations
  • Author
  • Message
Offline

dawman

  • Posts: 93
  • Joined: Fri Nov 29, 2013 2:12 pm

Re: Filter Design

PostTue Sep 23, 2014 6:50 am

Nice week for BCModular and friends.

ANkyu
Offline

jhulk

  • Posts: 75
  • Joined: Fri Apr 12, 2013 11:45 am

Re: Filter Design

PostFri Sep 26, 2014 2:54 pm

i have a few other designs that i will put here

the hammond 4 pole res filter uses a hipass filter in the feedback path at high resonance it reduces the bass frequencies being feedback and so there is no bass drop at these high settings

and the serge resonant eq which uses a crossfade mixer for the input and res feedback input because of this the level off feedback is limited to the cross fade mixer and so its not mixed back as conventionally but it mixes from the input to the output for feed back so at either extreme its either normal input or full res output and when mixed in the center both are mixed equally

this means that you dont get the res ringing of a conventional res filter but enough q to change the sound
Offline

dawman

  • Posts: 93
  • Joined: Fri Nov 29, 2013 2:12 pm

Re: Filter Design

PostSat Sep 27, 2014 1:05 am

Sweetness...
Offline

dawman

  • Posts: 93
  • Joined: Fri Nov 29, 2013 2:12 pm

Re: Filter Design

PostTue Sep 30, 2014 11:05 pm

I am really interested in the Hammond you mention as so many digital filters I have used in the past always lose the bass when high resonance is needed. HPF as a fix the fed back audio is pretty witty IMO.

Cheers
Offline

jhulk

  • Posts: 75
  • Joined: Fri Apr 12, 2013 11:45 am

Re: Filter Design

PostThu Oct 02, 2014 7:50 pm

that really easy to achieve jimmy

just run 2 lowpass filters on paralleled have the audio go into the same input on both then sum the the out puts to a sum mixer

if using a svf type then sum the lowpass together the bandpass together and the hipass together for the same effect if you want notch then sum the lowpass and high pass from the filter and you will have a notch signal then for both filters sum the notch sum

then run a control signal to cutoff to both

and only run resonance control to one of them now when you add high res the filter with out the res signal control will just have frequency control

so the frequency will alter the same but bass response is kept

the hammond filter actually came way out before bob moogs filter as did his resonators on the novacord bob moog is always given the accolade of the birth of the synth filter but its not the case but his cascaded ladder design is the first controlled filter design for specific synth moog modular and then the minimoog
Offline
User avatar

JoPo

  • Posts: 164
  • Joined: Mon Apr 29, 2013 4:43 pm

Re: Filter Design

PostMon Nov 10, 2014 10:42 pm

Great ! I learn a lot of things !
So .. If I understand something, you sort out frequencies thank to diode (wave rectifier) ? Right ? :geek:

And in warp69 case, how does it work with only mix & feedback modules ??

I don't have time now to study this, roll on the holidays !

Thanks for the lesson !
Offline

jhulk

  • Posts: 75
  • Joined: Fri Apr 12, 2013 11:45 am

Re: Filter Design

PostTue Nov 11, 2014 10:13 am

a diode used in a filter for feedback it turns into a dc type signal and how power supplies us ac and a diode bridge rectifier to get a dc signal

its not really a dc signal it still has a ripple to it they use capacitors in power supplies to smooth the wave

i diode feedback ring does the same type of thing but your not smoothing it with capacitors because of the rectification it adds distortion and because it is a diode ring the signal is feed back again and again which causes more distortion this signal is then feedback to a vca which is used to apply the signal back into the input the vca control is the q control

the circuit design is based on my hardware version a linear vca is used as a gain cell the way you implement the vca in the feed back path with resistors and capacitors creates a filter response this core block is the cascaded to create poles so the simplest filter is a 1 pole you then cascade them to create bigger filters an svf filter

in a 1 pole can do only lowpass and hipass if you summ the outputs you get notch this happens with frequency cancellation if you put the add each output to a leg of a pot from the wiper you will get a mixture of the 2 so you can sweep from lowpass to hipass this is an tom oberhiem design and was implemented on the sem

now to get bandpass you need 2 cores as its a serial lowpass to hipass so what would be the hipass output on the first core this same output on the second core would be bandpass as the lowpass output goes to the input of the second core and goes through the hipass circuit and its output is the bandpass output this gives us a 1pole hipass a 2 pole bandpass and a 2pole lowpass filter now on mine i used the sem filter crossfading on the hipass and lowpass and its output was also crossfaded with the band pass this gives you variable notch and bandpass just by turning these 2 knobs many filter topologies can be created and in real time

the matrix12 used the same technique but used pole mixing but it used switched sample and holds to mix the signals and so its hard to morph them with the filter mixing with the pots its summing filter outputs which makes it easy to morph

now resonance is always done on the lowpass output usually the final core stage in our case a 2pole one this goes through a gain stage so it can be controlled on my design i added the diode ring and had the feed back input go to a crossfade mixer so that i could choose if i want normal res or the distorted res from the diode ring

again diode ring feedback is nothing new the sem filter uses 2 diodes the ms20 uses a few more its well documented design

the chip that i use for my design is thatcorp 2162 dual blakmere vca this is a very good vca and was used in the most famous buss compressor by ssl this discrete vca was first made by dbx and was in a gold can thatcorp are the chip designers of the original dbx vca and you can substitute the gold cans with these blakmere vca

http://www.thatcorp.com/datashts/dn130.pdf

here are the design notes on the state variable filter

which might give you a better visual understanding
Offline
User avatar

JoPo

  • Posts: 164
  • Joined: Mon Apr 29, 2013 4:43 pm

Re: Filter Design

PostTue Nov 11, 2014 12:41 pm

Thanks a lot !

18 years ago, I go back at school and I learned a bit of analogic electronics, but for industrial applications. I remember we made low pass & high pass filters. Few years before I buy my first pc !

When I'll have time, I'll study all that stuff.. Thanks, jhulk !
Offline

jhulk

  • Posts: 75
  • Joined: Fri Apr 12, 2013 11:45 am

Re: Filter Design

PostTue Nov 11, 2014 1:54 pm

the vca control block is the most important on a synth as it is used for as contour control and also modulation control a vca can be used as audio or control signals

as both are voltages and because its gain is controlled by voltage you find a lot that vca are ac and dc coupled so that they can be used as mixers for audio and control signals

its also used in wavehsapes and oscilators for level controls for linear control use a linear vca for log control on the db scale use the bcmodular mix 1 module as sharc says it uses less dsp than the the exp vca

also if doing volume modulations you can use the volume modulators instead i have used these in waveshapers

if you use a oscilator with several outputs if you also invert each output and send all 6 outputs to a 6 input mixer which are log scale with a db control by adjusting each volume you can get more waveform outputs you can also do the same with a comparator which is used for pwm modulation where you put an normal and an inverted signal to the inputs and you have a threshold it will reach the threshold then flip to the other input channel this works well on ramp or triangle waveforms and the square wave will be different for each type of input my waveshaper uses 3 of the comparators and it has audio inputs and inputs that go direct to vca these vca are switched on and off by the comparator as the outputs from the comparators are changed from unipolar to bipolar so that they are just a on off state this then goes to the vca which switches the vca on and off the sum from the vca are added and the output is a switched waveform because this is done so fast it is a constant waveshape if instead of summing you go through a mixer stage then you can add variable amounts of volume which will make even more waveshapes

you can make complex lfo the same type way the vca gain cell is what makes the synth complex modulations

you can take a simple oscillator design and make it a power house complex oscillator with a few extra components in out case models

try this take simons multi osc this is based on the cem 3340 that i had him build for a memorymoog emulation it has 3 outputs tri ramp and pwm this osc can have fm and you can also crossmodulate it by using the output of a second oscillator to modulate the pwm modulation now set up few modules 1 full-wave rectifier 1 half wave rectifier

for each osc output and one invertor for each output set up 9 mix1 modules and summ the outputs to a sum mixer you will need 3 of these as you will have to use a 4 and 5 and a 2 now add controls to these to the bcpipe and the other async inputs take the audio out from the mix 2 module take frequency in through a sync input and take the pwm control to a async output and the sync signal through a sync output

now you have a very complex oscillator by varying the volumes of the mix1 modules you can change the oscilator waveform in real time and the great thing is that you can modulate the mix1 in real time making this oscillator an evolving oscillator like a scannable wavetable osc

ill be doing some clm workshops screen capture videos about doing things like this and saving them as a .mdl for reuse in other designs as what modules bcmodular does not have it has the building blocks to make them and how hardware modular modules are made with cores and mixers and vca for modulation
Offline
User avatar

cwmod

  • Posts: 283
  • Joined: Wed Oct 30, 2013 5:16 pm

Re: Filter Design

PostWed Nov 12, 2014 7:15 am

thanks for the explanations jammie!
this is excellent for me being a total ignorant when it comes to elctronic stuff :evil: :D
Previous

Return to BC Modular Patches

Who is online

Users browsing this forum: No registered users and 1 guest

cron