Pre & post filters in saturation (cascading waveshapers)

DSP, Plug-in and Host development discussion.
binaryoblivion
KVRist
44 posts since 18 Feb, 2016

Post Sun Apr 01, 2018 3:15 pm

I am working on a saturation module for a plugin, which is inspired by the design of tube amps. I am not aiming for an accurate amp emulation, moreso something that gets me in the ballpark. Currently I have cascading asymmetric sinusoidal waveshapers, with attenuation applied between each stage.

From my research into the topic I am given to understand that filters between the saturation stages play an important role in tube amps. I am hoping to get a better understanding of the role those filters play. Are the filters intended to “repair” the distorted sound, or alter its sonic character? For example, would a high pass filter be used purely to eliminate DC offset, or to noticeable remove low frequencies?

Any tips, advice or suggested reading is appreciated. If my entire approach is flawed, please let me know that too!

Thanks in advance.

matt42
KVRian
1068 posts since 9 Jan, 2006

Re: Pre & post filters in saturation (cascading waveshapers)

Post Sun Apr 01, 2018 7:36 pm

I think you're approach is only flawed if you really want to consider it a tube emulation.

As for saturation and distortion filters can play many important roles. Like you mention using a highpass to kill DC. You can also pre filter the signal to select how much distortion is applied to different frequencies. For examples cut some lows with a shelf filter to avoid everything turning to mush when really driving the saturation. Maybe boost some other frequency range that sounds good with more distortion. This is a matter of taste although you could try to emulate frequency specific saturation of a tube in this way. Then after the saturation you can apply post filtering to compensate the prefiltering, or just shape the overall frequency response.

Ivan_C
KVRian
1067 posts since 11 Aug, 2004 from Breuillet, France

Re: Pre & post filters in saturation (cascading waveshapers)

Post Sun Apr 01, 2018 10:51 pm


binaryoblivion
KVRist
44 posts since 18 Feb, 2016

Re: Pre & post filters in saturation (cascading waveshapers)

Post Sun Apr 01, 2018 11:09 pm

Thanks @matt42, that is very helpful.

I hadn't considered attenuating frequencies before clipping, then boosting the same frequency range after clipping, so that is something new for me to think about. I have been poring over the amp analysis articles at electrosmash.com today, and it seems like cutting low frequencies from the incoming signal is common in guitar amps, but for non-guitar inputs it seems undesirable to eliminate so much low end. Pre/de-emphasis shelving filters, as you've suggested, might be a good compromise.

@Ivan_C: I actually discovered your presentation today :) It is fantastic, and very accessible. The accompanying Juce plugin is super useful too.

Thank you both, you've given me lots to think on. I think some experimentation is in order...

Ivan_C
KVRian
1067 posts since 11 Aug, 2004 from Breuillet, France

Re: Pre & post filters in saturation (cascading waveshapers)

Post Mon Apr 02, 2018 11:31 am

Thanks and have fun experimenting !

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