Soft/Hard clipping live guitar input with waveshaper

[Taemin]

First of all, this is my first time using waveshaper. So basically I am trying to make a digital distortion/overdrive pedal using teensy4.0 and audio shield. I've read the description of waveshaper multiple times, did a ton of research, tried messing around with the example codes, posted a few questions, but I just can't understand how it works and how I should use it. Can someone explain some basics to me? Also an example code for a working clipping effect would be helpful.

[MarkT]

You pass a transfer function to the class as an array of floats, it applies that function to every sample with interpolation.
The length of the array needs to be one more than a power of two, so 3, 5, 9, 17, 33, etc.

For instance passing the array {-1.0, 0.0, +1.0} would be the identity transform since the first and last points are
the mapping of -1.0 and +1.0 samples (samples are conceptually in the range +/-1.0 in the audio lib.)

Another example: passing {0.0, 0.0, 1.0} would give a rectification mapping as negative samples all map to 0.0

[oddson]

....which I think means you have to provide a 'graph' for a plausible transfer function that (soft) clips.

The array you need will have a plateau at or near -1 at the start and at or near 1 near the end with a steeper than 45 linear segment and possibly some softening around the knees between the linear segments.

Minimally five points might work (i.e. {-1.0,-1.0,0,1.0,1.0} ) but it's likely you'll need something smoother and perhaps not so flat in the plateau regions.

Don't expect a 'tube' sound from something very simple.

It's the interpolation that will start to get expensive if you have a ton of points defining your transfer function.

I've been meaning to try this out for literally years but I've never even started. Good luck and hope to see your results.

[MarkT]

It's the interpolation that will start to get expensive if you have a ton of points defining your transfer function.

The interpolation costs the same whether its 2 or 8193 or whatever points. You might want thousands of points for a particular smoothly
changing curve in fact. I looked at the code.

[Taemin]

I tried lots of different arrays after seeing your reply, and I at one point I got something that actually sounded sort of like a tube amp with overdrive pedals. But that was pretty much based on luck. I'm having a hard time understanding the whole "map to" part. So I drew a simple sine graph, divided it into 8 parts, and cut off the top and bottom part. I wrote down the estimated y values and used that as the array. It looks like this: float waveshape[9] = {-0.7, -0.7, -0.4, 0, 0.4, 0.7, 0.7, 0.7};



But now that I think about it, I don't think this is how it works. Can you help me out?

[h4yn0nnym0u5e]

This thread reminded me I was going to implement a non-real-time audio plotter. I've done it now, see https://forum.pjrc.com/threads/69900...udio-debugging for my post, which you may find helpful to experiment with waveshaping. I even put a 5-point waveshaper in the demo...

As noted by MarkT, your array needs to have 2^N+1 entries (your 9-element one actually only initialises 8 of them, so it may sound a bit odd!). This divides the input into 2^N "slots" which are linearly mapped to output values. Say your array is 9 elements, {-1.0,-0.9,-0.7,-0.4,0.0,0.42,0.75,0.95,1.0}: input values from -1.0 to -0.75 fall in the first slot, and will map to values between -1.0 and -0.9; from -0.75 to -0.50 are second slot, and map to -0.9 to -0.7 ... and so on. Note I've deliberately made the array asymmetrical, because that's valid (and may even sound nice).

[Taemin]

Thanks. That helped a lot.

float ODShape[9] = {-1.0, -0.9, -0.7, -0.4, 0.0, 0.42, 0.75, 0.95, 1.0};
float DSShape[9] = {-0.9, -1.0, -0.9, -0.5, 0.0, 0.53, 0.95, 1.0, 0.94};
float FZShape[9] = {-1.0, -1.0, -1.0, -0.5, 0.0, 0.5, 1.0, 1.0, 1.0};

This was the closest I could get to the actual pedal sounds. I am pretty satisfied with it but if you have any suggestions that would be great.
I also modified your oscilloscope code to see if my code was actually working. That was also really helpful too.