u/kiwiberrydrink

I posted my chorus plugin here a couple weeks ago and someone told me allpass interpolation was the wrong tool for a modulated delay, since the recursive state goes stale while the delay length is moving under it, and that I should look at Lagrange. I was still on linear at that point. So I wrote a 4-tap cubic Lagrange interpolator by hand instead of pulling in a crate, mostly because I wanted to understand it rather than trust it.

What's actually in the thing now, so nobody has to guess from the post:

- 6 ms base delay with the LFO modulating around it, cubic Lagrange on the fractional read. Taps at -1, 0, 1, 2 off the integer index, each one wrapping the ring buffer separately.
- One-pole highpass at 160 Hz sitting inside the feedback loop so the low end doesn't stack up.
- One-pole lowpass on the wet path, cutoff swept by the LFO between 1k and 7k. Most of the character comes from that, not the delay.
- Separate voice struct per channel, LFOs offset 90 degrees. Collapses to mono without eating itself.
- Dry/wet is a plain linear crossfade. There's roughly a 3 dB dip at 50% and I'm fairly sure that's comb cancellation from summing a correlated wet signal with the dry, not bad crossfade math. If that reasoning is wrong I'd like to know.

One debugging note that saved me: check your coefficients at frac = 0.5. They should land on -0.0625, 0.5625, 0.5625, -0.0625. Mine didn't, and it was a typo in one of the products in the third coefficient.

At my defaults (1.6 Hz, about 4 ms depth) the improvement is small. A little less grit on held notes up top. I assume it opens up more if you push the rate.

Two things I still can't answer:

1. Any reason to go past cubic when the modulation is this slow, or is higher order mainly for pitch shifting and wide sweeps?
2. The HF droop moves with the fractional part, so slow modulation means a slow wobble on the top end. Does anyone correct for that in practice or is it under the floor at this depth?

It's free, GPLv3, VST3 and CLAP, mac/windows/linux. It's built for pitch-corrected vocals specifically, which is the only thing I use it on. If you'd rather hear it than read about it, it's at pyfessional.tech, the download button picks your OS for you and the source is linked from the same page. Would rather have someone install it and tell me it sounds wrong than get upvotes.

I’m a CS student and I’ve been teaching myself DSP for about a year, mostly by building one plugin over and over until it stopped being broken. It’s a vocal chorus called Crimson, written in Rust with nih-plug.

It’s at v1.0 and it sounds good to me, which is exactly why I’m posting. Mine are the only ears that have been on it and I’ve been in the code long enough that I can’t hear it clearly anymore. Would love other ears, especially from people in this DSP Reddit, your opinion means a lot to me.

It’s free, and I’d rather have it looked at than downloaded, if I’m being honest about which one I want more. The goal was a chorus that survives autotune, since most of what I tried either did nothing or moved the pitch enough to fight the tuning. So instead of sweeping the delay down from zero like a flanger, it sits at a 6ms base and modulates 0.5 to 6ms on top of that. That keeps pitch excursion somewhere around 3 cents at the defaults, which in my testing is roughly where it stops smearing. The rest is fairly plain.

Linear interpolation on the delay read with the standard wraparound. LFO shapes built additively from harmonic sines so the modulator itself isn’t aliasing. Feedback at 44% default with a one-pole highpass at 160Hz inside the loop, because without it the low end piles up fast. The thing that gives it any character at all is a one-pole lowpass on the wet whose cutoff is swept by the same LFO, roughly 1k to 7k, so the wet brightens and darkens in time with the modulation. I found that more than designed it. One voice per channel with phase offset for width. No oversampling, no FFT, small buffer, deliberately light.

Where I know I’m on thin ice:
The feedback path is my biggest worry. Linear interp acts like a lowpass, so the tail dulls a little more every pass around the loop. I’ve been working through Julius Smith’s section on first order allpass interpolation and it looks like the fix, but I don’t know whether allpass phase behavior causes new problems when the delay length is being modulated. If anyone has actually done that swap I’d love to hear how it went.

Smaller things I’m unsure about: whether a first order highpass is enough inside a feedback loop or whether a damping shelf would be more musical, and whether random-walk or noise-driven modulation is worth it at only two voices or if that’s something that only pays off at higher voice counts.

Where I want to take it: The part I’m most excited about for V2 is making the modulation react to the performance instead of running open loop. A chorus that listens, basically. Pulling back the wet on sibilance so it doesn’t splash on esses, letting depth bloom on sustained notes and tighten on fast delivery, that kind of thing. I don’t know yet how naive that is. My instinct is that the hard part isn’t the modulation, it’s getting a detector that’s stable and fast enough to be musical without chattering. If that’s a solved problem with a name I should be reading, please point me at it.

Source, GPLv3: http://github.com/MichaelAngulo3232/crimson-chorus
Builds for Mac, Windows and Linux, CLAP and VST3, free: http://pyfessional.tech
Not selling anything and not trying to pitch you. I’ve learned most of what I know from posts on this sub, so if something in here reads as “this guy read one blog post and started typing,” I’d genuinely rather hear it than not