Building a Lo-Fi Polysynth in Max (for Live)

[alanza]

Hi! I’ve been very slowly building a polysynth in Max. The goal is for me to learn more about working in Max and with digital audio signals in general, and also to make something that I’ll enjoy using in my own music. I finished a “version zero” a few months ago while working with @jasonw22 for LCRP, and I’ve been slowly working towards a version one.

I’m actually still a ways off from finishing, but since I do have something to share, I figured it might be fun to start a thread about it, and maybe if I’m lucky learn from your feedback about it, ala @billyhologram’s Nearness thread.

The high concept for the synth is to make something inspired by the sounds of early digital music like the Commodore64 or Yamaha FM chips in old cell phones, but without trying for fidelity.

---

The thing I have to share with you is a gen~ patcher containing (among other things) a variable-slope triangle wave oscillator.

copy me into a Gen patcher!

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So, I’ve been shamelessly pulling apart the Max for Live Poli synthesizer to learn how to construct a robust Max for Live device.

Version zero of my synth created “8-bit” wavetables naïvely and attempted to eliminate most of the aliasing by filtering, so naturally I turned to Poli’s oscillator code to see how they do it. Peter McCulloch’s code uses the Polynomial Transition Region method to produce anti-aliased saw and pulse waves. I did find a paper that proposes an algorithm along these lines for the variable-slope triangle wave, but since they only compute the first-order case, I ended up having to rederive the algorithm.

Aurally, I think it sounds quite good, especially with “pulse width” near the middle. Testing it out in Max, there is still some aliasing, particularly as you sweep the frequency, but A/B comparison with the Poli square wave leads me to think we’re in “good enough” territory. I suspect that part of the missing link here is that Poli never lets you completely bypass the filter.

For my future purposes, the above patch contains both Peter’s code for the pulse wave and mine for the triangle wave but outputs only one of the two according to an input. I’ve also included options for two-operator linear FM. I took a nod here from @Galapagoose’s design for FM pairs on Just Friends, but arbitrary FM should be possible.

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Questions I’m still working on that I would welcome input on

• FM works very well, but PM creates bizarre clipping and volume-shift problems. Why is this? (In fact, since shifting the fundamental frequency involves forcing a recalculation of all the polynomial coefficients and so forth, it seems like PM should be easier, since we’re just changing the input of the polynomials.)

• I tried to use clip and clamp to constrain the pulse width, thus avoiding having to divide by zero at the extremes. However, not only did this not seem to have any effect on the output, division by zero seems to be “fine” in the sense that the code does not crash.

• I’m not sure of the best way to implement hard sync; currently it seems like I would have to introduce some more state to keep track of whether we’re jumping to zero phase and how far our output is jumping.

• I have very little idea of whether what I am doing could be improved performance-wise. E.g., does the selector tell Gen~ to turn on and off the parts of the DSP automatically, or is the idiom different.

[andrew]

so excited to try this ! ! ! I’ve just being thinking about ways to computer something like my crap Yamaha keyboard and this just might be it (or something better)

[alanza]

Hard sync seems to “just work”? At least in my initial testing it doesn’t seem to sound obviously wrong. Mixing hard sync and FM also quickly makes some wild tones, as expected

[alanza]

Thanks to a suggestion from @zebra, a gen~ implementation of Andrew Simper’s State Variable Filter. It is (gasp), a linear model, but it sounds just fine for my purposes, and it even pings nicely.

inlet 1 is the signal input, inlet 2 expects a frequency value in Hz (shallow FM should work fine, I think it might dislike negative values though), and inlet 3 expects a resonance value between zero and one. Self oscillation at 1, pings work very close to 1, but resonance values above 1 result in loud noises.

copy me into a Gen patcher!

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[andrew]

agh - beautiful. so glad you implemented this as I am still not prepared to take in long signal processing papers. will be stealing soon 4 my own explorations ! hopefully this will also help me understand what the heck a filter is.

[eblomquist]

Is a gen patcher different from a max patcher?

Can you share this as an .amxd for those of us who are further back on the learning curve?

[vals]

not an answer to your questions, but by looking at your FM implementation I would suggest of taking the Mod.Width by multiplying the FM Index with the Mod.Frequency (in1 * in5 * in6 instead of in1*in6).
The equation is like this:

[Modulator Width = Modulator Frequency * Index]
or
[Modulator Width = Base Frequency * Harmonicity Ratio * Index]

[alanza]

oh, good catch! I’m so used to implementing PM rather than FM that I didn’t realize that I needed to account for the ratio that way!

[alanza]

When the whole polysynth is ready I will definitely share it as a Max for Live device!

A Gen patcher is made by making a gen~ object and opening it like you would a patcher object. The inside of Gen patches operate on the level of samples, rather than at Max control message or MSP signal vector speeds, so objects are a little different and so forth. Sadly playing around with the internals of my patch might not be super enlightening, but hopefully the resulting gen~ object gives an interesting alternative to the builtin filters Max comes with.

[eblomquist]

20 chars of thanks !

[oisin]

would also love to get a M4L version of this… gen objects require a full max license:confused:

[robertsyrett]

I just got Max 8 yesterday after I gave up on trying to learn 7 about two years ago. I do love node-based programming environments, but MAX is very confusing to me in so many regards.

I try to open new from clipboard with the link in the top post to no avail. Then I started a new patcher, created a gen~ object, and right clicked to try and open a Gen patcher and the open options I have are help and reference.

[alanza]

To open a gen~ object, do one of two things: you can lock the patch and then double-click on it, or you can double click while holding the Apple command key.

[robertsyrett]

it worked!

I still can’t figure out how to use it, but I got the patch to open

Regarding phase modulation, I have two approaches in Audulus (the node-based program I am most familiar with) and wondering how they relate to your polynomial coefficients.

One is just to use sin expression, since any value put into it will be between -1 and 1. It would be something like sin( phasor + phase ) I suppose you would have to scale the inputs by 2π if you want to get technical, but this works very well for DX-7 style sounds.

The other is using the fractional expression, which just takes any number as returns the non-integer portion, to prevent any glitches in reading a waveform mapped to a 0-1 floating point value. If there is no fract(x) expression, perhaps there is a floor(x) function for rounding down, in which case you could use floor(x)-x to create an equivalent function.

[ht73]

I still can’t figure out how to use it, but I got the patch to open

There are two modes in max, locked and unlocked. Command-click toggles between them.

Unlocked mode is the main mode when you’re editing the patch. Here you can move objects around, change their names, draw patch cables and so on. If you option-click an object you get its documentation which often includes a tutorial patch so you can try things out. The only editing move you cannot do – as you discovered – is double-click on a subpatcher and get it to open. You should see a grid of dots in the background when you’re in unlocked mode.

Locked mode is for when you’re done editing and just want to use the patch, such as in a live performance situation. Here you cannot move objects or change their names. Patch cables will generally be hidden to avoid a cluttered UI. You can hide objects as well. But you have to enable this hiding manually. While in unlocked mode, click on an object and select Object:Hide on Lock. Or command-K as a shortcut. (Command-L shows the object upon lock). The grid of dots will also disappear as this looks cleaner. The only confusing thing is again, you need to be in locked mode to double-click on subpatchers and open them.

Basically, when you’re editing a patch and use a structured design (with lots of subpatchers) you’ll be toggling quite a bit between locked and unlocked modes, so it’s good to get the hang of this at the very beginning.

[robertsyrett]

Thanks, I’m slowly getting all that. It’s just that my brain is so heavily acclimated to Audulus, I’m just not sure how to even hood up a midi controller to the patch and connect it to the output. Looks like I’ll be watching Max tutorials this weekend

Post any you recommend

[ht73]

The notein and ctlin objects should be useful, especially the latter, and if you option-click in unlocked mode you’ll get a tutorial patch. This built-in “learn by doing” is for me the most effective.

Unfortunately I am way out of date on max and don’t think I’ve even seen a video tutorial, which in the youtube age is kinda embarrassing to admit… I’m sure others can recommend their favorites!

The other things to know right off are the strict right-to-left ordering of events, and the common usage of the “t” (trigger) object to ensure you get the ordering you want. The rest is specifics, which are almost limitless, but I’d be happy to help if you run into a certain issue…

[robertsyrett]

I appreciate the offer and may message you. I don’t want to clutter up this thread too much unless it pertains directly to phase modulated poly synths.