Strange comment in wikipedia about TB303's square waves

[antto]

my highpass filter is 6dB (simple one) but it's actually part of the LowPass filter
and i get the same thing - with no resonance it looks like some wierd sawtooth (as in the picture that was posted earlier)
when resonance is raised - it starts to look more like a square (look at the waveform of the mp3 i posted)
so there are 2 highpass filters really, one is inside the lowpass filter (filtering the feedback) which makes the whole difference
the other highpass filter is outside, it filters the input, and it doesn't change the waveform dramaticaly (not in the way we are interested here)

both are 6dB
and yes, the one on the feedback is looking perfect at 20-36Hz (in my case)

[mistertoast]

Anyone for any interesting ideas of where the asymmetry in the resonance comes? It must be something clipping somewhere where there's DC offset (?) present because at least the samples on xox site seem to show that it stabilizes back to symmetry further into a long note.

edit: nevermind, probably a later stage... turns out the resonant frequency stays constant, which couldn't possibly happen (as far as I can tell) if it was different resonance.. so somewhere after the filter, but before the hipass it gets clipped asymmetrically..

Would the asymmetry do anything interesting to the sound? Or is it just an inconsequential weirdness?

[mistertoast]

my highpass filter is 6dB (simple one) but it's actually part of the LowPass filter
and i get the same thing - with no resonance it looks like some wierd sawtooth (as in the picture that was posted earlier)
when resonance is raised - it starts to look more like a square (look at the waveform of the mp3 i posted)
so there are 2 highpass filters really, one is inside the lowpass filter (filtering the feedback) which makes the whole difference
the other highpass filter is outside, it filters the input, and it doesn't change the waveform dramaticaly (not in the way we are interested here)

both are 6dB
and yes, the one on the feedback is looking perfect at 20-36Hz (in my case)

Are you going to have another release of your plugin soon? Sounds like you're still working on it?

[tomg]

The 303's square is strange-looking, no doubt about that. I never took a close look at it, but I always assumed its strangeness was probably mostly in the realm of unusually offset phases for the different partials.

However, the other day I was looking at wikipedia's 303 article and saw this:

The TB-303 has a single audio oscillator, which may be configured to produce either a sawtooth wave or a square wave. The square wave is in fact created by flipping every other cycle of the sawtooth wave upside down, giving it a specific, more hollow, sound a bit different from regular square waves[citation needed].

I don't think I believe that. Wouldn't a ramp flipped every other cycle simply be a triangle one octave lower?

This is not true. It overdrives a single transistor. It's more like a standard distortion than a sophisticated waveshaper. It's not really square either being a little more rounded that a real square ware.

[tomg]

well, wikipedia can give you a simple idea about something, but if you want to really know the "truth" then you will want to dig deeper, and not blindly trust wikipedia
that's what i think i'll do from now on

Absolutely! I spent another enlightning day reading Hal Chamberlin's book (errm bible?) to figure out where wp is just a tad too, hmmm, lacking. I love wp though for all the things I don't know about.

You should look into Horowitz and Hill's Art of Electronics.

[antto]

Are you going to have another release of your plugin soon? Sounds like you're still working on it?

yes, i'm still working on it
i don't want to use the beta for my own tracks
now that i'm back at work again, it's kinda hard (12hours a day) .. got like 6 hours to rest (or work on the synth) and 6 hours to sleep..

the asymmetry (ah, i can't spell that damn word) would have it's effect on the sound ofcourse..
a square sounds like a square (only odd harmonics) because it's symmetric
any small change and you'll have different sound (and thus, it won't be a square anymore)

in the TB-303, it had to be a square, but it's more like a Pulse

[uvacom]

I don't really know the method by which the TB-303 generates its square wave (although whatever it is, it has a unique sound). But I think the idea behind which this is suggested is more like the negative cycle is "reversed" rather than flipped. Basically odd harmonics are produced by symmetry and even harmonics are produced by asymmetry. A square, a triangle, and a sine wave are all symmetric about the X axis and so only produce even harmonics. A sawtooth is perfectly asymmetric about the x axis, so it produces odd harmonics. But flip the negative cycle and it becomes completely symmetric...

That's interesting. It makes a lot more sense than what was in the wikipedia article (since changed, of course).

Yeah - of course I'm not implying at all that the TB-303 did that to achieve its square wave. Just that depending on the technology (a design built around 74 series logic devices, for example), it can be a *very* cheap and fairly efficient method to produce a waveform that does sound square-like.

[mistertoast]

Are you going to have another release of your plugin soon? Sounds like you're still working on it?

yes, i'm still working on it
i don't want to use the beta for my own tracks
now that i'm back at work again, it's kinda hard (12hours a day) .. got like 6 hours to rest (or work on the synth) and 6 hours to sleep..

the asymmetry (ah, i can't spell that damn word) would have it's effect on the sound ofcourse..
a square sounds like a square (only odd harmonics) because it's symmetric
any small change and you'll have different sound (and thus, it won't be a square anymore)

in the TB-303, it had to be a square, but it's more like a Pulse

You spelled "asymmetry" fine, but in your concern over that word you got the wrong "its."

I look forward to your next release, whenever it happens to be.

[mistertoast]

well, wikipedia can give you a simple idea about something, but if you want to really know the "truth" then you will want to dig deeper, and not blindly trust wikipedia
that's what i think i'll do from now on

Absolutely! I spent another enlightning day reading Hal Chamberlin's book (errm bible?) to figure out where wp is just a tad too, hmmm, lacking. I love wp though for all the things I don't know about.

You should look into Horowitz and Hill's Art of Electronics.

That's a great book. Any other relevant recommendations?

[Gamma-UT]

You should look into Horowitz and Hill's Art of Electronics.

Somehow I think Horowitz and Hill would say "don't do this". Having said that, it's not on the bad circuits pages and if you squint a bit and knock off a few decoupling capacitor, you can see the guts of the example LM311 comparator circuit from the relevant section of H&H in the TB's waveshaper section.

[jupiter8]

Now someone should edit the VCF section where it claims it has a 18 Db/oct, 3 pole filter where in fact it has a 4 pole, 24 Db/oct filter and reference this page: http://www.timstinchcombe.co.uk/synth/d ... diode.html

That'll sure raise some hell and the biggest geekfight since multiple OS:es became available to the masses.

Who dares to risk eternal damnation amongst the 303 anoraks ?

[Gamma-UT]

Now someone should edit the VCF section where it claims it has a 18 Db/oct, 3 pole filter where in fact it has a 4 pole, 24 Db/oct filter and reference this page: http://www.timstinchcombe.co.uk/synth/d ... diode.html

That'll sure raise some hell and the biggest geekfight since multiple OS:es became available to the masses.

Who dares to risk eternal damnation amongst the 303 anoraks ?

You don't need a licence to edit Wikipedia (yet). You could, um, just go and do it. We'll watch your coat.

Mind you, it is good to see that someone has actually tried to do a SPICE simulation of the circuit instead of poking at it with a stick which seems to be what happens on most 303 modder lists. I've not had a chance to read it but the explanation suggests that the Roland engineers didn't care much for the textbook which makes the idea of it being a three-pole filter a bit moot: it's a filter circuit that, when pushed and prodded makes the right noise. No wonder it's so hard to emulate these things in software: you need to run SPICE to get close to what's going on.

[jupiter8]

Now someone should edit the VCF section where it claims it has a 18 Db/oct, 3 pole filter where in fact it has a 4 pole, 24 Db/oct filter and reference this page: http://www.timstinchcombe.co.uk/synth/d ... diode.html

That'll sure raise some hell and the biggest geekfight since multiple OS:es became available to the masses.

Who dares to risk eternal damnation amongst the 303 anoraks ?

You don't need a licence to edit Wikipedia (yet). You could, um, just go and do it. We'll watch your coat.

Hell no. I believe the 303 mafia has a local chapter here in Gothenburg.
I dare not risk it. I'd rather sit on the side with a nice bag of popcorn watching hilarity ensue. I'm a wimp.

[Gamma-UT]

Now someone should edit the VCF section where it claims it has a 18 Db/oct, 3 pole filter where in fact it has a 4 pole, 24 Db/oct filter and reference this page: http://www.timstinchcombe.co.uk/synth/d ... diode.html

That'll sure raise some hell and the biggest geekfight since multiple OS:es became available to the masses.

Who dares to risk eternal damnation amongst the 303 anoraks ?

You don't need a licence to edit Wikipedia (yet). You could, um, just go and do it. We'll watch your coat.

Hell no. I believe the 303 mafia has a local chapter here in Gothenburg.
I dare not risk it. I'd rather sit on the side with a nice bag of popcorn watching hilarity ensue. I'm a wimp.

Yeah, but you're already a marked man: you've pointed to heresy on a popular web forum. OK, they don't come here much cos it's mostly software, but someone's gonna squeal.

[brambos]