We rebuilt the TB-3 mk. II from scratch starting from the original synth schematics and circuit diagrams. We created hundreds of samples of the original TB-303 hardware along with more samples from a modified version of the x0xb0x so that we could study every nuance and characteristic of the true sound.
It starts with the oscillator signal which at first seem like a very standard saw and square wave, but while traveling down the circuitry, get transformed into very frequency dependent shapes. From there the signal enters the filter which is special resonant 4-pole design with an 18dB frequency response.
The most difficult part was calibrating the complex interactions of knob controls. For example, on accented notes, high resonance values actually soften the filter envelope while accent affects both overall amplitude and cutoff frequency. Just subtle difference in knob positions can dramatically change the sound.
Note the switch, in the top-right of that pic, which lets you toggle back to the old synth engine. This is for maintaining backwards compatibility, as well as allowing it to run smoothly on older hardware. RhythmStudio's developer, Pulse Code, has always kept an eye on supporting older iDevices and I really respect that.