Converting live vocals to discrete signals
Confusing question. You list vocals and Digital Instruments, then you ask which audio device provided pre-processing?

There are the auto tunes boxes you mentioned. Since you know about them I don't see how you would be asking about them.

A digital instrument might output midi, and midi is just a simple serial protocol, so there is no device needed.

So what is your question actually?

An auto-tune box does work that would be best suited to a laptop or desktop computer. It first must determine the intended pitch than apply a formant filter to shift the incoming signal to a correct pitch. It requires the use of heavy fast math, Fast Fourier Transforms, FFT.  The smaller boxes you see doing this are actually specialized computers. Do not confuse their size with their power.

Google 'digital tuner software' to get an idea of what goes on.  I do not know of a device that specifically output the 'corrected' midi for an auto-tune, but at that point it would be fairly trivial so it would not surprise me if many did it.  For sure the many guitar to Midi converts do just that, but a good one is several thousand dollars.