The VF-11 is an 11-band analog vocoder from German company MAM (Music And More). A vocoder lets you create robotic vocal effects. You basically feed two sounds into it, the sound you want to process (Synthesis Input) such as a synth lead or synth pad, and the sound that will do the vocoding (Analysis Input) such as your voice or a drum loop.
The Analysis input has mono Mic and Line inputs. The signal you plug in here will be separated into 11-bands which cover a frequency range of 50Hz to 12kHz. The unique frequency spectrum of the analysis input sound will have different amplitudes at each of the 11 analysis bands. So a low frequency specific sound will generate lots of voltages in the low frequency bands, but very little to no voltages among the higher frequency analysis bands. The amount of voltages generated by these bands depends entirely on the harmonic content of the Analysis sound input.