The Midac internal function corresponds to the top diagram which is the conventional approach to FT-IR spectroscopy. The transfer functions have been measured from the detector elements through to the ADC inputs. The new approach is to bypass these modules from the original instrument and substitute external ADC's and DSP. The signal processing is somewhat different in that both the laser and IR channels are digitized and recorded simultaneously. In conventional spectrometers the laser signal is used only to trigger sampling of the IR channel; the IR is then the only signal recorded.