¹²CO (J:1–0) frequency SM observation obtained at the IRAM 30-m telescope with the EMIR receiver and FTS backend. Top: standard folded spectrum after subtraction and calibration. Middle: original two phases calibrated independently. Bottom: two phases directly averaged after realignment without using noise-averaged gain or subtracting a baseline from the raw data. The baseline shows no ripples but the noise is three times higher than in the upper panel. The vertical axis has the same amplitude for all three boxes (16 K). The color of the rms figure corresponds to the color of the spectra to which it pertains.