This paper deals with the possibility of recovering the information contained in a spectrogram, in the case of line widths larger than the mean period of the granulation noise of the photographic plate. Suppose the spectrogram to be known by means of its microphotometric one-dimensional record, e.g. x(t), where x is the transmittance of the plate at abscissa (wavelength) t. The measurement of the position and shape of a line of the spectrogram, easy when the amplitude of the line exceeds the granulation noise, becomes very hard to do if the S/N ratio approaches unity. Nevertheless, as the power spectrum of the noise has normally a bandwidth larger than the spectrum of the signal, it is possible to reject all the noise power beyond the highest frequency in the signal spectrum, by means of suitable filters. This technique does not lose any useful information, since by definition the information is the signal, while on the contrary it betters the S/N ratio by diminishing the noise power. Of course, as both the signal and noise spectra are continuous, it is not practically possible to perform the filtering without signal losses. However, the technique may be advantageous in all cases, such as highly rotating stars, in which the spectrum of the signal has a cutoff much sharper than the noise cutoff. A simple criterion of applicability is the base width of the lines: if the base width is larger than the granulation noise period, the filtering may result in an improvement of the S/N ratio.