The colorimetric space HSI offers numerous advantages in the analysis and treatment of colour images (H and S attributes insensitive to variations of light setting, perception of colours analogous to those of the eye, robust chromatic segmentations which are well adapted to real images). In the meantime, cameras and other image captors deliver the standard RGB signals and the RGBHSI transformations at the rate of video is still far from being mastered (incomplete transfer of space, loss of meaning of the attributes H and S in some zones, aberrations owing to the quantification of the signals upstream). To attempt to resolve some of these problems, we have chosen an unconventional method of transformation which results in the development of a mixed electronic structure (analog-numeric). All the functions (except the trigonometrical relations) necessary for the conversion are sustained by the broad band analog operators (linear functions, minima tn-signals detection, normalization). In this way, the digitalization does not appear until the end of the treatment determining the hue. Results of this method are only limited by the signal/noise ratio of the RGB signals on entrance. The improvements obtained in relation to completely digital methods are on one hand, a considerable increase in the chromatic zone of the HSI space; on the other hand, the chromatic/achromatic transition presents improved continuity allowing acceptable colour differentiations even at low level. Lastly, a final format of 3*8 bits in HSI offers a total number of discernible colours analogous to those obtained by an identical quantification of the RGB cube.