Luminance and chrominance are combined using one of the coding standards — PAL, NTSC and SECAM — to make composite video. The process, which is an analogue form of video compression, restricts the bandwidths (image detail) of components.
Chrominance is added to the luminance using a visually acceptable technique but it becomes difficult, if not impossible, to accurately reverse the process (decode) into pure luminance and chrominance. This can cause problems, especially in post production.
Kodak: A Concise Glossary for the Non-Technician
Pank, B., Editor, 1998, The Digital Factbook, 9th Ed, Quantel, Newbury, UK