The colour television system used in the US from 1954 until digital conversion in the 2000s. The system needed to be compatible with the existing monochrome television transmissions, i.e. the colour transmissions were required to be receivable as black-and-white pictures on a monochrome receiver and allow reception of black-and-white pictures on a colour receiver. In the NTSC system colour information is encoded into the video signal in such a way that the transmissions occupy the same bandwidth as the existing black-and-white transmissions. The system was susceptible to variable colour quality attributed to small changes of differential gain and differential phase within the transmission chain. This problem was overcome in the European PAL colour television system.