A display apparatus comprising: an input device configured to receive a display signal comprising a video signal of N bits per colour, a graphic signal of M bits per colour, and an alpha value; a calculator configured to calculate effective bits of data consisting of effective bits of the video signal and effective bits of the graphic signal, the effective bits of data being used to perform alpha-blending based on an alpha value of each pixel from the display signal, wherein a number of bits per colour of the effective bits of data is less than M + N; an alpha-blender configured to receive only the effective bits of data from the calculator, and to perform alpha-blending on the effective bits of data calculated by the calculator and the alpha value and output an alpha-blended display signal; and a display configured to display an image generated according to the alpha-blended display signal, wherein the effective bits of the video signal consists of a number of bits per colour of the video signal in most significant bit order, starting with the most significant bit, and the calculator is configured to decrease the number of bits per colour of the video signal that are included in the effective bits of the video signal as the alpha value increases; and wherein the effective bits of the graphic signal consists of a number of bits per colour of the graphic signal in most significant bit order, starting with the most significant bit, and the calculator is configured to increase the number of bits per colour of the graphic signal that are included in the effective bits of the graphic signal as the alpha value increases. an input device configured to receive a display signal comprising a video signal of N bits per colour, a graphic signal of M bits per colour, and an alpha value; a calculator configured to calculate effective bits of data consisting of effective bits of the video signal and effective bits of the graphic signal, the effective bits of data being used to perform alpha-blending based on an alpha value of each pixel from the display signal, wherein a number of bits per colour of the effective bits of data is less than M + N; an alpha-blender configured to receive only the effective bits of data from the calculator, and to perform alpha-blending on the effective bits of data calculated by the calculator and the alpha value and output an alpha-blended display signal; and a display configured to display an image generated according to the alpha-blended display signal, wherein the effective bits of the video signal consists of a number of bits per colour of the video signal in most significant bit order, starting with the most significant bit, and the calculator is configured to decrease the number of bits per colour of the video signal that are included in the effective bits of the video signal as the alpha value increases; and wherein the effective bits of the graphic signal consists of a number of bits per colour of the graphic signal in most significant bit order, starting with the most significant bit, and the calculator is configured to increase the number of bits per colour of the graphic signal that are included in the effective bits of the graphic signal as the alpha value increases.