Luma (video) - Rec. 601 Luma Versus Rec. 709 Luma Coefficients

Rec. 601 Luma Versus Rec. 709 Luma Coefficients

For digital formats following CCIR 601 (i.e. most digital standard definition formats), luma is calculated with the formula Y' = 0.299 R' + 0.587 G' + 0.114 B'. Formats following ITU-R Recommendation BT. 709 use the formula Y' = 0.2126 R' + 0.7152 G' + 0.0722 B'. Modern HDTV systems use the 709 coefficients, while transitional 1035i HDTV formats may use the SMPTE 240M coefficients (Y' = 0.212 R' + 0.701 G' + 0.087 B'). These coefficients correspond to the SMPTE RP 145 primaries (also known as "SMPTE C") in use at the time the standard was created.

The change in the luma coefficients is to provide the "theoretically correct" coefficients that reflect the corresponding standard chromaticities ('colors') of the primaries red, green, and blue. However, there is some controversy regarding this decision. The difference in luma coefficients requires that component signals must be converted between Rec. 601 and Rec. 709 to provide accurate colors. In consumer equipment, the matrix required to perform this conversion may be omitted (to reduce cost), resulting in inaccurate color.

As well, the Rec. 709 luma coefficients may not necessarily provide better performance. Because of the difference between luma and luminance, luma does not exactly represent the luminance in an image. As a result, errors in chroma can affect luminance. Luma alone does not perfectly represent luminance; accurate luminance requires both accurate luma and chroma. Hence, errors in chroma "bleed" into the luminance of an image.

Due to the widespread usage of chroma subsampling, 'errors' in chroma typically occur when it is lowered in resolution/bandwidth. This lowered bandwidth, coupled with high frequency chroma components, can cause visible errors in luminance. An example of a high frequency chroma component would be the line between the green and magenta bars of the SMPTE color bars test pattern. Error in luminance can be seen as a dark band that occurs in this area.