HDMI - video levels and color space
Posted: Sat Jun 18, 2005 10:08 am
HDMI is derived from DVI which originated from the computer world. There are three standards for HDMI digital video streams.
RGB 4:4:4 sampling rate
YCbCr 4:4:4 sampling rate
YCbCr 4:2:2 sampling rate
Any display that accepts YPbPr analog component is supposed to be setup for YCbCr HDMI at either encoding rate. That means when you connect your HDMI source to your HDMI input the source is supposed to provide YCbCr rather than RGB since in most cases the display has component video inputs.
RGB is based on DVI-PC for your computer display and uses a video range of 0-255
RGB can also be DVI-video for your home theater and uses a range of 16-235.
YCbCr is based on DVI-video for video content and uses a video, Y, range of 16-235 and a color, Cb and Cr, range of 16-240. It also can use the color space of SD or HD depending on the selected SD or HD scan rate.
The correct output type and levels must be set correctly by the manufacturer for the intended application of the product whether source or display. If not you will have problems such as crushed blacks or crushed peak whites or the opposite of low peak light output and washed out blacks or in the case of color space the wrong colors. While you may be able to compensate for these errors using the display controls in practice there will likely be a penalty in imaging by doing so. The best alternative is that the display decoding is setup properly for the two different types and provides a manual control to select RGB or YCbCr digital encoding for the HDMI input in case there is a failure of the source to properly identify the HDMI type.
Another potential problem is color decoding. SD and HD video uses two different standards because HD provides a wider color space. This means HD has a slightly deeper green, red and blue primary than SD provides. When HDMI uses the expected YCbCr encoding it must also use the correct color space based on whether the source is HD, ITU Rec. 709, or SD, ITU Rec. 601. For HD only sources such as satellite and cable receivers this presents a problem for SD that is being converted to HD scan rates such as 720P and 1080I. For DVD players it is a problem because when the player is set for 480I or 480P the display will use SD color space and when it is set for 720P or 1080I the display will use HD color space. On one hand this would seem correct as it is following the specifications. In reality you will get the wrong color. The only way to correct this error is for the display to provide a manual selection of SD or HD color space which is not common.
Improper color space selection results in improper color saturation and tint. Will you notice? Hard to say as the perception will be viewer dependent without a reference point but if you are making comparisons of products this is critical to your evaluation.
HDMI adds yet another dicey consideration if you are using a DVI adaptor. In this case the HDMI source is supposed to detect this fact and provide RGB encoding from the native YCbCr that is provided on a DVD movie for DVI since DVI is for the most part RGB for displays. This brings us back to a potential color space problem if the player is not setup to output the ITU Rec. 601 standard for SD sources regardless of the selected output scan rate. In this case outputting the HD ITU Rec. 709 color space when selecting 720P or 1080I output would be an error on the part of the source since color decoding is a process that occurs prior to any RGB delivery. For a display to compensate for this error would require it convert the incoming RGB to YcbCr, make the necessary color space correction and convert it back to RGB for the final stage of driving the imaging technology. Based on specifications the display has no reason to provide this capability.
RGB 4:4:4 sampling rate
YCbCr 4:4:4 sampling rate
YCbCr 4:2:2 sampling rate
Any display that accepts YPbPr analog component is supposed to be setup for YCbCr HDMI at either encoding rate. That means when you connect your HDMI source to your HDMI input the source is supposed to provide YCbCr rather than RGB since in most cases the display has component video inputs.
RGB is based on DVI-PC for your computer display and uses a video range of 0-255
RGB can also be DVI-video for your home theater and uses a range of 16-235.
YCbCr is based on DVI-video for video content and uses a video, Y, range of 16-235 and a color, Cb and Cr, range of 16-240. It also can use the color space of SD or HD depending on the selected SD or HD scan rate.
The correct output type and levels must be set correctly by the manufacturer for the intended application of the product whether source or display. If not you will have problems such as crushed blacks or crushed peak whites or the opposite of low peak light output and washed out blacks or in the case of color space the wrong colors. While you may be able to compensate for these errors using the display controls in practice there will likely be a penalty in imaging by doing so. The best alternative is that the display decoding is setup properly for the two different types and provides a manual control to select RGB or YCbCr digital encoding for the HDMI input in case there is a failure of the source to properly identify the HDMI type.
Another potential problem is color decoding. SD and HD video uses two different standards because HD provides a wider color space. This means HD has a slightly deeper green, red and blue primary than SD provides. When HDMI uses the expected YCbCr encoding it must also use the correct color space based on whether the source is HD, ITU Rec. 709, or SD, ITU Rec. 601. For HD only sources such as satellite and cable receivers this presents a problem for SD that is being converted to HD scan rates such as 720P and 1080I. For DVD players it is a problem because when the player is set for 480I or 480P the display will use SD color space and when it is set for 720P or 1080I the display will use HD color space. On one hand this would seem correct as it is following the specifications. In reality you will get the wrong color. The only way to correct this error is for the display to provide a manual selection of SD or HD color space which is not common.
Improper color space selection results in improper color saturation and tint. Will you notice? Hard to say as the perception will be viewer dependent without a reference point but if you are making comparisons of products this is critical to your evaluation.
HDMI adds yet another dicey consideration if you are using a DVI adaptor. In this case the HDMI source is supposed to detect this fact and provide RGB encoding from the native YCbCr that is provided on a DVD movie for DVI since DVI is for the most part RGB for displays. This brings us back to a potential color space problem if the player is not setup to output the ITU Rec. 601 standard for SD sources regardless of the selected output scan rate. In this case outputting the HD ITU Rec. 709 color space when selecting 720P or 1080I output would be an error on the part of the source since color decoding is a process that occurs prior to any RGB delivery. For a display to compensate for this error would require it convert the incoming RGB to YcbCr, make the necessary color space correction and convert it back to RGB for the final stage of driving the imaging technology. Based on specifications the display has no reason to provide this capability.