HDMI 1.3
Dear Gary:
There seems to be a lot of information regarding the new HDMI 1.3 standard and display devices that will be released with this standard in the near future. When it comes to picture quality, would an HDMI 1.3 display have a better picture than an HDMI 1.2 display using next-generation HDMI 1.3 Blu-ray Disc and HD DVD formats? If so, why?
Mark Hoepfl, Singapore
mailto:mark.hoepfl@panalpina.com
Video Technical Editor Greg Rogers Comments:
All versions of the HDMI standard include 165 Mpixels/sec transmission over single-link cables, which is sufficient to transport 1080p60 video. Although some chip manufacturers produce HDMI (and DVI) parts that are incapable of that pixel rate, there are plenty of HDMI parts that are capable of 1080p60 video rates for equipment manufacturers that choose to provide those signals.
The HDMI standards prior to version 1.3 provide RGB 4:4:4, YCbCr 4:4:4 and YCbCr 4:2:2 formats. The 4:4:4 formats were limited to 24-bits-per-pixel (8-bits-per-component). The YCbCr 4:2:2 format allows 8-bit, 10-bit, or 12-bit components, because the Cb and Cr color components are only sent with every other luma (Y) pixel. Hence, the 165 Mpixel/sec rate is still sufficient for 1080p60 YCbCr 4:2:2 video with 12-bit components.
The new HDMI 1.3 standard extends the bit-depth of the RGB 4:4:4 and YCbCr 4:4:4 formats to 30-, 36-, and 48-bits-per-pixel (10-, 12-, and 16 bits-per-component). Color depths greater than 24 bits are called Deep Color modes. The Deep Color modes are an optional feature, and do not need to be supported by HDMI 1.3-compliant devices. However, each Deep Color mode that is supported must include RGB 4:4:4, and optionally may include YCbCr 4:4:4. The Deep Color modes do not affect the existing YCbCr 4:2:2 formats.
The HDMI clock rate is increased to transport the Deep Color formats with more than 24 bits/pixel. The clock rate increases by the ratio of the pixel bit depth to 24 bits, i.e. the clock rate doubles to transport 48-bit Deep Color video.
All existing consumer digital video sources are encoded using 8-bit per component YCbCr video. That includes broadcast and satellite sources, as well as standard-definition DVDs and high-definition optical discs. Therefore, the HDMI 1.3 Deep Color modes provide no advantage when transporting consumer video in its original format between products.
However, when video is processed, such as scaling or gamma correction, it is necessary to perform the mathematical operations with additional precision, and it is then highly advantageous to maintain 10- or 12-bit component values all the way to the final display engine. Prior to HDMI 1.3, that was only possible by using the YCbCr 4:2:2 format to transport video from an external processor to a display.
Now with HDMI 1.3, an external processor can convert incoming YCbCr 4:2:2 signals to YCbCr 4:4:4, perform scaling and gamma correction in the 4:4:4 format, and then transport the results to the display in the RGB or YCbCr 4:4:4 format with more than 8-bits-per-component. Of course, in order to achieve the desired image quality improvements, the display must also perform 4:4:4 video processing and imaging at the higher bit-depths. Consequently, real image quality improvements will not be achieved until all of the processing and display products in the video chain are able to take advantage of higher bit-depth 4:4:4 video signals. When that happens, the HDMI 1.3 Deep Color modes will eventually provide a significant video quality advantage over previous HDMI standards.
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