SECRETS Blu-ray Player HDMI Benchmark

The Secrets Blu-ray Player HDMI Benchmark - Part I

ARTICLE INDEX

Introduction

It's hard to believe that we are already into the second decade of the 21st Century, and all future Blu-ray players will have to deliver their HD content digitally over HDMI only, with analog component video outputs limited to 480 lines of resolution (480p). Since all of the content will be transferred digitally to your display, this should result in perfect picture quality with no issues since "bits are bits", right?  Unfortunately the reality is far more complex than this, and there are many factors that can come into play.

This article is an introduction to the way we will be changing how we benchmark Blu-ray players, since we will be taking measurements of the digital signals through the HDMI output instead of analog data measurements through the component video outputs. There is a lot to discuss, so start reading now, and you won't be confused when we begin publishing Blu-ray Player Benchmark Reviews with our new methods shortly.

In the current article, we will lay out what these factors are, how we are going to test for them, and provide results showing what this data will look like. The reason that "bits are bits" doesn't apply for Blu-ray is that the storage medium and our display devices work in different languages. HDMI will transfer all of this data between the devices correctly, but how the devices and displays handle that data is what we are investigating.

All current display devices use RGB to display color on the screen. White is the sum of all those colors, and black is the absence of all that color, with the remaining spectrum coming in-between. The easy way to store this on Blu-ray would be in RGB as well, but unfortunately Blu-ray isn't close to large enough to save raw RGB data for a whole film to a disc. Because of this, data is stored on Blu-ray in YCbCr format using compression.

The compression that Blu-ray uses is very effective because it values information that we can observe more easily – black and white (luma) over color information (chroma), which the eye isn't as sensitive to. Blu-ray saves all of the luma data while disregarding much of the chroma information to save space, and then compressing the data. Additional information on this storage format can be found here: http://en.wikipedia.org/wiki/Chroma_subsampling. This difference in how data is stored compared to how it is are displayed leads to the two main issues that we can run into with Blu-ray players and that we are going to investigate and report on.

Color Conversion: As no consumer display can handle the YCbCr 4:2:0 data that is stored on a Blu-ray disc, they must first be converted to another colorspace. The colorspaces that you will see on consumer equipment are 4:2:2, 4:4:4, and RGB. While in theory this should be a conversion that everyone should do correctly, there is the ability to make a mistake and have data remap incorrectly in the process.

Color Space Handling: If you have looked at our recent receiver and processor reviews, you have seen that we have started to grade them on how they handle certain color spaces. While ideally every component would be able to handle 4:2:2, 4:4:4 and RGB without issue, some components cannot and lose color data or resolution along the way. Additionally, we have found that some consumer displays might only display one or two of the three formats correctly, and so unless all of your source components can display that format, you will never get the full resolution out of your sources.

Because of these color space issues, we believe that all Blu-ray players should offer the option for the user to select between 4:2:2, 4:4:4, or RGB color spaces for output and that it should do the color conversion to these from 4:2:0 correctly.

Additionally, players should offer a Source Direct mode that outputs the data as closely as possible to how it is displayed on the disc. In Source Direct, the only alteration made to the data is a conversion from 4:2:0 color space to 4:2:2, but no adjustment in resolution or anything else. In this case, as the video output is bypassing the image processing of the Blu-ray player, any color errors introduced are often the fault of a hardware component and are often not able to be fixed in a software update since it is a hardware flaw.