HTPC Blu-Ray essentials

Page 2: resolution and refresh rates

what displays do we watch Blu-Ray movies on?

Blu-Ray movies are encoded in 1920x1080 pixels. This very high resolution cries for big displays, ideally even for front projection. Only that way you can get the full benefit of the high resolution.

Of course it's also fun to watch Blu-Ray movies on smaller computer monitors. But we should concentrate on what Blu-Ray is really meant for: Big consumer displays like LCD TVs, Plasmas, rear projection, front projection with a size of at least 40" (the bigger the better). So reviews should primarily be done on such displays, if they want to accurately evaluate the quality of the Blu-Ray playback experience.

optimal graphics card output mode

Blu-Ray movies are stored on the disc in a resolution of 1920x1080 pixels with 23.976 frames per second (to be exact it's 24/1.001). Obviously a perfect playback of Blu-Ray movies should be done in the exact same resolution and frame rate to avoid any conversion artifacts. The connection format between the graphics card and the display should consequently be 1920x1080 with 23.976 Hz (this is usually called "1080p24"). Sadly some older displays don't support this connection format.

alternative output modes

Some front projectors don't accept 1080p24 but 1080p48 instead, which means that each movie frame is simply shown twice. Alternatively the graphics card could also output 1080p60. However, you can do the math yourself. When the graphics card outputs 60 frames per second (60/1.001 to be exact) while there are 24 frames per second stored on the Blu-Ray disc, the motion will not be totally smooth, anymore, because 24 and 60 simply do not match that well.

European Blu-Ray content can be stored on the Blu-Ray disc in 1080i50. So a graphics card fit for Blu-Ray playback should also support 1080p25 and 1080p50 output modes, because only that allows smooth playback of such content.

displays with lower physical resolution

Newer displays perfectly match the Blu-Ray resolution of 1920x1080. However, there are many older HD displays out there which have a lower physical resolution. Common resolutions are e.g. 1280x720, 1366x768 and 1080x768.

Some of these displays can still accept 1080p24, although they have a different physical resolution. They will then internally resize the incoming 1080p24 signal so that it matches the physical resolution of the display.

Unfortunately some displays do not accept 1080p24. So the graphics card should also be able to output 1280x720 (which is the "official" smaller HDTV resolution) in all the refresh rates mentioned above.

"EDID"

Most displays are able to communicate which formats they accept. They offer a small record of information which source devices (e.g. the graphics card) can read. This information record is called "EDID". The problem is that the EDID is not always complete, or sometimes even plain wrong. So while it's nice if a graphics card makes use of the EDID information, it's even nicer if the graphics card doesn't depend on that the EDID is both correct and complete.

There's another problem which needs mentioning: The EDID block has room for two custom resolutions with full timing information. Unfortunately the pixel clock information is limited to 2 decimal digits, which is not enough to correctly specify the pixel clock needed for 24/1.001Hz or 60/1.001Hz. So the timing information in the EDID block should be disregarded by the graphics card. The CE devices (e.g. Blu-Ray players) do that. They simply use the standard timing for 1080p24 and 1080p60 output instead of the timing promoted by the EDID block. The graphics card needs to do that, too, or else we won't get smooth motion with 24p and 60p output modes.

ATI rating

At this point in time ATI mostly follows what the EDID sais. Resolutions not promoted by the EDID are not offered for selection. Timings listed in the EDID are followed to the last letter. There is an option in the control panel to activate some resolutions which are not listed in the EDID. But the list of modes is badly chosen. Nobody who has any experience with HTPCs uses interlaced output because it just doesn't work well in the HTPC world. So the interlaced output modes offered by ATI are not really all that helpful. The only helpful modes are 720p50, 720p60 and 1080p60. Where are all the other modes? What about 1080p24 and 1080p50 at least, which are absolutely essential modes? And while we're at it, ATIs interpretation of 24p and 60p is totally incorrect. ATI actually uses 24.000Hz and 60.000Hz. Hello!? Has nobody at ATI heard that TV, DVD and Blu-Ray all use 24/1.001 respectively 60/1.001 frames per second??

Here's a list of things wrong with ATI's current implementation:

• It's not possible to activate several essential output modes if the EDID is incomplete.
• The refresh rate for 24p and 60p is flat out wrong (1.001 divisor missing).
• EDID pixel clock information is used which is known to be bad. Standard timings (see EIA/CEA-861-B specification) should be used instead.

ATI scores 20 points for 720p50, 16 points for 1080p50 and 720p25 and 4 points for 1080p25 support. Unfortunately no scores for any 24p, 30p or 60p mode because the timing is all wrong.

NVidia rating

NVidia cards make use of the EDID information. But in addition to that latest drivers allow creation of custom resolutions with detailed timing information, which is a very good thing. But (AFAIK) if the EDID already lists some modes you can not redefine those by using the custom resolutions. So the custom resolution functionality is only useful for resolutions which are missing. Which means that it's still essential for NVidia to get those modes right which are listed in the EDID. Here it's especially important that the pixel clock information listed in the EDID is not used. Unfortunately NVidia falls short of our expectations in this case. Here's where there's still a need for improvement with NVidia's drivers:

• It's possible to manually create missing output modes. However, most end users will not have enough knowledge to do this correctly. Especially they will lack the information needed to create the missing modes with the correct standard timing. So it would be very useful if NVidia would make all the important standard modes with proper timings available without having to manually create them.
• EDID pixel clock information is used which is known to be bad. Standard timings (see EIA/CEA-861-B specification) should be used instead.
• There's no information available for 3rd party software to tweak NVidia graphics cards. E.g. the well known utility PowerStrip does not work with NVidia cards due to lack of documentation.

It's a bit tough to evaluate NVidia's performance cause theoretically you can create all necessary custom resolutions by hand. However, this method fails to work for modes listed by the EDID. So if we take a usual EDID block of a good display (e.g. Pioneer Kuro), NVidia scores 20 points for 1080p50 and 720p50 and 5 points for 1080p25 and 720p25. Unfortunately we cannot give any points for 24p and 60p modes listed by the EDID cause NVidia is using incorrect timings (pixel clock) for those. However, some modes are usually not listed by EDID blocks, so we can give 5 points each for 1080p30, 1080p48, 720p24, 720p30 and 720p48.
 

rating guidelines:

If the output mode is supported only if the EDID lists it, the score is reduced by 20%.

The first 3 digits of the (rounded) frame rate must be correct. Otherwise no points are given.