UHD TVs with Subpixel Rendering May have Certification Issues

Consumer Reports Shapr Quattron Plus Zone plate result

Display Metrology –The CEA recently announced a new logo program for UHD TVs. Like most logo or certification programs, one of the criteria is a resolution of 3840 x 2160. For UHD TVs with a traditional RGB stripe pixel architecture that should not be a problem, but many sets now offer non-RGB-stripe pixel architectures where pixels are created with subpixel rendering algorithms. It is not a simple matter to draw a line around a set of sub-pixels in this case to count the pixels. So how will resolution be measured and will this create problems for TV makers?

For example, Sharp’s UHD TVs with RGBY pixel architecture, as well as Samsung’s and LG’s TVs with RGBW subpixel architectures, are all non-traditional RGB stripe pixels that use subpixel rendering.

First of all, CEA’s new logo program drops the insistence upon using the term Ultra HD and now adopts the terminology “4K Ultra HD” – an acknowledgement that 4K is becoming synonymous with Ultra HD (even though the resolutions are different of course). The use of the logo will be voluntary, but to get the 4K Ultra HD logo, sets must include at least 3840 x 2160 resolution, have the ability to upscale content, offer 24-bit color images and display UHD content at 60 fps in the Rec 709 color space. The 4K Ultra HD Connected logo has the same requirements with the addition of the ability to decode HEVC, receive and display multichannel video (quad viewer capability), receive IP-delivered UltraHD content through WiFi, Ethernet or other appropriate connection, and support this delivery through apps and services on the manufacturer’s chosen platform.

Readers should note that the UHD-1 specification developed by the EBU calls for UHD content to support 30-bit color and the BT 2020 color gamut. The CEA Logo program will set the bar much lower allowing 24-bit content (which many feel is inadequate) and the Rec 709 color gamut (even though most sets will be able to display content much beyond this color space).

But the question of resolution remains. CEA sources noted that sets that do not meet the horizontal and vertical resolution methods using subpixel rendering will not be allowed to use the logo. But will anyone measure or validate the specs?

Other industry sources suggest that CEA is considering a self-certification process with a neutral third party asked to perform adjudication should a complaint be lodged about non-conformance. My sources suggest that this is still an idea in-process and any agreement will have to be approved by the CEA’s Video Division Board.

Lodging of complaints could come soon if this policy is adopted. For example, we recently read a Consumer Report review of Sharp’s Quattron Plus TV. This TV uses the RGBY stripe format with subpixel rendering to deliver what they say is an image with 3840 x 2160 pixel resolution. I am sure Sharp will want this TV to carry the 4K Ultra HD logo, but does it meet the resolution criteria?

In the Sharp Quattron Plus TV, the pixel architecture is 1920 x RGBY x 2160. If sent UHD content, the processing engine will use the RGBY subpixels to create two luminance peaks – one with the red and green subpixels and the other with the blue and yellow subpixels, to effectively double the horizontal resolution to 3840. In the testing that Consumer Reports did, they were able to verify the reproduction of the horizontal pixels using the subpixel rendering algorithm. So far so good. But they did not verify the vertical resolution of 2160 lines. Instead, they measured about 1080 lines of resolution (see zone plate measurement in photo). As a result, they concluded the TV did not perform like a UHD TV.

The moire in the vertical direction indicates problems in displaying 2160 rows of content at full resolution

We were baffled by this result as Sharp claims it has 2160 addressable rows. So, we contacted Sharp to ask them about this result. The answer? The TV should be set to Mode 2 to get the full vertical resolution. It turns out Consumer Reports set the TV to Mode 1 for the testing as that was the default setting and they had not seen much difference between Mode 1 and Mode 2.

Consumer Reports was not able to retest the set to verify the desired result, but Claudio Ciacci, Project Leader, Technical Div., Electronics, made a good point – Why would Sharp make Mode 1 the default to deliver only 1080 lines of resolution when they can deliver 2160? Good question.

Subpixel rendering expert Candice Brown Elliot says that the best way to confirm the display’s ability to deliver the claimed resolution is to use the tests developed by ICDM. Section 7.8 titled, Resolution from Contrast Modulation, described the technique. This section also details the difference between addressability and resolution.

“Addressability refers to the number of (complete) pixels that can be separately and adequately controlled. Resolution refers to how well those pixels can appear separate and distinct to the eye. Describing resolution with simple numbers, such as 1920 x 1200 pixels, is an approximation to a complicated subject. We define resolution here as the number of alternate black and white lines that can be displayed with a stated minimum contrast modulation (Michelson contrast), the threshold contrast modulation CT. If the display fails to meet this criterion for a specified addressability, then the addressability is not the same as resolution in describing the display – the actual resolution would be lower than the addressability”.

This is a topic we will have a lot more to talk about in the coming months and years. – Chris Chinnock