The emergence of HEVC and 10-bit colour formats

In case you haven’t seen the latest headlines, Imagination recently announced PowerVR D5500, the world’s first multi-standard 10-bit video decoder that is able to handle the H.265 (HEVC) standard at 4K (Ultra HD) resolution. H.265, which was officially ratified earlier this year, aims to provide roughly double the data compression of the H.264 standard currently employed for digital HDTV broadcasting.

HEVC is set to be rapidly adopted and deployed by manufacturers and content providers in direct relation to 4K distribution; for example, Italy wants all DVB-T2 tuners to provide H.265 / HEVC support from 2014.

A breakdown of how H.265 (HEVC) works

The H.265 video coding divides a frame into small blocks (referred to as ‘CU’ – or ‘coding units’) and the prediction and transformation coding of the images is conducted within each CU.

HEVC offers variable blocks that can handle up to 64×64 pixels, changing the size according to texture, while the previous generation H.264 standard relied on a fixed macro-block size that has a maximum of 16×16 pixels.

H.264 vs. H.265 (HEVC): larger block sizes enhance encoding efficiency*H.264 vs. H.265 (HEVC): larger block sizes enhance encoding efficiency*

This allows HEVC to achieve higher compression or higher resolution and improve parallel processing efficiency compared to H.264/MPEG-4 AVC. To get a better understanding of the differences between MPEG-2, H.264 and H.265, you can find a comparison of the tools employed by each standard in the table below:

MPEG-2, HEVC and H.264 tools comparison

ITU-R BT.2020: 10-bit colours become the norm

The recently announced standard for 4K (and, believe it or not, 8K) TV called ITU-R BT.2020 makes recommendations for a wider selection of colours and increased bit-depth, which will result in noticeably better pictures on TV. Although BT.2020 is still under development, the advantages it offers will drive a fast, widespread adoption of 10-bit colours in the 4K TV and HEVC space.

4K needs 10-bit colours

According to ITU-R BT.1361 and BT.2020 recommendations, H.265 (HEVC) transcoding at 4K resolutions needs 10-bit colours

To get some idea on what this means, the current HDTV Rec. 709 standard specifies the following X-Y coordinates for red, green and blue:

R(ed) = (0.640, 0.330); G(reen) = (0.300, 0.600); B(lue) = (0.150, 0.060)

These are specific coordinates for the locations of those colours, as depicted by the smaller triangle on the chart below. The colour space described by these coordinates is reasonable but certainly not as realistic as we experience in the real world; anything outside the triangle is not guaranteed to be captured by the camera, encoded on the disc, or accurately shown on your TV.

HEVC CIE BT 2020

A visual representation of Rec. 709 and BT.2020 standards**

Contrast this with BT.2020 that recommends coordinates of

R = (0.708, 0.292); G = (0.170, 0.797); B = (0.131, 0.046)

These coordinates describe the larger triangle on the chart; this extends the range of colours to be captured and rendered on display devices.

The second aspect about how BT.2020 defines colour is bit depth. The number of bits used to represent each colour determines the range of shades one can generate. Current systems use 8 bits per colour, resulting in 28=256 different shades for each primary colour (red, green and blue, respectively). The current Rec. 709 standard can support a total of 16.78 million colours (256 R x 256 G x 256 B).

BT.2020 expands this range significantly by defining a minimum 10-bit colour depth. By adding another two bits to each colour representation, the number of overall possibilities increases to 1.07 billion (230=1024 x 1024 x 1024). This extended range has numerous advantages when representing a greater dynamic range between the lightest and darkest areas of a frame.

Our PowerVR video IP has been architecturally designed to make optimal use of 10-bit precision. Cores such as the PowerVR D4500MP and E4500MP enable increased colour depth, providing up to 10 bits of precision, which allows over a billion colours to be represented, compared with only 16 million available to traditional 8-bit systems.

This increase in colour depth provides a threefold advantage for all modern SoCs:

  • maintain colour fidelity for the latest generation of 10-bit and OLED displays
  • ensure maximum performance due to a simplified and more efficient transcoding pipeline
  • reduce banding phenomenon, which typically affects 8-bit systems.

PowerVR video IP: multi-standard support, low power consumption and reduced area

For 4K-ready video hardware encoding and decoding platforms, our PowerVR Series4 video IP cores, including the D4500MP, E4500MP and the newly announced VP8-capable encoder, provide an entry-level, multi-standard solution that supports H.264, VP8 and many other standards.

At launch, all PowerVR Series4 video IP cores uniquely introduced a scalable, multi-pipe architecture able to support 10-bit colours as well as YCbCr 4:2:2 and 4:4:4 colour resolutions.

PowerVR D5500 PowerVR D4500 - PowerVR video IP roadmap

Platform architects demanding HEVC, which is highly likely to become a requirement for 4K broadcast services, can now become lead partners for our next-generation PowerVR Series5 video decode and encode IP, which we have already covered in a former article.

If you want to stay ‘in the know’ about 4K, HEVC and our PowerVR video IP, follow us on Twitter (@ImaginationPR) and keep coming back to our blog.

 

* Images courtesy of Elemental Technologies, HEVC webinar viewable on-demand.

* Image courtesy of CNET.com, all rights reserved

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