The road towards 4K
Resolution of video pictures has always taken an important part of the evolution of the broadcast industry.
Standard definition was commonly used until end of the twentieth century, and viewer get used to that quality level.
Then, HD formats took the relay, and were seen as a real innovation, bringing up to 5 times more pixels to each single displayed picture. 1080i and 720p formats, then 1080p ones, became the reference for nowadays video transmissions.
About ten years after the massive adoption of HD television, the industry kicked off a new revolution with the introduction of 4K resolutions. The term “4K” comes from the amount of pixels per video line; depending on the exact resolution, this can be 3840 or 4096. These formats are also referenced as 2160p referring to the quantity of video lines per frame.
The increase in quality is just stunning! A 4K picture contains four times more pixels than a HD one, and hence up to 20 times more pixels that a standard definition picture.
On the other hand, increasing the picture resolution brings additional requirements to keep a pleasing viewer experience. While the human eye was satisfied with standard definition pictures moving at a frequency around 25 or 30 hertz, it starts distinguishing artefacts when HD pictures are moving at that frame rate, and the experience even becomes uncomfortable with 4K content. The industry quickly ratified 50 and 60 hertz as the recommended frame rates for 4K moving pictures.
But this quality jump has a cost : while carrying on an uncompressed digital SD video signal requested a bandwidth of 270Mbps, a 4K feed running at 60 frames per second now requires 12Gbps, multiplying the interface speed by a factor of 44.4!
On the other hand, while a color depth of 8-bit was sufficient for a satisfying experience in SD and even HD formats, ones agree that at least 10-bit of depth are necessary in 4K – and even that high dynamic range pictures in 12-bit should be the natural evolution for those formats.
There actually exists two families of 4K formats :
- 3840x2160 – aka UHDTV1 or simply UHD – is the 4K resolution issued from the broadcast television world. It corresponds to four times the resolution of a HD picture (1920x1080). UHD image format is defined by SMPTE 2036-1. UHD formats are generally associated to broadcast frame rates (25, 29.97, 30, 50, 59.94 and 60 frames per second) but also exist in digital cinema frame rates
- 4096x2160 – aka DCI – is the 4K resolution coming from the digital cinema industry. It corresponds to four times the resolution of a 2K picture (2048x1080). 4K DCI image format is defined by SMPTE 2048-1. 4K DCI formats are generally associated to cinema frames rate (23.98, 24, 47.95 and 48 frames per second) but also exist in broadcast frame rates
The broadcast industry prematurely moved to 4K before the standardization bodies defined how to carry on such content.
The first adopted transport method was naturally to split the 4K picture into four HD quadrants, and to carry them on four synchronized SDI signals – four times HD-SDI for 4K at up to 30 frames per second, then four times 3G-SDI for 4K at up to 60 frames per second.
Later on, SMPTE introduced the 425-3 and 425-5 standards. They define a new method for mapping a 4K picture onto four HD sub-images using a two-sample interleave division rule, so that each of the four sub-stream carries on a quarter-resolution picture. These new standards still use four HD-SDI signals to carry on 4K at up to 30 frames per second, or four 3G-SDI signals to carry on 4K at up to 60 frames per second.
With the evolution of serial digital interfaces speed, those quad-link mappings can now be replaced by dual-link 6G-SDI equivalent, or even by a single 12G-SDI connection.
To achieve that, SMPTE recently released ST 2081 standards for 6G-SDI carriage, and ST 2082 standards for 12G-SDI transport.
DELTACAST goes 4K
Since the early adoption of 4K, DELTACAST committed to follow the evolutions of the industry requirements, and to propose solutions accordingly, right at the time the market needed them.
The following table summarizes the DELTACAST products and developments supporting 4K formats :
(1) With some limitation
(2) In 4-2-0 only (HDMI 1.4)
Obviously, the story does not end up here.
Market requirements evolve, and new standards arise constantly. The next step will most probably be about higher dynamic ranges, and DELTACAST already works on the subject.
Stay tuned for future announcements around 4K!