For some time now the consumer electronics industry has been grappling with how to improve the performance and efficiency of display interfaces, especially in light of more recent increases in display resolution. Through the eras of DVI, LVDS/LDI, HDMI, and DisplayPort, video has been transmitted from source to sink as raw, uncompressed data, a conceptually simple setup that ensures high quality and low latency but requires an enormous amount of bandwidth. The introduction of newer interface standards such as HDMI and DisplayPort have in turn allowed manufacturers to meet those bandwidth requirements so far. But display development is reaching a point where both PC and mobile device manufacturers are concerned about their ability to keep up with the bandwidth requirements of these displays, and their ability to do so at reasonable cost and resource requirements.

In order to address these concerns the PC and mobile device industries – through their respective VESA and MIPI associations – have been working together to create new technologies and standards to handle the expected bandwidth requirements. The focus of that work has been on the VESA's Display Stream Compression (DSC) standard, a descriptively named standard for image compression that has been in development at the VESA since late 2012. With that in mind, the VESA and MIPI have announced today that DSC development has been completed and version 1.0 of the DSC standard has been ratified, with both organizations adopting it for future display interface standards.

As alluded to by the name, DSC is an image compression standard designed to reduce the amount of data that needs to be transmitted. With DisplayPort 1.2 already pushing 20Gbps and 1.3 set to increase that to over 30Gbps, display interfaces are already the highest bandwidth interfaces in a modern computer, creating practical limits on how much further they can be improved. With limited headroom for increasing interface bandwidth, DSC tackles the issue from the other end of the problem by reducing the amount of bandwidth required in the first place through compression.

Since DSC is meant to be used at the final transmission stage, DSC itself is designed to be “visually lossless”. That is to say that it’s intended to be very high quality and should be unnoticeable to users across wide variety of content, including photos/video, subpixel text, and potentially problematic patterns. But with that said visually lossless is not the same as mathematically lossless, so while DSC is a high quality codec it’s still mathematically a lossy codec.

In terms of design and implementation DSC is a fixed rate codec, an obvious choice to ensure that the bandwidth requirements for a display stream are equally fixed and a link is never faced with the possibility of running out of bandwidth. Hand-in-hand with the fixed rate requirement, the VESA’s standard calls for visually lossless compression with as little as 8 bits/pixel, which would represent a 66% bandwidth savings over today’s uncompressed 24 bits/pixel display streams. And while 24bit color is the most common format for consumer devices, DSC is also intended work with higher color depths, including 30bit and 36bit (presumably at higher DSC bitrates), allowing it to be used even with deep color displays.

We won’t get too much into the workings of the DSC algorithm itself – the VESA has a brief but insightful whitepaper on the subject – but it’s interesting to point out the unusual requirements the VESA has needed to meet with DSC. Image and video compression is a well-researched field, but most codecs (like JPEG and H.264) are designed around offline encoding for distribution, rather than real-time encoding as part of a display standard. DSC on the other hand needed to be computationally cheap (to make implementation cheap) and low latency, all the while still offering significant compression ratios and doing so with minimal image quality losses. The end result is an interesting algorithm that uses a combination of delta pulse code modulation and indexed color history to achieve the fast compression and decompression required.

Moving on, with the ratification of the DSC 1.0 standard, both the VESA and MIPI will be adopting it for some of their respective standards. On the VESA side, eDP 1.4 will be the first VESA standard to include it, while we also expect DSC’s inclusion in the forthcoming DisplayPort 1.3. MIPI in turn will be including DSC in their Display Serial Interface (DSI) 1.2 specification for mobile devices.

With the above in mind, it’s interesting how both groups ended up at the same standard despite their significant differences in goals. The VESA is primarily concerned with driving ultra high resolutions such as 8K@60Hz, which would require over 50Gbps of uncompressed video and something not even DisplayPort 1.3 would be able to achieve. MIPI on the other hand is not concerned about resolutions as much as they are concerned about power and cost requirements; a DisplayPort-like interface could supply mobile devices with plenty of bandwidth, but high bitrate interfaces are expensive to implement and are typically very power hungry, both on an absolute basis and a per-bit basis.

Display Bandwidth Requirements, 24bpp (Uncompressed)
Resolution Bandwidth Minimum DisplayPort Version
1920x1080@60Hz 3.5Gbps 1.1
2560x1440@60Hz 6.3Gbps 1.1
3840x2160@60Hz (4K) 14Gbps 1.2
7680x4320@60Hz (8K) >50Gbps 1.3 + DSC

DSC in turn solves both of their problems, allowing the VESA to drive ultra high resolutions over DisplayPort while allowing MIPI to drive high resolution mobile displays over low cost, low power interfaces. In fact it’s surprising (and almost paradoxical) that even with the additional manufacturing costs and encode/decode overhead of DSC, that in the end DSC is both cheaper to implement and lower power than a higher bandwidth interface.

Wrapping things up, while DSC enabled devices are still some time off – the fact that the standard was just ratified means new display controllers still need to be designed and built – DSC is something we’re going to have to watch closely. Display compression is not something to be taken lightly due to the potential compromises to both image quality and latency, and while it’s unlikely the average consumer will notice it’s definitely going to catch the eyes of enthusiasts. The VESA and MIPI are going in the right direction by targeting visually lossless compression rather than accepting a significant image quality tradeoff for better bandwidth savings, but it remains to be seen just how lossless/lossy DSC really is. At a fundamental level DSC can never beat the quality of uncompressed display streams, but that doesn’t rule out other tradeoffs that will make compression worth the cost.

Source: VESA

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  • p1esk - Wednesday, April 23, 2014 - link

    In 6 years, you will be lucky if average Joe will finally stop buying 720p@60Hz TVs at BestBuy.

    Also, in 6 years all I hope for is 4k@120Hz, at reasonable price ($2k).
  • BMNify - Wednesday, April 23, 2014 - link

    its been stated many times by NHK the inventors and bbc R&D that 2020 was the goal for full broadcasting service, the most resent here, theres a reason the specs called "rec 2020" 10bitpp/12bpp real colour when it was ratified.
    "A statement from Japanese public broadcaster NHK on March 17 stated again that it was readying for test transmissions in 8K to start in 2016, “and full broadcasting service in 2020, the year of Tokyo Olympics and Paralympics.” NHK says there is an air of heightened anticipation for the evolution of a broadcasting style that gives an elevated sense of reality in various genres, such as sport, live music, film and drama. “8K is the next step in this evolution,” says NHK."
  • p1esk - Wednesday, April 23, 2014 - link

    Sure, it's possible - in Japan.

    Average internet speed in US right now is 10Mbps. Let's be very optimistic and imagine in 6 years it will be 100Mbps. That's still barely enough for streaming 4k@60Hz. In fact, judging by the fact that streaming Netflix currently at 1080p is pretty much impossible regardless of your home internet connection speeds (even ignoring the fact that their 1080p is horribly compressed).
    I will be impressed if by 2020 in US we will be able to stream movies in present BluRay quality.
  • BMNify - Wednesday, April 23, 2014 - link

    "Average internet speed in US right now is 10Mbps" i can see how that might be a problem for you in the us , (didnt i read somewhere your govt payed a new subsidies plan for more fiber again), but everywhere else in the main land EU/UK/Ireland etc can get fast 50/100+ Megabits and theres also the latest DVB-T2 over the air broadcasting too that we know NHK/BBC r&d demoed again successfully
  • p1esk - Wednesday, April 23, 2014 - link

    Actually, according to all the sources I could find with a quick google search, the avg internet speeds in UK are roughly the same as in US (which is surprising to me actually, comparing the size of the area to cover).
  • BMNify - Wednesday, April 23, 2014 - link

    well don't believe everything you Google on the internet :)

    i can tell you for sure i run 100+ Megabits from uk on virgin and also 50 Megabits in Ireland and depending where i was at the time anything from 30 megabits to 1Gb/s , anyway we are going off topic , UHD is coming very soon and 120Hz is the spec we need in any real UHD-1 rec 2020
  • simonpschmitt - Friday, May 9, 2014 - link

    I actually have to interject here. I'm sitting here in Germany but it is much the same in many EU Countrys including the UK:
    Yes, we can get 50MBit/100Mbit Internet in many locales (as I have 50MBit in an metropolitan area)
    But, many areas are lucky if they get 16MBit (A-DSL, as my sister has with ~2MBit)
    Aditionally, many people who can get 50MBit/100MBit opt for 16MBit because they don't want to Pay more. even though it is just a 5€-10€ premium.
    So the average speed in the EU is depending on country in the 5MBit to perhaps 15MBit range an on par with other industrial nations.
  • Silma - Wednesday, April 23, 2014 - link

    You should be less forgiving with marketing bullshit and tell a lie when you see one.
    Visually lossless is a contradiction in terms and is highly deceiptive.
    Even the sound industry didn't dare to go so far in bullshit.

    DSC is lossy. Even if it is 100 % indistinguishable from a non compressed image by the average joe, it is lossy.

    VESA loses all credibility spreading this BS. VESA, just say the truth.
  • Guspaz - Wednesday, April 23, 2014 - link

    The term "visually lossless" is not a contradiction, it's a useful descriptor. It has a very specific meaning: the loss is not visible to humans.
  • Murloc - Wednesday, April 23, 2014 - link

    it means that it's not visible to humans, not that it's lossless. That so hard to understand? The image goes to the monitor and then gets in your eyes and your brain elaborates it.
    What is important is the final result, what's in the brain, not the outside world.

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