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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  • afree - Tuesday, April 22, 2014 - link

    Let me clarify a few things with my above post:
    1. If the manufactures are concerned they won't be able to keep up, perhaps that indicates they would be able to approximately double displayport 1.4 bandwidth in something like 5 years.
    2. When I mention 8K at 2inches, I meant a slight reduction in compressive resolution wouldn't be much of a problem.
    3. I am not 100% certain that 2 displayport 1.3 equals 8K @ 60 hertz
    4. Perhaps I was hasty in judging 5 years for 8K, could be 2 years could be 10.
    5. LCD to LED screens are imo a significant improvement which is associated to bit depth, while there are certain colours that humans have trouble discerning and LED screens obviously improve in critical areas; 24 bit to 8 bit bit depth seems a bit much to me.
    Reply
  • p1esk - Wednesday, April 23, 2014 - link

    I really don't understand why no one introduced an optical cable for video. We have had optical audio cables for audio for many years - you can buy one for a few bucks.

    On the other hand, I can see the value of a stream compression standard, because of the desire to stream content to a monitor wirelessly. It's likely that in 5 years most of us will want to stream high resolution content to our displays from our mobile devices, and that's when standards like this will become useful.
    Reply
  • The Von Matrices - Wednesday, April 23, 2014 - link

    That optical audio cable transmits at most ~5 Mb/s (24bit/96kHz/2ch). The display standard is 50 Gb/s, 10,000 times faster.

    The reason optical audio cables are cheap is that they don't need to carry a high bit rate of data so plastic fibers with poor optical properties are more than adequate. You won't get 50 Gb/s through a plastic fiber used for TOSLINK.
    Reply
  • p1esk - Wednesday, April 23, 2014 - link

    Multimode fiber can do 100Gbps at distances up to 150m [1]. Good luck trying to do that with copper.
    You have to keep in mind that Average Joe is perfectly happy paying $20+ for a cable at BestBuy. Especially when he's buying a brand new 4k TV. And from the marketing point of view, a lossy video link is just not very appealing to someone who's ready to dump a sh!tload of money for the latest, highest res TV.

    [1] http://www.fols.org/fols_library/white_papers/docu...
    Reply
  • willis936 - Wednesday, April 23, 2014 - link

    Fiber TxRx costs more than copper in pretty much all cases. Sure it's a better medium and at these bitrates it'd even be lower power but do you really think they'll switch to a more expensive medium when they're actually damaging data in order to save money? Reply
  • p1esk - Wednesday, April 23, 2014 - link

    They are damaging data because some exec said "we gotta do this over copper".

    It's not obvious that fiber is more expensive. With fiber you don't need a decoding chip in a display, only the optical to electrical converter. Let's see what costs more - a new, very high data rate decoder chip, or a converter that has been mass produced for the last 30 years?
    Reply
  • willis936 - Thursday, April 24, 2014 - link

    Undoubtedly the optics. Take a look at the numbers. Fiber is expensive. Really expensive. Tens of thousands of dollars expensive. 40g is expensive. 100g is expensive. Pushing 20g over copper is a miracle. There's a barrier they're pushing against here and they took the cheaper route because that's the world we live in today. Reply
  • sheh - Wednesday, April 23, 2014 - link

    I don't think 8K will be mainstream for at least another 10 years. Probably more.

    People don't watch 2 inch away from the screen, of course.

    8 bpp (bit per pixel) isn't about color depth, it's a measure of average data rate. For example, high quality Bluray movies are around 1 bpp, or less, though it's a compression format with different goals and requirements than DSC.

    What do you mean LCD to LED screens? LED screen is a misnomer, unless you mean OLED. "LED screens" (LCD screens with LED backlighting) generally use the same panel technology as "LCD screens" (LCD screens with CCFL backlighting), just the backlight is different. Arguably LED backlights were inferior on introductions and for a while afterwards. Even now it might not be better.

    One thing that is getting slightly more common is 10-bit and extended range panels. This should be a real improvement in quality, but it's still far from mainstream. In particular, the software and graphics content have to catch up. On the other hand, many TN panels, and other types as well, are still actually 6-bit per pixel.
    Reply
  • extide - Wednesday, April 23, 2014 - link

    The effective bit depth with still be 24, 30, whatever it was in the first place, so the amount of colors is NOT reduced. It's (very mild) compression. People need to realize that codecs like H264 usually get like 1:15 - 1:30 compression ratios. Yeah that means the output file is 15 - 30 times smaller. The lower end of that is Blue Ray quality.

    THIS compression is LESS THAN 1:2. The output size on this is 66% of that it was in the first place. (Compared to 3-6% in typical H264)

    It will be extremely difficult to notice any compression artifacts!
    Reply
  • psychobriggsy - Wednesday, April 23, 2014 - link

    It'll allow you to daisy chain three 4K monitors off a single port at 60Hz, instead of one 4K monitor, even with DP1.2. Reply

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