Cables for 8K
We thought it was about time to summarize where we’re up to on cables for 8K display use. As some readers may want to know ‘what do I need to buy?’, we’ll cover that first and then dig into some options and special cases.
Video Interface Cables
There are two current output protocols from consumer devices dedicated to video and graphics to support 8K: HDMI and DisplayPort. These have their own dedicated connector ports, but they can also be used over other physical port interfaces like USB Type-C and Thunderbolt. These latter can add additional functionality such as data transfer, power, and other signals. It can get very confusing and users have to be very careful to understand what source (sending) device supports and what the sink (receiving) device supports.
The quality of cable that you need depends on the amount of data that you need to send. That is affected by the resolution, the frame rate, the colour depth (HDR typically uses more than SDR) and whether the image is of computer information or video. If you are showing computer images, it is important that all the data is transmitted so the format, in the jargon, is sent as 4:4:4 quality.
However, if you are just using the cable for video, a technique called chroma sub-sampling can be used. It’s beyond the scope of this article to cover this, but you can find information on the technique here. Basically, the chroma sub-sampling allows the video system to throw away data that the eye won’t see in motion video, so you can send higher resolutions at lower bit rates.
In this article, we have assumed that if you are using DisplayPort, USB Type-C or Thunderbolt to connect to an 8K device, it is some form of computing device or game console and you will want 4:4:4 quality as when you show static computer graphics, chroma sub-sampling can lead to visible artifacts.
HDMI is the leading consumer connector found on TVs, set-top boxes, Blu-ray players, etc. The kind of cable that you require will depend on whether you want to show computer graphics, or just video.
To support 8K, you would ideally have HDMI 2.1 with the maximum bit rate of 48Gbps. That level will support up to 8K 120Hz if the device and display support Display Stream Compression (DSC) and up to 8K 60Hz 4:2:0 if DSC is not supported. To support 48Gbps, you need an Ultra High-Speed Cable (48GB Cable).
We recommend checking to verify that the cable is certified to be sure it will support this bandwidth over the cable length. Wyrestorm for example, offers Ultra Hish Speed Certified HDMI cables in 10 and 15m lengths as well as active fiber cables for longer distances.
If you are watching video on the device, you may be able to use a lower quality and cost cable. Consumer video devices with 8K support will use 4:2:0 chroma sub-sampling. The bit rate that you need for different resolutions is as follows.
If you have a device or display with HDMI 2.0 that supports 8K at 30Hz (just with 4:2:0 video, so not optimized for computer graphics), you would need a Premium High-Speed HDMI Cable or Premium High-Speed HDMI Cable with Ethernet. Those cables support up to 18Gbps, so would all support up to 60fps in devices that support DSC. However, for that frame rate without DSC or for 120 fps with DSC, you would be better off with a cable that meets the full 48Gbps specification.
What about length?
There is no formal specification for the maximum length of HDMI cables, and it depends on the details of the cable and construction. However, if you choose a certified cable, the maker is required to validate correct operation over that specific length.
However, HDMI Licensing expects passive copper-based cables up to 5 meters (16’) to be available. (We’ll cover active and fiber-based cables later).
DisplayPort 1.4/1.4a can support up to 8K at 60Hz with 10-bit HDR using DSC. As with HDMI, there is no standard maximum length for DisplayPort cables. VESA introduced a DP8K DisplayPort cable that can support up to 32Gbps (HBR3). That’s enough for the support of 8K at 30Hz without DSC and 8K at 60Hz with DSC.
The introduction of DisplayPort 2.0 has been delayed due to disruptions caused by Covid. DisplayPort 2 pushes things a long way, especially compared to HDMI. The 80Gbps maximum bit rate means support for uncompressed 8K at 60Hz. (for more on DisplayPort 2,0 – go here)
To go with the new version, two new cable labels have been developed – DP40 and DP80 with 40Gbps and 80Gbps, respectively.
The table below shows what cable you need for SDR or HDR full chroma support (4:4:4 color sampling) for HDMI 2.1, DP40, and DP80 cables. Note that support for SDR or HDR video (4:2:0 color sampling) requires less bandwidth and would change the cable needs in this table. Hopefully, we will see DisplayPort 2.0 devices and cables in the market soon.
USB Type-C Cables
Up to USB-4
USB Type-C cables carry graphics data and often the power to charge devices, so for safety, it’s essential to get a suitable cable for that use. The USB Implementers Forum has a certification program for cables that carry charging power of 60W or 240W. However, below USB4, USB Type-C cables can support the full performance DisplayPort 1.4 and also HDMI 2.1 via adaptors so that they can support 8K at 30Hz, 10-bit 4:4:4.
USB4 requires USB Type-C connectors and can (but doesn’t have to) also support either Thunderbolt or DisplayPort up to level 2.0 in what is called an ‘Alt mode‘, where some of the pins are re-allocated to make dedicated display connections.
Thunderbolt can support displays with up to 40Gbps allocated for display, but the maximum length at that speed is 0.8m (2’7″) for passive copper cables. 40Gbps would be enough for 8K at 30Hz or 60Hz with DSC with 4:4:4 color sampling. 8K video at 60 Hz and 4:2:0 sampling can fit in a 40 Gbps connection without DSC.
Now, although you can get DisplayPort certified cables for the display alone, at the moment, there is no certification system for the USB cables in Alt-mode. We contacted VESA (which manages the DisplayPort specification), and we heard that it is working with the USB-IF (which controls USB) to develop a labeling scheme, so, as the saying goes, “watch this space”.
There are two basic ways to extend the cable length for all of these specifications, but you must go beyond the simple passive copper connection. The two fundamental approaches are
- Active electronics acting as a kind of relay amplifier, in the form of amplifiers and receivers included in the connectors or as separate boxes,
- Fiber-optic technologies.
When the active electronic system is integrated into the connectors, the active electronics typically draw some power from the interface to support the operation. In contrast, a separate amplifier may also need a power connection. DisplayPort, for example, can provide 3.3V at up to 500mA to power active cables.
Active copper cables with amplifiers can typically extend the typical 3m (10′) maximum of HDMI or DisplayPort passive cables up to 8-10 meters (26′ – 33′) or even longer. However, there are no standards or approvals for these cables, so a reputable supplier and precise specifications are needed when buying to be sure of correct operation.
Fiber Optic Cables & Hybrid Cables
Fiber optic cables include integrated circuitry to convert the signals to optical signals for transmission and back at the other end. Fiber optic can carry very high bandwidth, so there is no problem with extended cables supporting 8K up to 30 meters (100’) or more.
One downside of fiber optic cables is that they cannot carry power and are typically unidirectional – sending video data from the host system to the display. DisplayPort also has a facility for using the Aux pins to detect what the cable is connected to and to transfer some signals back to the host system. Rather than make the optical connection bidirectional, it is possible to use copper for the signals that may need to come back to the host while using high-speed optical fiber for the display data, which is unidirectional from the host to the display.
Again, there are no standards for hybrid and optical systems, so you need to check that they will do what you want before buying. However, a recently announced example is from professional supplier Extron, which combines 8K optical video with copper to support 8K over USB-C while allowing up to 60W of power delivery over the cable. The cables can be up to 30′ (9.1m).
So, in summary, there are lots of cables out there that might be able to support 8K transmission over different distances, but getting a reliable supplier and, if it’s available, a certification should ensure correct operation the first time.