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November 3, 2022

USB4 to Double Output – USB-C & Thunderbolt 5 to Support Dual 8K Monitors

A few weeks ago, the USB Implementors’ Forum (USB-IF) made the surprising announcement that they had been able to double the effective speed of USB4 interfaces from a maximum of 40Gbps to 80Gbps using existing cables. However, new transmitters and receivers will be needed. There will also be a new active cable specification to support 80Gbps over longer lengths. The group has revealed more about the specification and the labeling scheme. Further, a new USB Type-C specification that allows speeds of up to 120Gbps across three 40Gbps lanes has been confirmed.

VESA has reacted by announcing a version 2.1 of DisplayPort to make the most of USB4 and, not to be out-done, Intel said that future Macs would use Thunderbolt 5, which will support 80Gbps in two directions, or 120Gbps for external displays. That would allow dual 8K 60P displays to be supported on forthcoming Macs. Thunderbolt 5 is built on top of USB4 Version 2 and DisplayPort 2.1.

USB4 Version 2

Although the next USB version should be able to use current cables, it will need new interfaces as the encoding scheme has been changed and is now based on PAM-3 encoding (there’s a brief video here on PAM3 signaling from Teledyne Lecroy). PAM encoding is used in other high-speed interfaces, such as versions of PCI Express. The system uses three different amplitude levels to signal -1, 0, or +1 values, and the interface combines two sequential bits to generate a number from 0 to 7 using this information. This way, rather than just supporting numbers 0-3 with two bits of data, the interface can support 0-7, thus doubling the throughput.

The PAM-3 Encoding Scheme

The USB-IF told CNET that new hardware should arrive around the end of 2023.

The critical change for 8K displays is how the four lanes of USB4 can be reconfigured. Each of the lanes can support 40Gbps. When a connection is made, the interface defaults to dual 40Gbps lanes in each direction for 80Gbps each way. However, there is a defined mechanism for the lanes to be configured as three lanes for output from the host device and a single 40Gbps lane back, allowing 120Gbps one way and 40Gbps the other.

Asymmetric Connections Help Displays

That kind of asymmetric connection is ideal for display applications where more bandwidth is needed one way than the other. With 120Gbps, you can support dual 8K 60P 4:4:4 images with 10 bits of color for HDR without DisplayStream Compression (DSC). Of course, you could also keep frame rates up to 120Hz for a single display, and that is very interesting as many believe that 8K benefits from higher frame rates.

USB-IF has a new labeling scheme, and the new labels are shown below.

Source: USB-IF

DisplayPort 2.1

USB4 supports the tunneling of DisplayPort data, and VESA has updated DisplayPort to V2.1 to ensure that the tunneling works more efficiently with USB4. The DisplayPort 2.1 specification mandates the use of DSC to reduce the bandwidth required by more than 67%. The standard also allows the support of Panel Replay technology over USB4. This technology allows a system unit to update only part of an image that has changed rather than the whole frame and can reduce power where the display has memory to store the image. (this is similar to the Panel Self Refresh capability in Embedded DisplayPort (eDP) – although we don’t know of any current monitors or displays that support Panel Replay yet).

VESA said that existing DisplayPort 2.0-approved products have been tested and shown to be capable of supporting version 2.1.

Thunderbolt 5

Intel has not yet released details of version 5 of its Thunderbolt technology but has confirmed that the interface will be in future Mac computers from Apple and that they will support dual 8K 60P displays using the architecture that supports 120Gbps using three lanes. Note that a “consumer” 8K/60p signal is 4:2:0 color sub-sampling, which can be sustained over a 40Gps interface, so is it possible to support three 8K displays?

Active Cables

We looked at the topic of cables for 8K on the Discover8K website and highlighted that there were no standards for active cables at the time. On this basis, it’s good to see a standard being developed by USB-IF. One of the challenges of developing cables is that a critical feature of USB4 is the support for power delivery. To extend the distance that fast interfaces can support, moving the data delivery to optical fiber is logical. However, the optical fiber does not support power delivery, so cables have to be ‘hybrid’ with copper for the power and optical fiber for the data.

This development will take away a significant roadblock for further improvements to 8K display development by enabling multiple and high frame rate displays. Even better, existing copper cable technology will be usable, avoiding the need for more expensive active connections.

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