Introduction
Cable lengths in video conference rooms is one the more complicated areas in ensuring a robust system. Some systems provides end-to-end cabling, but in many instances you need to expend on that since it is too limited.
USB-C also add another complicating factor, but in terms of the actual USB-C cable to the laptop, but also how it indirectly effects downstream cabling over HDMI and USB-A.
Different extender technologies exists to circumvent limitations of coppar cables, but these can also add other complications.
This article explains some of the underlying parameters and a set of best-practices for cabling video conference systems.
Solution Overview
The picture above shows the different standard and cables involved in a standard USB-C BYOD solution. The USB-C cable includes four different parts: DisplaPort for video, USB 2 and USB 3 for communication and charging of the laptop.
The USB-C cable
Since the USB-C cable carries multiple protocols and charging the max length is factor of the most length-sensitive payload. The max lengths for the internal protocols are:
- DisplayPort: 2m
- USB 2: 5m
- USB 3: 3m (both for 5 Gbps and 10 Gbps)
- Charging: Depends on the voltage drop over the cable which is based on the available coppar-thickness of the VBUS pin + induced power in the un-shielded data pairs. For a regular, high-end cable this gives 2m = 100W, 3m = 60W.
So for all practical purposes, the max length of a passive 100W USB-C cable is 2m.
At Ochno we are continuously evaluating various alternative cables that tries to extend these limitations, but so far we have only found one 3m with electrical redriver that works with 100% the laptops and USB-C docks we have tested with. More information here.
For longer than 3m an hybrid active optical cable (Hybrid AOC) is needed. In these cables the high-speed lanes carrying DisplayPort and USB 3 is using optical pairs instead. USB 2, charging (obviously), USB PD and DisplayPort Aux is still using copper. Electrical redrivers can be used to extend these as well. For an AV-installation it is still not recommended to use these due to:
- The connector is rather large and cable is heavy so there is a very big chance that either the laptop’s USB-C port or the connecter itself breaks.
- There are many situations when combining certain laptop models with certain USB-C / USB-devices and when the connection is lost or other signal quality issues. In princip Ochno has not found a single product on the market that works in 100% of the tested cases.
Example of Hybrid AOC USB-C cable on the left. On the right an example of an HDMI AOC cable with chipset from Silicon Line. The fiber pairs are combined with electrical wires. The actual optical transievers are around 2 x 2mm in size.
USB-C vs USB-C
In an installation where laptops are docking with USB-C you typically wants to use Alt-mode DisplayPort as method of transmitting the video since that doesn’t need any drivers to be installed (as is the case with DisplayLink). There are many longer USB-C to USB-C cables that only supports USB 2 and USB 3 but not AltMode DP so be sure the read the detailed specifications.
The HDMI cable
Most newer laptops support DisplayPort 1.4 so the laptop is capable of sending up to 32 Gbps of video over a single USB-C port. In most cases however, half of the available capacity of the USB-C cable is allocated for USB 3 so 16.2 Gbps is available for video. When converted to HDMI 2.0 this can in practice go up to 18 Gpbs, supporting resolutions up to 4k at 60Hz at color-depth of 4:4:4. In 2023 DisplayPort 2.1 will be more broadly deployed in new laptops, which at half capacity can deliver 40 Gbps. Combined with HDMI 2.1 converters this will allow for 48 Gbps HDMI video output in a USB-C docking scenario.
In a conference room with USB-C docking and no EDID emulation, the laptop will negotiate video resolution directly with the TV. There are then a couple of issues that might happen:
- If there is a cable or extender technology that doesn’t support 18 Gbps HDMI, the laptop can still select a resolution that requires up to 18Gbps. This is because the normal signal-training and down-grading of the resolution which happens in HDMI-to-HDMI cabling doesn’t work so well when USB-C and DisplayPort is also involved.
- If the extender solution doesn’t support HDCP pass-through, similar issues can happen.
- In many cases, the HDMI-port of a laptop supports lower resolutions than the USB-C port does. So when testing with connecting the laptop to the HDMI port of the laptop it works since it only goes up to 4k 30Hz, but when going through the USB-C port it goes up to 4k @ 60Hz and there could be a signal problem.
- The timing of how the signal training is initiated for the HDMI-connection might conflict with the signalling of the USB-C protocol and the video signal training of the DisplayPort video. Typical example of this is that the laptop discovers the external screen but no picture is shown or just a 640×480 picture is sent.
- The TV doesn’t support start on signal and instead need an HDMI CEC command to start which is not supported natively by USB-C.
Some recommendations on how to deal with these issues:
- If no EDID emlation is used, it is not recommended to have longer than 5m passive HDMI cables. Absolute max is 7.5m but that is stretch to cover all possible combinations of TVs and laptops (and their signal quality)
- If optical or Ethernet-based extenders are used, ensure they support 18 Gpbs HDMI and HDCP pass-through.
- Try not to use AVoverIP due to possible issues with HDCP and MacBooks. If used, ensure to configure the system correctly in regards to HDCP. Test with both Windows and Macbooks.
- If it is not possible to support 18Gpbs HDMI, you need to use an EDID emulator to pull down the resolution, typically to 1080p.
- If you think you have problem with number 4 or number 5 above, use Ochno Power Conference which has unique features to deal with these issues.
The USB-A cable
With USB-A cable here we mean the cable going from the USB-C dock to the USB-devices like videobar, camera or audio DSP.
First, in USB there are three main standards:
- USB 2.0 aka HighSpeed at 480Mbit/s,
- USB 3.1 aka Gen 1 aka SuperSpeed at 5 Gbps
- USB 3.2 aka Gen 2 aka SuperSpeed+ at 10 Gbps
In a video conference room, it is recommended to run the video camera using USB 3 (5 Gpbs) to allow for higher framerate and less heavy video compression. Different models of cameras deals with USB 2 video compression more or less good or bad, but at USB 3 speeds, most delivers a very good video quality. 10 Gbps is basically not needed since it is not supported by any commercial video conference system as of today.
For speakers and microphones, USB 2 is more than enough bandwidth-wise and many products doesn’t expose the audio-function via USB 3 regardless.
Max lengths for passive cables are:
- USB 2.0 Only (480 Mbps): Max length: 5m
- USB 2.0 + USB 3.1 / Gen 1 aka SuperSpeed (5Gbps): Max length: 3m
- USB 2.0 + USB 3.2 / Gen 2 aka SuperSpeed+ (10 Gbps): Max length: 3m
Any cable longer than this need either a redriver in the cable or connected to a device with built-in redrivers.
The normal color-coding of a USB-A cable that supports USB 3 is a blue connector, with USB 2 have white or black. This is however not a 100% requirement, more of a best-practice.
A cable that has at least one USB-A plug in one end supports maximum USB 2 and USB 3. A cable with USB-C in both ends can support basically anything (only USB 2, USB 2+USB3 or USB2+USB3+DisplayPort). Many USB-devices ships with 5m USB-C to USB-A cables and these are only USB 2.0 cables and will therefor lower the video camera image quality. So if you for instance have a Logitech MeetUp or Poly Studio, video quality will be improved if you change from the included USB 2 cable to a USB 3 solution instead.
When longer USB-cables are needed than the standard allows for, please keep these things in mind:
- Pure optical USB cables usually only supports USB 3, not USB 2. There are many scenarios when this is a not a good idea, so not recommended to use these.
- When using hybrid active optical cables, USB 2.0 signaling is usually done over copper and with redrivers. For distances over 10m that technology doesn’t work super good.
- Longer USB cables will not be able to carry 5V at max current (0.9A for USB 3.0) so assume that you can’t power anything using long USB cables.
- Active electrical USB cables normally implement USB 3 extension using redrivers (which are transparent to the rest of the system), but the USB 2 extensions are done by adding USB 2 hubs every 5m. Since the USB standard doesn’t allow for more than 5 hub ”hops” between host and device that can introduce problem in larger installations since both USB-C dock any many USB-device have internal hubs as well.
- As long as you don’t extend the 5 hubs-limit, there is no problem in adding additional USB hubs between laptop and devices to optimize cabling.
- There are basically no practical solution for running USB 3 over Ethernet cables. Any solution that uses Ethernet from the wall to the table uses USB 2 internally (e.g. Logitech Rally, MTR Touch screens, etc).
- When using HDBase-T 3.0 solutions with support for both HDMI 18 Gpbs and USB 2.0 in the same Ethernet-cable, ensure that it support up to 480 Mbit/s on the USB 2-side otherwise camera will start behave strange and might disconnect.
- When using Ethernet or AVoverIP extenders, ensure to test sound using both asynchronous and isochronous mode. The selection of mode is done by the laptop and device, but isochronous mode is very sensitive to signal quality problems.
- When using hybrid AOC or active electrical USB cables, use a USB-C dock that can reset the power on the USB port when connecting the laptop. This is important since some cables might not support switching of hosts very well if they are continously powered up.
- If you are unsure of why some USB device doesn’t work or is not accessible from the laptop, download the great tool USB Device Tree Viewer. There you can see the full USB topology with USB 2 and USB 3 connections seperatly.
