Engadget Outlines Limits of USB-C to HDMI Adapters

The publication described the connection as a relay. The device's USB-C port transmits a video signal through the HDMI adapter, which converts it into an HDMI signal the display can read; an HDMI cable then carries that signal to the TV or other display. Engadget said the quality of the connection is determined by the weakest link, whether that is an incompatible port, an old cable or an overheating cheap adapter.

A central limitation is the USB-C port. Engadget said not all USB-C ports support video; some can only charge a device and conduct data transfers. USB-C ports can carry video output only if they support DisplayPort Alternate Mode, which uses the same DisplayPort digital display interface found on video cards. The level of DP Alt Mode affects video quality. DisplayPort 1.4 signals can reach 8K resolution at 60 Hz, while DisplayPort 1.2 can achieve 4K at 60 Hz, according to the report. 8K displays are expensive and barely in use.

Engadget said DP Alt Mode is an optional feature that manufacturers can add to standard USB-C ports. Thunderbolt uses the same USB-C connection but comes with guaranteed data transfer rates depending on the generation and includes video output. The report advised checking the manual or online specification page to confirm a port's capabilities.

Even with DP Alt Mode, a basic USB-C charging cable will not transmit video signals, Engadget said. A fully featured cable is required. If the receiving device does not have a USB-C port for video input, users need a specialized USB-C to HDMI cable adapter with a chip that converts a DP Alt Mode or Thunderbolt signal into an HDMI signal. The quality of that link depends on the type of HDMI signal the cable and adapter support.

The report also said ideally a laptop will have at least two USB-C ports so one can be used for charging while the other powers a display.

HDMI versions introduce another set of limits. Engadget said not all HDMI signals are the same. Since the standard was introduced in 2002, it has gone through several iterations, each adding bandwidth for higher resolutions and faster refresh rates. A connection is limited to the oldest generation supported by either port, the cable or the adapter.

HDMI 2.2 has been approved but is not really in use, according to the report. The highest-quality option widely available is HDMI 2.1, also known as Ultra High Speed. Engadget said it can carry up to 48 Gbps and deliver 8K at 60 Hz and 4K at 120 Hz. It also supports variable refresh rates, wider HDR signal compatibility and eARC with enough audio capacity for Dolby Atmos and DTS:X.

HDMI 2.0 cables and adapters are a tier lower. Engadget said they can still handle 4K at 60 Hz and should meet most streaming needs, but their 18 Gbps bandwidth is noticeably lower than HDMI 2.1, making them not ideal for high-end gaming.

The report advised avoiding earlier-generation HDMI adapters and cables because they will noticeably drop signal quality below current standards. HDMI 1.4 can still reach 4K resolution, but at a reduced 30 Hz refresh rate. Engadget said that unless a user has held onto a very old adapter, these older versions are unlikely to be an issue.

For permanent setups, Engadget said top-rated USB-C hubs are often the best option, or users can choose a single USB-C to HDMI cable instead of an adapter. The report said its advice largely applies to all three options: adapters, hubs and single cables.