USB-C Alt Mode: How Video, Audio,

One Cable, Multiple Signals

USB-C can carry power, data, video, and audio simultaneously through a single cable. How? Through a technology called Alternate Mode (Alt Mode). Alt Mode repurposes some of the USB-C connector’s high-speed data pins to carry non-USB signals — most commonly DisplayPort video.

At USBToUSB, our MCY 12-in-1 Hub and 3-in-1 USB-C to HDMI Hub rely on Alt Mode to output video to external monitors. Here’s how it works.

USB-C Connector Pin Layout

The USB-C connector has 24 pins (12 on each side). In standard USB mode, these pins carry:

• 4× Power (VBUS and GND): Deliver up to 240W
• 4× High-speed data (TX1+, TX1-, TX2+, TX2-): 10-40 Gbps data
• 2× USB 2.0 data (D+ and D-): 480 Mbps backward-compatible data
• 2× Configuration Channel (CC1, CC2): Cable detection and PD communication
• 2× Sideband use (SBU1, SBU2): Auxiliary signals (analog audio, debug)
• Other: VCONN (cable power), RX1/TX1 (redundant high-speed pairs)

Alt Mode repurposes the four high-speed data pins (TX1+/TX1-/TX2+/TX2-) to carry non-USB signals. The CC pin negotiation determines which Alt Mode (if any) is activated.

DisplayPort Alt Mode: The Most Common Alt Mode

DisplayPort Alt Mode (DP Alt Mode) is by far the most widely used Alt Mode. It repurposes the USB-C connector’s high-speed data pins to carry DisplayPort video signals. This allows a single USB-C cable to output video to an external monitor while simultaneously carrying USB data and power.

How DP Alt Mode Works

1. Connection: USB-C cable connects device to monitor (or hub with HDMI)
2. Alt Mode negotiation: Device and monitor negotiate via CC pin — device offers DP Alt Mode, monitor accepts
3. Pin repurposing: TX1+/TX1- pins carry DP Main Link Lane 0 and Lane 2; TX2+/TX2- pins carry DP Main Link Lane 1 and Lane 3
4. Video output begins: DisplayPort video signal flows through the repurposed pins
5. USB data continues: USB 2.0 data (D+/D- pins) continues to work simultaneously — you can use USB peripherals on the hub while outputting video

DP Alt Mode Versions

• DP 1.2 Alt Mode (HBR2): 4 lanes, 4K at 30Hz or 1440p at 60Hz. Most common in current USB-C hubs.
• DP 1.4 Alt Mode (HBR3): 4 lanes, 4K at 60Hz with DSC (Display Stream Compression). Found in premium hubs and Thunderbolt docks.
• DP 2.0 Alt Mode: 4 lanes, 8K at 30Hz or 4K at 120Hz. Rare in current devices.

How Many Lanes Does Alt Mode Use?

USB-C has 4 high-speed lanes. DP Alt Mode can use either all 4 lanes or just 2:

4-Lane DP Alt Mode

All 4 high-speed lanes carry DisplayPort signals. This provides maximum video bandwidth (4K 60Hz with DP 1.4) but disables USB 3.0 data — only USB 2.0 (480 Mbps) remains available for peripherals.

2-Lane DP Alt Mode

Only 2 lanes carry DisplayPort signals. The other 2 lanes remain available for USB 3.0 data (5 Gbps). This provides video up to 4K 30Hz (DP 1.2) while maintaining full USB 3.0 speed. Most USB-C hubs use 2-lane DP Alt Mode to balance video and data.

Other Alt Modes

While DP Alt Mode is the most common, USB-C supports other Alt Modes:

HDMI Alt Mode

Carries HDMI signals through USB-C. Rare in practice — most “USB-C to HDMI” products actually use DP Alt Mode and convert DP to HDMI internally. True HDMI Alt Mode was never widely adopted.

MHL Alt Mode

Carries Mobile High-Definition Link (MHL) signals. Extremely rare — MHL technology has been largely abandoned in favor of DP Alt Mode.

Thunderbolt 3/4 Alt Mode

Thunderbolt uses all 4 high-speed lanes for combined PCIe and DisplayPort tunneling. This is the most capable Alt Mode, supporting 40 Gbps total bandwidth. Thunderbolt ports always support DP Alt Mode as well.

Analog Audio Alt Mode

Repurposes the SBU (Sideband Use) pins to carry analog audio signals. This allows USB-C headphones to receive analog audio — but this feature is being phased out in favor of digital USB audio.

VirtualLink Alt Mode

An Alt Mode designed for VR headsets, carrying video, data, and power through a single USB-C cable. Not widely adopted after the VirtualLink consortium disbanded, but some VR headsets still use it.

How to Check If Your Device Supports Alt Mode

Not all USB-C ports support Alt Mode. A USB-C port that only supports data and charging cannot output video, no matter which hub or adapter you use. Here’s how to check:

Look for Icons Next to the Port

• DP (DisplayPort) icon: Port supports DP Alt Mode
• Thunderbolt (lightning) icon: Port supports Thunderbolt Alt Mode (includes DP)
• No icon: Port may be data/power only — check documentation

Check Manufacturer Specs

Look for “USB-C with DisplayPort Alt Mode” or “DP Alt Mode” in the port specifications. If it’s not mentioned, the port likely doesn’t support Alt Mode.

Test with a Hub

Connect a USB-C hub with HDMI (like our 3-in-1 USB-C to HDMI Hub). If the monitor receives a signal, your port supports Alt Mode. If not, the port is likely data/power only.

Common Alt Mode Issues

No Signal on Monitor

Likely causes: (1) USB-C port doesn’t support Alt Mode, (2) cable doesn’t support Alt Mode (USB 2.0 cables can’t carry Alt Mode signals), (3) hub doesn’t support the device’s Alt Mode version.

Monitor Shows 4K 30Hz Instead of 60Hz

This is a bandwidth limitation. Most USB-C hubs use 2-lane DP Alt Mode with DP 1.2, limiting output to 4K 30Hz. For 4K 60Hz, you need a hub with DP 1.4 Alt Mode or a Thunderbolt dock.

Video Drops When Connecting USB 3.0 Devices

In 2-lane DP Alt Mode, USB 3.0 and DP share bandwidth. Connecting multiple USB 3.0 devices can reduce available video bandwidth, causing flickering or signal loss. Use USB 2.0 for peripherals when possible.