How ATI Implemented Audio Over DVI to HDMI Dongles

Starting with the Radeon HD 2000 series, ATI (later AMD) introduced a method to deliver full digital audio through standard DVI ports using specialized DVI-to-HDMI adapters. Rather than using external jumper cables, ATI combined an integrated on-die audio processor, standard TMDS video signaling, and an internal EEPROM handshake inside a proprietary dongle to switch the DVI port into a native HDMI transmitter.

The Challenge of DVI and Audio

The Digital Visual Interface (DVI) specification was created solely for video transmission. While both single-link DVI and HDMI utilize Transition-Minimized Differential Signaling (TMDS) across three data channels and one clock channel, standard DVI does not officially support audio. HDMI, by contrast, embeds audio and auxiliary metadata packets into the "data islands" located within the horizontal and vertical blanking intervals between video frames.

Transmitting HDMI-formatted audio down a standard DVI connection risks sending unsupported packet formats to legacy DVI monitors, which often results in a corrupted image, pink/green tinting, or no signal at all.

On-Die Audio Controller

Prior to ATI's approach, PC graphics cards capable of routing audio over digital outputs required an internal S/PDIF cable connected from the motherboard or sound card directly to the graphics card.

ATI bypassed this limitation by embedding a hardware High Definition Audio (Azalia-compliant) controller directly onto the graphics processing unit. The operating system recognized the GPU as a distinct, standalone audio device. This allowed the GPU to ingest, process, and synchronize uncompressed LPCM or compressed multi-channel bitstream audio internally, without routing extra cables across the motherboard.

Proprietary Dongle Detection via EEPROM

To ensure that audio-bearing HDMI signals were only transmitted to capable displays, ATI introduced custom-keyed DVI-to-HDMI adapters. Standard passive DVI-to-HDMI adapters only remap physical pins, leaving the GPU to assume a legacy DVI monitor is attached.

ATI's proprietary adapters contained a tiny onboard EEPROM chip wired to the Display Data Channel (DDC/I2C) lines. During the initial connection handshake:

  1. The graphics driver queries the I2C bus over the DDC pins.
  2. The driver checks for a specific ATI vendor signature stored inside the dongle's EEPROM.
  3. If the signature is absent, the GPU restricts the port to standard DVI signaling with no audio output.
  4. If the signature is verified, the driver enables native HDMI signaling over the DVI physical connector.

TMDS Multiplexing and Pin Passthrough

Once HDMI mode is enabled, the GPU multiplexes the audio data from the on-die sound controller directly into the TMDS data blanking intervals alongside the video data.

Because single-link DVI-D and standard HDMI use identical differential signaling schemes (Data 0, Data 1, Data 2, and Clock), no active digital-to-analog or signal conversion is required inside the dongle. The adapter physically routes the corresponding DVI pins directly to the HDMI connector pins. The connected HDMI television or receiver interprets the stream as a standard, native HDMI signal carrying synchronized digital audio and high-definition video.