HDMI signals

AVI infoframe

Auxiliary Video Information infoframe and VIC

A television cannot measure what it is being sent. It is told, once per frame, in the AVI infoframe, and when that packet is wrong the picture looks wrong for reasons no cable swap will fix.

AVI stands for Auxiliary Video Information. It is one of the infoframes defined by CEA-861 and carried in the blanking of an HDMI stream, repeated at least once per frame so a display that starts watching part way through is never in doubt for long. It arrived with the earliest HDMI versions and removed the guesswork a DVI monitor had to do about the signal reaching it.

Its fields are short and consequential. VIC, the Video Identification Code, names the timing from the numbered list in the standard, so a display knows which format it is looking at without measuring the timing itself. Colorimetry bits say BT.709, BT.2020 or something older. Quantisation range bits say limited or full. Others carry the pixel encoding, RGB or YCbCr 4:4:4, 4:2:2 or 4:2:0, plus aspect ratio and scan information. A display reads all of this before deciding how to interpret the numbers arriving on the video lines, which is why one wrong colorimetry bit gives you washed out or oversaturated colour from perfectly good video.

AVI infoframes on the HDfury Diva

The HDR and DV section of the web server both reads and writes them. CAPTURED AVI METADATA shows the infoframe arriving from the source, with the hex deciphered and the VIC, colorimetry and range exposed in readable form. Underneath it the custom AVI generator has dropdowns for VIC, Primaries and Range: choose the values, click Create IF to build the hex, then Send AVI.

Use custom AVI

Sends the infoframe held in the custom AVI box instead of the source's own, and persists over hot plugs and power cycles. The IP command is set avicustom on.

Disable AVI

Stops the source AVI infoframe reaching the sink at the Diva output and cancels any custom one. Meant for DVI displays and monitors that behave badly when they receive an infoframe. The IP command is set avidisable on.

Send AVI

Changes the values being injected rather than stopping injection, which avoids a resync on the television.

The common reason to build one is to activate BT.2020 mode in a television whose incoming content is not sending the correct infoframe. The pro tips page lists strings you can start from: 40:E8:64:5D:00 for BT.2020 YCbCr 4:4:4, 20:E8:64:5D:00 for BT.2020 YCbCr 4:2:2, and 20:88:00:10 for 1080p 4:2:2 BT.709 with no BT.2020.

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