HDMI signals

Infoframe

The data packets sent alongside the picture

A television switching into HDR mode is reacting to an infoframe, not to the pixels. The Diva reads every infoframe that arrives and can rewrite or suppress the important ones.

An infoframe is a small packet of data that a source sends between frames of video, in the blanking interval where there are no pixels to carry. The format comes from the CEA-861 standard that HDMI adopted, and each type is a short run of bytes with a type tag, a version, a length and a checksum. None of it is picture. Infoframes exist so that the sink knows how to interpret the pixels that follow.

The ones that matter in a cinema room are the AVI infoframe, which describes video format, colorimetry, range and aspect; the audio infoframe, which describes the sound arriving; SPD, which names the source product; the vendor specific infoframes that carry signalling such as 3D and Dolby Vision; and the dynamic range and mastering infoframe added with HDMI 2.0a, which is what puts a display into HDR mode and states the mastering brightness.

They are repeated at least once per frame, so a display that misses one recovers on the next. That repetition is also what makes them worth intercepting: replace the packet and the display behaves differently while the video underneath it is untouched.

Infoframes on the HDfury Diva

The HDR and DV section of the web server is where they are read. CAPTURED HDR METADATA, CAPTURED AVI METADATA and CAPTURED INPUT DOLBY VISION METADATA show what the source is sending as hex and deciphered into human readable form, with the VIC, colorimetry and range exposed separately for the AVI packet. Further boxes on the same page report the rest as they arrive.

SPD INFOFRAME

The captured Source Product infoframe.

AUD INFOFRAME

The captured audio format information. For PCM this usually gives the sample rate and speaker count; for bitstream it usually reports the default, refer to stream header.

HVS INFOFRAME

The HDMI Vendor Specific infoframe.

VSI INFOFRAME

The HDMI 2.0 version of the vendor specific infoframe.

HFV INFOFRAME

The HDMI Forum vendor specific infoframe, shown when the source sends one.

Building and blocking them

The same section carries two generators. Set your values, click Create IF, and the packet appears in the text box as hex. A separate tick box decides when it is actually sent, and all of them are unchecked by default. Use custom AVI sends your AVI packet in place of the source's; Disable AVI stops any AVI infoframe reaching the sink, which is what DVI monitors that misbehave on receiving one need. The HDR side has the same pair, with variants conditional on the input being HDR10 or HLG. Over RS232 or IP the switches are set avicustom on, set avidisable on and set hdrcustom on.

Injection persists over hot plugs and power cycles, and Send HDR or Send AVI only changes the values rather than starting or stopping injection, which avoids a resync on the television. Unchecking the box is the off switch.

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