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Comment by phh | Hacker News Reader

Based on what you said, you didn't actually look at HDMI protocol, only the protocol exposed to your HDMI encoder chip. You could have such a protocol on Thunderbolt 3 encoder chip.

FWIW, yes HDMI is still pretty simple, but not as simple as you describe it. Even though there are 3 pairs of data it's not one pair R, one pair G, one pair B (highest-bandwidth HDMI uses 4 pairs), it's just one data bus. The data pairs aren't only used for data colors, but also conveys audio, and info frames (which will include various stuff like HDR or VRR metadata). Of course there is the matter of DRM: the content will often be encrypted (but negotiation of that encryption happen separately, over I2C)

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That's only the case in the (relatively) shiny new 2.1 spec with FRL. Prior versions are TMDS with 3 channels for red, green, blue, along with a clock. The audio and InfraFrames slot into the data islands in blanking periods on those signals.

Thanks for the complementary infos. YCbCr420 (which you can't just mux on the channels directly?) dates back to HDMI 1.4 though

As the source device, you can simply choose not to send YCbCr420.

Absolutely - YCbCr 4:2:2 and 4:2:0 use different encodings across the lines to balance bandwidth. Complexity then continues to stack with deep colour pixel packing. There's a beautiful pocket of simplicity for RGB 4:4:4 8bpc which is essentially a direct digital encoding of ye oldy RGBHV signals. More than OK for any retro-comp needs, and a fun starting point for general hacking.

> Based on what you said, you didn't actually look at HDMI protocol, only the protocol exposed to your HDMI encoder chip.

You are correct. Though as part of my studies (and curiosity, of course) I did end up analyzing the signalling protocol. The side-effect of standardizing line protocols is that it offers an abstraction for the engineers working with it. I didn't have to understand the signalling methods per se to use HDMI in my project.

> FWIW, yes HDMI is still pretty simple, but not as simple as you describe it.

I should have added that at the time I used it, HDMI was still in the 1.0 spec (1080i, 60Hz max), which was effectively DVI. Much has changed since then.

While that's true, you really only need to blast out RGB data to get an image on screen. Most of what you are talking about is layered on top and optional.

I did a tiny HDMI implementation in an FPGA for a project, the TMDS implementation was what took the longest.

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