How does HDMI work?

[bluejay]

How does HDMI work? It seems nothing like composite or RGB. I'm almost certain it's more complicated as well, with 19 pins, and I wonder what would take to make something like an Atari 2600 output HDMI.(No, I'm not going to mod it to output HDMI really, I just want to figure out how it works) Thanks!

[+Stephen Moss]

There are AV to HDMI converters out there that take a composite and audio input and convert to HDMI but results can vary, I tried using one with the PS2 & Jaguar, the PS2 image was not a celar as it was in my 14" LDC analogue TV one (too blocky, could not read shirt names in Madden) and with my Jaguar audio was OK but I just got a black screen. Maybe a differnt adaptor would have produced better results.

So obviously if you wanted to connect a 2600 via HDMI using one of those you would first need to do a composite mod on it.

 

I did look at trying to do a HMDI mod for the Jaguar as it has all the necessary signals (8 bit RGB & I2S audio) but with IC's being surface mount it would be extremely difficult for end users to retro fit.

There are many HDMI converter IC's out there if you wanted to build your own solution but IIRC first for the video you would need convert the RGB or YCrCb signals to 8 bit digital values, get signals for Hsync, Vsync and a video clock. Then convert the audio to an 16bit I2S stream, provide an I2S clock and Left/Right channel select signal.

Then you just need to figure out how to use the HDMI IC's to connect the your inputs the correct device pins and the device outputs to the correct HDMI pins.

 

Again it was some years ago I looked at this by IIRC the HDMI video output used a pair of pins per colour and for the video clock, where the signal on on pin is the opposite voltage to that of the other, i.e. if one is 5V the other is -5V but if one is 0 both are 0, using a differential pair like that provide better noise immunity and allows for longer cables to be used.

I cannot recall if the audio is also a differential but I think it is just plain single pin serial data and clock as it is transmitted at a much lower speed, so all of that takes 10 of your 19 HDMI pins, then there is grounds and depending on the HDMI version Audio Return Channel (ARC) and I believe networking not that you would need to concern yourself with networking and ARC.