Electronic – Making a HDMI switch basic question

HDMI is just "DVI with knobs on" on the video side.

As to "can it be done"... my first question is "what FPGA"? Some of them can create HDMI/DVI signals with the IO blocks, others just fundamentally can't.

DVI uses TMDS signalling, which is an encoding on top of a Current Mode Logic (CML) differential pair. CML is actively pulled down by a current source for a '0' and floats high with a termination resistor at the far end for a '1'. It might be emulatable for a hobby project by using a bidirectional LVDS pair driven low and using the tristate line to drive and release (a bit like doing an open-drain drive).

Then you have to encode and serialise the data. TMDS describes how to encode the data bits, and then you you "just" have to serialise the data bits across the data pairs. The specification can be found here - see section 3:

Digital Visual Interface Spec

The TFP410 chip data sheet also has a reasonable description of what goes on:

TFP410 - TI PanelBus™ DIGITAL TRANSMITTER

Don't implement the HDMI standard on a DSP, the standard isn't public and it's a waste of your time.

There's two ways to do this that I can think of. The "hard" way: get a receiver and a transmitter IC (ADI sells them, for example). Demod the HDMI data with a reciever, and pipe the video directly to the transmitter. Push the audio into a DSP like the ADI Blackfin, do your audio magic, then send it to the transmitter. This is not a trivial task: you need to design a PCB and you'll have to deal with high-frequency design concerns.

The "easy" way: just get a small Mini-ITX PC with an HDMI capture card and write some software do do what you want.

In both cases, you'll probably have to contend with HDCP, so you won't be able to use all media.