Reddit mentions of StarTech.com PCI Express HD Video Capture Card 1080p - HDMI/DVI/VGA/Component TV Tuners/Video Capture (PEXHDCAP)

Smashing Grounds uses one of these, coupled with a component splitter so as to output to the TV and the capture card at the same time with no input lag.

Here's a sample of what it looks like

I don't have any code that does it, but I do have a broad understanding of web technology so I'm confident that I could figure it out. I have a CS degree and I've been employed as a web developer for 10 years now, so perhaps that skews my opinion on the difficulty of such a project.. I would probably attempt it with Java since that's what I'm most familiar with, but you could do it with any general purpose language that supports processing streams of data.

The goal is to keep a buffer of the previous 10 seconds of AV data that had been streamed and to automate packaging it up, uploading and submitting it to reddit along with a meaningful title with one command.

One aspect would be in obtaining the input stream of AV data. I reckon the best bet for this would be to use the HDMI from the Sky box and plug that directly into your PC. A special PCI card would be required for this, as the HDMI ports you get on PC graphics cards are for video out only. Something like this would give you a stream of AV data that you could work with. I'm getting out of my comfort zone here, but I assume there would be a way to convert whatever format the sky box is providing into something compatible with web tech (ie mp4). Then you would need to maintain a buffer of streamed data which represents the video which will make up the gif. Posting it would be possible through standard HTTP requests.

It sounds like a mildly interesting pet project so maybe I'll try it one day if I have nothing better to do

All of the above is relatively doable for any proficient developer. There are probably hundreds of thousands or even millions of people out there that could do it. The other end of the difficulty spectrum would be a program that automatically posted goal gifs without any human interaction. That would have to somehow processes the AV stream and understand that a goal has been scored, work out which team scored it, and for maximum difficulty points which player scored it. I wouldn't know where to begin on something like that. The number of people that could do that is probably in the thousands, and they're probably working on systems like Tesla's autopilot and earning $200k a year

Use a capture device for capturing something like a game console. I use a StarTech capture device with my Wii U.

For PC, I use Open Broadcaster. It can be used for recording to your computer and for live streaming.

Try finding one of these and connect it to XBOX and as an audio-in on your PC and configure Windows to play its sounds to your headphones.

Alternatively you could try looking for one of these to have HDMI in while using Windows. It's a bit pricy and overkill for what you're looking though.

I'm not sure if either of these will work though, it's a shot in the dark.

Something like this?

I'd seen that, but the 'capture card' title threw me off a bit since I'm more interested in just playing rather than capturing. But I assume there's software that can just play the incoming video instead of capturing it?

You will need a capture device such as this

http://www.amazon.com/StarTech-com-Express-Video-Capture-1080p/dp/B007U5MGBE

I have no idea if that one is good or not, but this is the kind of device you are looking for. There really is not other good alternatives as far as I know.