Good day!
I am engaged in development of hardware-DVB scrambling. Currently I am interested in the possibility of acquiring IP Core for Altera which allows you to decode MPEG-4(video, audio) and then the flow encode to MPEG2(video, sound). The input stream is MPEG-4(video, audio) - 3 Mbit/s, the output MPEG-2 stream(video, audio) - 3-4 Mbit/s. Required 1 the Altera chip to implement transcoding from 1 to 6 streams MPEG-4 to MPEG-2. Prompt: 1. What is the Altera chip should I use?

That sounds like a fun project! I hope all goes well for you with it.

As for what Altera part to purchase, isn't that a function of which core you put on it? I mean, each core is going to have some minimum number of flip flops, look up tables, block rams, etc that it needs. I would ask the vendor that you are purchasing your core from what it requires.

Don't forget to allocate extra! You'll need more than just the MPEG core to handle your encryption, any bus level command and control, as well as any debug/scope functionality you need to put into the part before it's working.

I might also recommend baby steps: get one transcoder working in non-realtime, then in real-time, then work on getting multiple coders running at once on the same chip. Sadly, this may that the board you get the first transcoder working on cannot handle the full six you wish to do, but at least you'll know that by the time you get the first one going. Remember to plan for failure: you will have a couple along the way, don't get surprised by them.

Cheers, and I hope all works out for you!