I am currently a University student at Sydney. I am interested in writing an Ogg decoder, but I have no idea where to start.
Do I need to know DSP concepts? If so, what topics? The documentation on Xiph.org seems to need DSP concepts as pre-requisites.
Or do I start by looking at the current code?
I guess you should have a rough idea of how a codec like this works, but you don't need to know the tiny details unless you really want to write your own decoder from scratch. But that would be quite useless because you can simply use the vorbis libraries to decode Ogg data. So if you just want to make something that works, you only need to learn which library functions you need to call, and how.