DSP for computer engineers
DSP is a domain mixed between EE/CE and Math/CS. Hardware must be built, but also software written.

Some CS programs do touch DSP at the software level. An applied math program would touch even more the algorithmic side, since pretty much all DSP techniques are based on certain mathematical areas such as Fourier analysis, Complex analysis and Numerical analysis. An EE program might have a specialisation on DSP, covering both the hardware and software sides but at a more "applied", less "proof mathematical" level, than what mathematics programs do.

The field is that wide though that it might also be better to focus on only one thing and do it well, rather than think in a "jack of all trades" way, i.e. too broadly. That is, if your interest is in software and mathematics, then focus on those. If your interest is in hardware, then focus on that. Applied math is sort of a middle ground. It's both theoretical, but it will lack practice such as electronics design, even if it'd give you conceptual tools for understanding electromagnetism and digital logic.