Yeah, people already have. That's what I'm basing my design on--the iriver H340. I was pretty much inspired by everything by rockbox' work on disassembling and documenting all the hardware in the DAPs; I realized that there the differences among most DAPs were minimal, and that as long as if we had the software to run everything (rockbox) the hardware wouldn't be that difficult to
design (of course, sourcing all the parts and miniaturizing/reducing size would not be as easy). For example, look at the Iaudio X5/M3 and Iriver H3xx/H1xx; the basic components used in those players are virtually identical in function, and those players actually use the same processors as well. The other rockbox players, including the iPod, Gigabeat, and H10 also use pretty much the same portalplayer ICs (which are not accessible to us).
Hardware info is available via
http://www.rockbox.org/twiki/bin/view/Main/WebHome. Click on the player you're looking at under "Ports" then look for the link labelled "[player name]hardwarecomponents" or the schematics, if available (didn't want to post 15 different links)
Anyway, heres the basic idea of what i'm going to be working on:
a) first I'll probably prototype a (mostly) non-portable wall-powered unit based on the MCF5250, the microcontroller used in the H3x0. It'd really just be a quick prototype/feasibility check project.
b) then I'd start on a portable version. I might still use the MCF5250 first, but I think the base power consumption of that chip might be too high. I might look for a lower power ARM chip.
c) completely radical design. might pick some exotic ultra-low-power architecture or see if I can get some of the dedicated DAP chips. Might run a custom firmware instead of rockbox.
of course, everything would be designed with audiophile quality in mind.
The primary things that are preventing me from going forward at the moment are:
a) need a way of programming a coldfire microcontroller. The standard way is to use a BDM programmer, but those cost $250+ minimum and I don't see myself using coldfire ICs after this project so I'd rather make one myself. The only diy coldfire bdm pod I've seen requires programming cplds, which I don't know how to use either. There are some DIY BDMs for other kinds of microcontrollers, but I don't know if they'd work for the coldfire platform. The coldfire microcontroller also has a JTAG interface, but I'm still not entirely sure if I could use that for programming, as it is usually used for testing during production.
b) prototyping PCBs. It's not really a problem specific for this project, but currently I've got another project on my mind for making prototype PCBs which might go first to ease the wait time for the DAP project
c) MCF5250 IC. Waiting for samples--hopefully freescale accepts my request (I put legitimate info, of course). If they don't, I'm screwed because the only other place that has MCF5250 (or scf? i still can't get this straight) ICs available has a pretty large minimum order quantity, which also poses some problems if any of these designs really work and become published. May need some group buys and/or redistribute parts