I had an good meeting with AKM some time ago and went over this application with Joel. He said that he could help with recommendations of suitable DSPs to do the necessary processing. They really are neither expensive nor power hungry. I'm using several in a Bluetooth headphone project now that can do 192 KHz or better running on 3.3V and using no power. We just need to be able to load the DSP code. I can get direction on either TI or ADI parts that can do this (and much more). We do not need FPGA's. They are nice but not necessary and way overkill in development for this application (and most audio ones today).
Look at the DSP's in this block diagram. Block Diagram (SBD) - Pro Audio Mixer - TI.com I'll check with TI to see which ones to use. There is enough power to do some processing in the DSP before it comes back to the system.
Also an option is the ADI Sigma Studio stuff: SigmaDSP Processors | Processors and DSP | Analog Devices
I have both TI's and ADI's dev tools (free downloads) and except for things like Dolby, DTS and other uninteresting stuff its all free.
The current need would be to mix two or four ADC's to get better SNR. I'm not sure what else but anything from level detection to DC offset. Being software it can always be added later.
How small? Some are in chip scale packages. Or smaller in function? Many options.
My guess is that we would need 2-4 I2S in and 1 I2S out. Needs to run 192 KHz sample rate, lower power and cheap. I'll see what's possible in the next few days.
We are playing with a TI part that could do both USB and DSP, but working code is some ways off for the USB.
A FPGA would be nice. However, one could also consider changing the CPU as mentioned before. ST has a cortex M4 variant STM32F407VGT6. It's reasonably priced and could perhaps be an option. As far as I remember it only has one I2S port.
I don't have the time to learn a new language. If I had the skills I would consider it.
The SDR/Audio widget is open source but it is protected. Unless the owners want to switch processors I don't think that's going to happen.
Here's my suggestion: Don't let the need for summing rule this out. Let's come up with a nice PSU and analog design. I can handle the FPGA parts and organize a group buy of assembled units.