This weekend has been spent poring over parts catalogs and datasheets.

In the end, I'm more confused than when I started ;)

Parts Chosen

• 2x8 header, 2.54mm pitch - as used by FreeDSP. For my first run of boards at least. I'd prefer a more user-friendly connector, but I can use a keyed socket to help a little.
• 74CBTLV3245 - 8 bit bus switch, 75c. Again, a first run quick and dirty choice, in a 2x2 matrix if I can figure out the routing.
• TCA9546A - 4 channel I2C bus switch. Yes, you can do this with a normal bus switch chip, but I want to switch control independently from the audio. This specialty part is worth the $1.75 to have in-band control anyway. (And there's an 8 channel variant for 25c more)

Learnings

• TDM (Time Division Multiplexing) - as well as being an generic term, this is apparently a specific method of transmitting 8 channel audio over only a few wires.
• Bus switches are not that cheap if you want to do crossbar switching.
• This project will end up being be a lot more software than hardware to get the level of user experience I want.

Next Steps

• Check if TDM is commonly supported by DACs & ADCs used in readily available modules (I fear it is not).
• Check if TDM frequencies will be difficult to work with (48k samples/sec, 32 bits, 8 channels... it's too late at night)
• Figure out if a CPLD is practical for a bidirectional crossbar switch, or if the "FP" aspect of FPGA is worth it (the CPLD or FPGA will likely to be under $10 @ qty 1, but the FPGA needs memory etc.)

Open to suggestions of CPLD/FPGA - I have used Xilinx in the past, but I see Altera has a third-party usb programmer for like $2...