ok... when my mixer truncates a 24-bit input to 20-bits... it basically chops off the last 4 bits of data... with the resulting, though possibly insignificant, loss of data.
FOR INSTANCE:
This is how I set up my shit to mix w/ my mixer: (ps... for you, substitute bi-directional 8ch lightpipe connects for TDIF)
I have a digital mixer.. a tascam... it's got 20-bit in, 24-bit out. so I run my mics into it... run a TDIF line to my computer, and record to hard disc, in 24-bits.
I then can run OUT from my comp on the TDIF line to my mixer.. 8 channels at a time (well, I can do 16.. but lets say 8) .... and Mix at will, returning anywhere from 0 to 8 lines BACK to my comp... mixed.
The catch: my 24-bit signals get truncated to 20 bit on the way back into the mixer... then expanded back to 24 in the mixer.
You, too caould do this.. with an ADAT Sync card (??)... or any card with light pipe ins and outs..
xoxo