The Mixer plug-in for Pro Tools v5.3.1 will use a few more TDM timeslots when you must grow beyond a single DSP (more than 68 channels to a stereo pair), just because it will now send 2 timeslots worth of data (instead of 1) from the "sub-mixed" DSP's to the final summing point. For example, if you have a 90 channel mix to output 1-2 on HD, it will have 2 DSP's for the sub-mixes, and require 8 24-bit TDM connections in between the sub-mixes and the final summing mixer (L&R for each sub-mix at 48-bits each). Previously it would only require 4 TDM connections. As far as DSP usage, though, the new scheme uses a similar amount of DSP since the summing mixer doesn't have to gain/pan the inputs (even though it must sum them at 48-bits). Also FYI, the surround mixer will also use the new method when "splitting" across DSP's.