A large session would take quite a bit more DSP to do a mix than stereo, because there are a lot more output destinations that the mixer has to deal with. Pro Tools 5.01 (or earlier) includes a "24-bit Optimized" mixer that does mono or stereo mixing. Version 5.1 software includes a new "Surround Mixer" Plug-In in addition to the stereo 24-bit optimized one you're familiar with. The older 24-bit optimized stereo mixer can work with a maximum of 59 inputs (a 59x2 mix) per DSP. The new surround mixer can have a maximum of 17 inputs for 5.1 (17 x 6) or 12 inputs for 7.1 (12x8). If you do the math, you can see that this is very efficient, since there are many more connections that this mixer has to deal with (in the case of 5.1 mixing, 102 to be exact) So if you create a mix with 17 disk tracks feeding 5.1 panners to 6 discrete outputs, you will take up a whole DSP. (So a single DSP is required to do 5.1 mixing, not many.) In practical situations, you can save a lot of DSP if you're actually just hitting an output pair or a single output by assigning only that output. Behind the scenes, Pro Tools would use less DSP to do the task (even potentially using the stereo mixer). Also, many sends you might use in a mix would not be surround sends, but would be mono or stereo (which would use the stereo mixer). Certainly surround mixing does take more DSP (as you would expect), but the new mixer gives us good performance when creating all the mixers necessary to do the work we give Pro Tools "behind the scenes." If you were doing large mixes in stereo today, and wanted to do the same work in 5.1 surround, you may very well be needing some more DSP (depending on your present configuration). But keep in mind that there are other additions to Pro Tools v5.1 that help you save on DSP such as the ability to run RTAS versions of Plug-Ins on disk tracks, and the new Multi-Shell II technology. Multi-shell II let's almost of all of Digidesign's Plug-Ins share DSPs (and coming soon, third-party ones as well). On large sessions these two features will really offer some significant DSP savings.You mentioned "TDM time slots," which are the number of audio "streams" that a TDM system can send between DSPs (and MIX cards). The limit is still 256, and that limit is governed by our present generation of hardware. While 256 may not seem like enough for some applications, an important point to remember is that you don't necessarily use additional time slots every time you make a connection. For example, sends: once one send is created, any other sends can use the same mixer "tap off point." So assigning 5 sends on a channel uses 1 TDM slot, not 5. Also, v5.1 software offers other ways to save on time slots. You can set inputs or outputs to "none," which effectively saves time slots. Of course it's possible to run out of time slots if you have a very large mix with many different routing variations and veritably all inserts on all mixer channel strips in use, but this "brick wall" isn't commonly hit While we may offer newer systems in the future that have more time slots (and I've got nothing to tell you today about when that might be ;-), today's system is doing an effective job for many people in their work I know that I've been able to construct some pretty large surround mixes with a lot of Plug-Ins, sends, etc. without running out.