Both the "Stereo Mixer" and the "Stereo Dithered Mixer" plug-ins are 24-bit; there is no 16-bit Mixer plug-in with Pro Tools v5.3.Pro Tools v5.3 (with Pro Tools|HD hardware) defaults to the "Stereo Mixer" plug-in (installed in your System Folder>DAE Folder>Plug-Ins folder). You can replace it with the "Stereo Dithered Mixer" plug-in if you wish. The "Stereo Dithered Mixer" plug-in can be found in System Folder>DAE Folder>Plug-Ins Unused Folder.The "Stereo Dithered Mixer" takes more DSP. Some people claim that it sounds different (perhaps better). Others claim it is an inaudible difference. We provide both so that you can make your own decision.