Mercury Transmit is a software interface that Adobe applications use to send video frames to external video device. Video device manufacturers such as AJA, BlackMagic Design, Bluefish444, and Matrox provide plug-ins that route the video frames from Mercury Transmit to their hardware.
Video preview using Mercury Transmit sends the contents of the Composition, Layer, or Footage panel viewer to an external monitor. The external monitor can be one of the following:
The setting of the Resolution menu in the Composition panel determines the resolution for the external video preview.
Mercury Transmit does not send overlays such as user interface controls, guides, warning banners and other items drawn by OpenGL to the external monitor. This also means that Fast Draft and Wireframe preview modes do not send image data to the external monitor.
You can set the following video preview preferences by choosing Preferences > Video Preview:
If you are previewing an image that does not exactly match the preview monitor's pixel dimensions, the third-party I/O device scales the image. The manner of scaling differs between devices and is in some cases controllable through the Setup options for the device. The setting of the Resolution menu in the Composition panel determines the resolution for the external video preview.
If you experience slow RAM preview frame rates when Mercury Transmit is enabled, try one or more of the following:
Tim Kurkoski's blog contains more information about Mercury Transmit.