The link ScottW posted is certainly a complete discription of the Alpha channel but may be a bit much to start with if you are hoping to use it.

There are two major methods for using the alpha chanel: One, for example, is to overlay a name of a talking head. The other, also for example, is to use it to produce a ghost like effect on a clip.

For the overlay effect I use Adobe PhotoShop Elements. Do this: Display a transparent page and put the graphic on it that is to be an overlay. Save it as a .gif or .tif and save it as Transparent.(Be sure to save it as Transparent.)

In VMS drag the image to the Video Overlay track above the video clip that requires the overlay. When you review the clip you will see the graphic ovelayed while the rest of the frame will be transparent. You can fade the overlay in and out just as you would a video clip.

For the ghost effect: As an example, I wanted some buildings to lmply they were burning: The buildings were placed on the video track and a clip of fire was placed on the Video Overlay track. If you place the mouse cursor on the top of the frame of the "fire" clip it will turn into a hand. Hold down the left button and drag toward the bottom and the clip will become increasingly transparent so you see the clip on the Video track.

That's two examples of the use of the Alpha channel.

Hope that helps.

Thanks to both of you. It really helps. Could you now explain further the particularities and differences of the 4 Alpha Channel types that can be selected in the VMS Media Properties box : Undefined, Straight (unmatted), Premultiplied, and Premultiplied (dirty). There is no Help in VMS detailing this...

If you haven't already done so, you might want to download the Vegas (big brother to Vegas Movie Studio) manual. It's a much bigger and more complete manual. Haven't checked it for this topic in particular, but I go to the Vegas manual every once in a while to get details on something the Movie Studio manual skims over or skips entirely.

The manual is available for free in the downloads section of the Sony site; you might have to register but if you're on this forum chances are you already have.