HVD is an optical disc technology that can store up to 3.9 terabytes using a technique called collinear holography. It employs two laser beams - a red beam for information and a green beam for reference. The green laser reads holographic interference fringes while the red laser reads servo data from a bottom CD layer. HVD has a similar structure to CDs and DVDs but differs in its use of lasers and pulse speed, and can hold over 5,500 times more data than a CD while transferring data 128 times faster. It is not yet commercially available but promises a major increase in storage and is intended to replace DVDs.