 Known as the vertical blanking interval (VBI), these extra lines historically served no purpose other than to contain the vertical synchronizing pulses, but in more modern implementations they are used to carry or convey different things in different countries; for example closed captioning.Macrovision's legacy analog copy protection (ACP) works by implanting a series of excessive voltage pulses within the off-screen VBI lines of video.These pulses were included physically within pre-existing recordings on VHS and Betamax and were generated upon playback by a chip in DVD players and digital cable or satellite boxes.