Trick mode

Trick mode, sometimes called Trick play, is a feature of digital video systems including Digital Video Recorders and Video on Demand systems which mimics the visual feedback given during fast-forward and rewind operations that were provided by analogue systems such as VCRs by manipulating the video stream to include only a subset of frames.

Implementation Issues

With an analogue system the visual fast-forward/rewind effect was generated simply by transmitting the frames faster and/or in reverse; there was an inevitable loss of frame synchronization or 'tearing' but this was accepted as the norm. With a digital system it is unlikely the decoder can process the digital stream significantly faster than normal, and certainly not backwards. Hence only a subset of frames can be presented to the decoder.

This is made harder by the fact that in video compression systems such as MPEG-2 and H.264 many frames are dependent on previous frames for display, and hence cannot be decoded independently. Only certain "I" or "Intra" frames are independent. Hence a system offering trick mode has to select only those frames for display, which requires analysis of the digital stream either at the time of playback or in advance.

In a single-user disk-based system it is possible to scan the media file in real time to locate the independent frames, but in a network-based video-on-demand system the bandwidth allocated to the client is fixed, so the server has to use pre-generated 'hint' or 'index' information to locate suitable frames, and then play them out at the selected fast-forward or rewind speed within the original bandwidth envelope. Alternatively the server may pre-generate an entirely new video stream at at least one forward and backward speed and switch to it when requested.