digital stereo PCM sound on a special reserved track. Digital8 is the most recent 8mm video format. It retains the same physical cassette shell as its predecessors, and can even record onto Video8 (not recommended) or Hi8 cassettes. However, the format in which video is encoded and stored on the tape itself is the entirely digital DV format (and thus very different from the analog Video8 and Hi8). Some Digital8 camcorders support Video8 and Hi8 with analog sound (for playback only), but this is not required by the Digital8 specification. In all three cases, a length of 8mm-wide magnetic tape is wound between two spools and contained within a hard-shell cassette. These cassettes share similar size and appearance with the audio cassette, but their mechanical operation is far closer to that of VHS or Betamax videocassettes. Standard recording time is up to 180 minutes for PAL and 120 minutes for NTSC. (The cassette holds the same length tape; tape consumption is different between PAL and NTSC recorders.) Like most other videocassette systems, Video8 uses a helical-scan head drum (it having a small 40mm head) to read from and write to the magnetic tape. The drum rotates at high speed (one or two rotations per picture frame—about 1800 or 3600 rpm for NTSC, and 1500 or 3000 rpm for PAL) while the tape is pulled along the drum's path. Because the tape and drum are oriented at a slight angular offset, the recording tracks are laid down as parallel diagonal stripes on the tape. The heads on the drum of a Video8 recorder move across the tape at (a writing speed of) 3.75 meters per second. Unlike preceding systems, 8mm did not use a control track on the tape to facilitate the head following the diagonal tracks. Instead 8mm recorded a sequence of four sine waves on each video track such that adjacent tracks would produce one of two heterodyne frequencies if the head mistracked. The system automatically adjusted the tracking such that the two frequencies produced were of equal magnitude. This system was derived from the dynamic track following (DTF) used by the Philips Video 2000 system. Sony rechristened the system as automatic track following (ATF) as the 8mm system lacked the ability of the heads to physically move within the head drum. The main disadvantage of the ATF system was that unlike in the case of a control track,