物理

[VHS]Tape speed about NTSC vs PAL

One rotation of the video head drum writes a frame (each head (2) writes a field (with some overlap)). Since PAL runs at 25fps (50Hz) the head needs to spin at 1500rpm.(50*60/2=1500) NTSC requires the head at 1800rpm (actually I think it's 1798.2rpm or something like that because NTSC is 29.97fps (59.94 fields/sec)). That causes the necessary linear tape speed for PAL to be 83.4% that of NTSC. However, NTSC tracks are also wider (58microns) vs. PAL tracks (49microns), yielding a 16.5 percent less space required for PAL tracks meaning less space between tracks required and thus a lower linear tape speed. As to why the PAL tracks are narrorwer? I can only presume that PAL tracks require less guard band to prevent chroma cross talk because of PAL's alternating phases. Other theories are that head design improved more by the time PAL decks become more common (not sure how much water that holds though).