物理 一个有趣的问题
这里有一个问题，同样的sony e120磁带，在NTSC,SP模式下录制时间是1:32:09,在PAL,SP模式下录制时间为2:02:10。
但是，在NTSC,LP模式下可以录制5:01:09，在PAL,LP模式下可以录制4:03:01。
为什么在SP模式下，NTSC录制时间比PAL短，但在LP模式下PAL录制时间比NTSC短？
如果确实如传言所说，NTSC消耗的磁带长度大于PAL，那为什么LP模式下PAL消耗的磁带长度大于NTSC？
问题成功的解决。
NTSC的LP实际是SLP，PAL的LP还是LP。
NTSC时间*3，PAL时间*2
PAL 4hr=NTSC 3hr
PAL-LP=PAL*2=4hr*2=8hr
NTSC-LP=NTSC*3=3hr*3=9hr
当LP模式，NTSC>PAL
当SP模式，PAL>NTSC
这就是结局。
For PAL, the VHS linear tape speed is 2.339 cm/s, for NTSC-VHS it is 3.335 cm/s. Can someone explain that to me?
To get the 50 half-frames per second for PAL, the video heads spin with 50 rps. For NTSC, the head rotates with 60 rps. This is 20% more. However, the NTSC tape speed is 42% over the PAL-VHS tape speed.
There's also matters of resolution. While there are distinct vertical scanlines, analog storage is essentially continuous resolution on the horizontal, so it's limited really by analog bandwidth. For NTSC VHS, the bandwidth, IIRC, is about 3.2 MHz. Divide this by fps (29.97), and by the number of lines per frame (525), and you get about 200-ish cycles per line of video. Assume that about 20% are used for sync info, and only the remaining 80% are used for actual video content, and you get 160 cycles per line.
I don't know offhand how it works out for PAL, but I imagine that a fair bit of difference is showing up here and the fact that you've got 625 lines per frame and 25 fps.
It's actually quite simple... 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. 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).
NTSC LP = 90*3 = 270
PAL LP = 120*2 = 240
问题成功的解决。
NTSC的LP实际上是SLP，PAL的LP还是LP。
NTSC时间*3，PAL时间*2
PAL 4hr=NTSC 3hr
PAL-LP=PAL*2=4hr*2=8hr
NTSC-LP=NTSC*3=3hr*3=9hr
当LP模式，NTSC>PAL
当SP模式，PAL>NTSC
这就是结局。