Click here to purchase the entire book in PDF format. Time Code Encoding There are two methods by which the time code signal is recorded onto a piece of videotape, which in turn determine their names. In order to get a handle on why they're called what they're called and why they have different advantages and disadvantages, we have to look a bit at how any signal is recorded onto videotape in the first place. We said that your average NTSC television is receiving 525 lines of information every second - each of these lines is comprised of light and dark areas (or different colours, if your TV is newer than mine...) This means that a LOT of information must get recorded very quickly on the tape - essentially, we require a high bandwidth. This means that we have to run the tape very quickly across the head of the video recorder in order to get everything on the tape. Well, for a long time, we had to do without recorded video because no one could figure out a way of getting the tape-to-head speed high enough without requiring more tape than even a small country could afford... Then one day, someone said ``What if we move the head quickly instead of the tape?'' BINGO! They put a head on a drum, tilted the drum on an angle relative to the tape and spun the drum while they moved the tape. This now meant that the head was going past the tape quickly, making diagonal streaks across it while the tape just creeped along at a very slow speed indeed. (Sidebar: the guy in this story was named Alexander M. Poniatoff who was given some cash to figure all this out by another guy named Bing Crosby back in the 40's... see Bing wanted to tape his shows and then sit at home relaxing with the family while he watched himself on TV... Now, look at Alexander's initials and tack on an ``excellence'' and you get AMPEX.) Back to videotape design... The system with the rotating heads is still used today in your VCR (except that we call it ``helical scanning'' to sound fancy) and this head on the drum is used to record and play both the video information as well as the ``hi-fi'' audio (if you own a hi-fi VHS machine). The tape is moving just fast enough to put another head in there which isn't on the drum - it records the mono audio on your VCR at home, but it could be used for other low-bandwidth signals. It can't handle high-bandwidth material because the tape is moving so slowly relative to the head that physics just doesn't allow it. The important thing to remember from all this is that there are two ways of putting the signal (the time code, in our case) on the tape - longitudinally with the stationary head (because it runs parallel with the tape direction) and vertically with the rotating head (well, it's actually not completely vertical, but it's getting close, depending on how much the drum is tilted). Keep in mind that what we're discussing here now is how the numbers for the hours, minutes, seconds and frames actually get stored on a piece of tape or transmitted across a wire.