While you're enjoying all your 720- and 1080-line television broadcasts, this weekend it's time to reach back and remember the first "high-definition TV" that went into service at Alexandra Palace in London 75 years ago. Back in the '30s "HD meant an upgrade over the original systems, to resolutions of more than 240-lines. When the BBC debuted service (to some 400-odd viewers) it actually used two systems, one with 405 interlaced lines, and another mechanical system with 240 progressive lines of resolution. The studios are still there and open for tours so if you're in town you may want to stop by and pay respects Saturday or Sunday. If you're looking for more historical background from a distance, there's a magazine article with a breakdown and plenty of pictures of old cameras and sets linked below.