In relation to R. Inglis' enquiry - sorry, but Don Larsson and Lang Thomson are reproducing a widely understood but essentially incorrect myth here. Early sound on film systems needing 24fps (i.e. a foot and a half of 35mm film per second) consolidated the use of 24fps as a standard, but that wasn't what started it off. Bear in mind that Vitaphone worked at any speed you like - the only crucial parameter was the speed of the record. What really did it was projector shutter design. The bottom line is this - to fool the human brain into thinking that a lot of still pictures shown in quick succession are in fact one continuously moving picture, you need at least 48 expose and obscure cycles per second. That means a picture has to be projected, a shutter blocks the beam and it opens again 48 times every second. Now most projectors have what's called a two-bladed shutter. That is a concentric disc with two holes punched out of the edges. That disc rotates, driven by a shaft which is also linked to the projector's intermittent mechanism. But the film is only pulled down in one of every two obscures, hence 48 cycles per second but only 24 frames of film per second. Back in the 20s, it was common to have three bladed shutters. Same principle as above, only three blades and three holes. Three cycles per shutter rotation means you only need 16fps of film movement (16 x 3 = 48). But, because more of the shutter surface area per rotation is blocking light, the projector light source is less efficient, and that is the real reason why projector designers gradually moved from three to two bladed shutters during the 1920s. The picture palaces of the 20s really stretched projector lamp technology and the designers needed every speare foot-candle they could get. You need a third more light for a picture to look as bright with a three bladed shutter as it does with a two. Hollywood complied by upping camera speeds. Bear in mind that if the projector mech speed goes significantly below that crucial 48 expose/obscures per second, you'll see a horrid flicker on the screen. Anyone who has tried to watch BIRTH OF A NATION or a German silent (which were shot very slow right until the conversion) at 16fps on a two-blade projector will have the headache to prove it. Under pressure from exhibitors, Hollywood relented and increased shooting speeds, basically to make their pictures look better on the projectors that most exhibitors were using. By the time sound came along (and it is true that the De Forest system really needed 1.5 feet per second, even though Vitaphone didn't) the argument was over and that just cemented it. Feel free to email me off list if you want any references for the points mentioned above (havan't got 'em to hand, will dig 'em out if you wan't em, can't be bothered if you don't!). L ------------------------------------ Leo Enticknap Technical Manager City Screen Cinemas (York) Ltd.. 13-17 Coney St., York YO1 9QL. United Kingdom Telephone: 01904 612940 (work); 07710 417383 (brainfryer) e-mail: [log in to unmask] (work); [log in to unmask] (home) www.picturehouse-cinemas.co.uk ---- Screen-L is sponsored by the Telecommunication & Film Dept., the University of Alabama: http://www.tcf.ua.edu