Friday, March 18, 2016

telecine 2:3 pulldown

Films are shot and projected at 24 frames per second. NTSC based televisions and monitors display 60 pictures per second. The technique to convert from film fps to telivision fps is called telecine process.

First each input frame is divided into two fields, odd and even scan lines. This is called interlacing. It reduces the bandwidth to transmit by half.

Next it follows a field duplication method called 3:2 pull down. Since 60/24 = 2.5 = 5/2, we need to create five pictures out of every two frames. It repeats the first field two times and second field three times and so on for the odd and even number pictures.

If the input is already interlaced for broadcasting and we want to display on computer monitor, we need to deinterlace at the least to properly display it. It is not simple summing the fields into one picture and need some averaging of some pixels from adjacent fields. Pictures are not static and they are half pictures of two consecutive pictures of motion. If it is telecined, then deinterlacing is more than the averaging the adjacent pictures and involves finding the right pictures to average.

The term pulldown is actually about slow down. It was mandatory to have color television broadcasting to allow old black and white televisions to continue to work. They continued to transfer luminescence or lightness or brightness information along with chrominance or color information. They were also concerned about additional bandwidth required to transmit color information to black and white telivsions. They slowed down the frames per second by transmitting only 1000 frames during the interval 1001 frames were transmitted earlier.

Wikipedia page 3:2 pulldown process
Apple tutorial telecine process
Miscrosoft doc temporal rate conversion
Framerate follies trick

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