Premiere CC and '25i'. What is 25i?

[Cary Knoop]

Originally Posted by Steve Game

- particularly for those not old enough to understand the history of interlacing as a method of spatial and temporal compression inherited from analogue television.

It has nothing to do with compression, 25p and 25i has exactly the same bandwidth requirements. Arguably 25p has the potential to compress better than 25i.

[Steve Game]

Originally Posted by Cary Knoop

It has nothing to do with compression, 25p and 25i has exactly the same bandwidth requirements. Arguably 25p has the potential to compress better than 25i.

All video is spatially compressed before coding, e.g. I don't have eyes limited to 1080 lines and horizontal resolution of 1920 lines. There are a number of expert opinions on the eye's actual resolution that tend to agree at 576 Megapixels. Here's one:
Clarkvision Photography - Resolution of the Human Eye
There is some argument about the relevant angle of vision, but none of them say that 2MP comes anywhere near it.
Consequently, all video (and film) systems apply compromises in spatial and temporal dimension of the scene to make capture of the image possible. Film has a limited resolution owing to grain
Colour TV uses:

1) the minimum luminance resolution to present an acceptable represenatation of the detail in the scene
2) a chrominance resolution that is lower than the luminance (I'm not talking about 4:4:4 cinema video) commensurate with the human eye's acceptance of lower detail in colour component of the scene
3) the minimum dynamic range to present an acceptable representation of the gamut of luminance and colour in the scene
4) an scene image sample interval that is near to or longer than human sight's persistence of vision

At sensor level, ALL video cameras compromise on the above parameters. After all this visual compression in camera, compression (in the digital representation of the video) then follows.

Interlacing is a method of further economising on the raw bandwidth of moving image video by using human vision's ability to integrate adjacent scene samples into a moving image by means of the phi phenomenon. This discarding of either analogue or digital image information is a compression technique that is engineered such that most of the time the viewer accepts/can't detect it's shortfalls. The benefit goes to the distributor (broacaster/DVD maker/streamer) in terms of reduced bandwidth required for perceptually the same quality, or conversely, a better quality for the same bandwidth.

[Juris Lielpeteris]

25i is a technical term and 50i is a commercial name of the same video with 25 frames and 50 fields per second.

[Steve Game]

Exactly, - commercial interests have confused the frames/fields issue as it creates a bigger number for what is the same thing. That looks better to the marketeers, (and non-technical customers lap it up). Fortunately, this board's posters normally avoid causing confusion by using the correct engineering terms to discuss technical matters.