Frame size, Frame rates, Progressive or Interlaced
By Editing Team (Eyefish.tv)
A High Definition picture is defined by 3 parameters:
|Frame Size||1080x1920 or 720 x 1280 pixels|
|Scanning Method||Interlaced, Progressive or Progressive Segmented Frame (PsF)|
|Frame Rate (Progressive)||24,25,30, 60, 23.98 or 29.97 frames per second|
|Frame Rate (Interlaced)||50 or 59.94 fields per second|
You will see several different ways of describing the same High Definition format.
1080i59.94 = 1080 59.94i = 1080i29.97 These are all the same. 1080i29.97 means 29.97 interlaced frames per second. The"i" tells us that each whole frame is made up from 2 fields ie. 2 x 29.97 = 59.94 fields per second as in the 1080i59.94 notation.
The BBC and other Broadcasters are adopting a standard notation system which references whole frames only and not fields, so 1080i59.94 becomes 1080i29.97 and 1080i50 becomes 1080i25. This is the notation we use within Eyefish.
1080i29.97 means a frame which is 1080 lines high x 1920 pixels wide recorded with 59.94 interlaced images (fields) per second.
1080i25 means a frame which is 1080 lines high x 1920 pixels wide with 50 interlaced images (fields) per second.
1080p25 means a frame which is 1080 lines high x 1920 pixels wide with 25 progressive frames per second.
720p25 means a frame which is 720 lines high x 1280 wide recorded with 25 progressive frames per second.
Progressive or Interlaced
Cameras can be set to record pictures in one of 3 scanning modes: progressive, interlaced or progressive segmented frame (PsF).
Progressive: The entire image is scanned and recorded as one complete frame. This is often seen as being more 'film like' because it mimicks the 24 or 25 frames per second frame rate used by film cameras.If you're shooting in progressive, you must have the camera's shutter set to 1/50 or 1/60 sec.
Interlaced: Odd numbered horizontal lines are recorded as one image followed 1/50 sec (or 1/60 sec for NTSC) later by the even numbered lines as a second image. Each separate image is called a field and two fields are combined to form a frame.
PsF: Records the entire picture as 2 separate fields but differs from interlaced because both fields are of the same image taken at the same instant in time. Pictures recorded in PsF mode are compatible with interlaced pictures and can be fed directly into machines used to seeing interlaced video as they still offer 2 separate (but identical) fields per frame.
High Definition is its own standard and is separate from PAL and NTSC
However, because it has to be transmitted over existing analogue networks, it needs to be compatible with NTSC and PAL systems and this is why there are different frame rates. For the NTSC system programmes are usually transmitted in 1080i29.97 and for the PAL system in 1080i25. The transmission format has an effect on your choice of aquisition format.
These are the different frame rates currently being used:
For the USA (to accomodate the NTSC system): 23.98p, 24p, 29.97p, 30p, 59.94i, 60i
For the european PAL system: 24p, 25p, 30p, 50i, 60i.
Europe is moving towards standardising HD broadcasting to 720p and later 1080p.
Note: there is no such thing as 720i.