Progressive vs Interlaced Frames
Progressive (usually denoted by p as in 24p or 1080p, etc) frames are whole frames. If I’m transmitting 32fps progressively, I’m transmitting 32 full frames per second. There’s no catch. Film is shot at 24 progressive frames per second.
However, when interlaced, each frame is no longer whole, or full. So is it still a frame?
No, to distinguish a cut-up or interlaced frame from a full frame, it got a new name. It’s called a Field.
Fields are denoted with the symbol i, as in 50i or 60i.
Whenever you see something like 50i what it means is the frame rate is 25 fps at 50 fields per second. The video is playing at 50 or 60 fields per second. The frame rate is still 25 or 30 fps.
Whenever you see something like 60i what it means is the frame rate is 30 fps (actually 29.97 fps) at 60 fields per second (actually 59.94i).
Theoretically, you can have 1i or 120i or any other number you like, but in practical terms there’s only 50i and 60i. There is no such thing as 25i or 30i in standard broadcast.
Frame and Field Rates used today
The most common broadcast systems for SD and HD are as follows:
Believe it or not, 24p is actually a rare standard for TV. Most of the time, 24p is broadcast as 23.976 fps.
As far as interlaced footage goes, there’s only 50i and 60i – period. It is understood that 50i means 50 fields per second at 25 fps, and 60i means 60 fields per second at 29.97 fps (or in some cases 23.976 fps).
A very important fact to note is that even when some manufacturers claim to have true 24 progressive, they might really mean 23.976 progressive. This is also the case for 30 fps, which usually means 29.97 fps.
This is typically because, to keep costs down, the sensor is designed to send out only one ‘type’ of data stream. In many ‘prosumer’ cameras, the sensor is reading out at 50i or 60i.
It is relatively easy for a manufacturer to convert a 50i or 60i readout to pseudo 24p, and you wouldn’t even know it. But they stick to 23.976 fps for the same reason they use 29.97 fps – that is the broadcast standard, and it makes workflows easier if the footage is already in a delivery standard.
Odd and Even Fields
One thing you might have noticed is that the lines on two subsequent fields is not the same – otherwise when you add them how will they match?
The field that has scanned the odd lines (1, 3, 5 and so on) is called the Odd field, and the field that has scanned the even lines (2, 4, 6 and so on) is called the Even field.
Remember our earlier example:
Clearly, Fields 1, 3 and 5 are the even fields and Fields 2, 4 and 6 are the odd fields.
What happens if we mistake Fields 2, 4 and 6 as even fields? Instead of our expected result:
We’ll end up with something like this:
Not the same thing, right? When working with interlaced footage, keep an eye out for weird line effects – it’s usually caused by the wrong choice of odd or even fields when conforming footage.
Usually, when one uses software that is said to support a given camera, it usually means that the software can correctly interpret the footage from the camera, and this sort of mistake doesn’t happen. However, if one changes the parameters in the software to deviate from standard practices (lots of reasons why one should do so), then it becomes the responsibility of the user to correctly interpret the footage.