- [Instructor] If you're working in video,
you're probably already pretty comfortable
with the idea of a frame.
It's a single, complete image,
one of many displayed in a continuous series
that creates the illusion of movement.
A single frame of video is literally an image,
little different to any photo or graphic you might have.
But there's one difference, I suppose,
which is that the compression systems
used to store video information
often include a temporal factor, a time factor.
They can compare two frames or more
and only store the differences, for example.
Still, regardless of the way a frame is stored,
it's still just an image ultimately like any other.
Except in one particular instance.
If your video frame is recorded, stored,
and played back as a whole, single frame,
it's referred to usually as progressive.
There's another way to record frames of video,
which allows for the appearance
of smoother movement, called interlacing.
The principle is quite simple.
Let's imagine you're recording NTSC video
at 30 frames per second.
I know we normally think of NTSC as 29.97 frames per second,
but that's another story for a lesson on frame rates.
If you were recording progressive video,
you'd have 30 frames per second.
But if you were recording interlaced video,
each frame would be constructed
from first all the odd lines,
and then all the even lines as separate images.
This means you'd actually capture 60 images a second,
with each image being half a frame's resolution.
This gives you smoother movement,
particularly when the camera pans or tilts.
And it was developed as a system
to help earlier cameras and TVs record and display video.
One set of lines from an image,
whether it's the odd lines or the even lines,
is referred to as a field.
And the order in which the fields are recorded
can vary from system to system.
For this reason, interlaced video is described
as upper field first or lower field first,
or odd field first or even field first.
The order in which the fields are recorded
is defined largely by the video technology
used to capture it.
It's not something to choose arbitrarily,
but you will sometimes find the order
is displayed the wrong way while working with footage.
Editing applications have options
to specify the field order for playback,
and the good news is that it's very easy to fix
if it's wrong.
Here's a video clip with the field order set the wrong way,
and notice the way movement
appears to stagger across the screen.
That's because we're jumping backwards and forwards in time,
as the fields are displayed in the wrong order.
There are other technical issues that can cause this effect,
but if you come across video
that looks fine until something moves,
the field order would be the first thing to check.
Just for the record, if you ever see a frame rate
followed by an I or a P,
you're seeing an indication
that the footage is interlaced or progressive.