Interlaced NTSC SD (601) digital video
We're just going to experiment with some different flavors of SD to get an idea of what's going on with interlacing and how to live with it. I won't get into the issues around field order here, as that depends largely on your source material and edit system. These examples use A as upper field and B as the lower field.
The source frames at the bottom of the page were used to create a side by side comparison of various manipulations with the fields and resulting frame. Tthe sources were created in Photoshop as 24 bit TIFF files. I've used the square pixel aspect ratio 720:540 (which works out to be 4:3 when imported to an editing system). The fields were interlaced in Eyeon's Digital Fusion, but After Effects or other field capable compositing tools can do the same things. The rendered images were brought back into Photoshop for text and JPEG outputs.
I chose to work with the primary RGB values. Field A is 0,0,255 blue, while field B is 255,0,0 red at the top, 0,255,0 green at the bottom. When blended together, this should result in magenta - 255,0,255 at the top and cyan - 0,255,255 at the bottom.
The first column has the actual source fields - 540/2=270 pixels high. The next two columns are the same fields stretched to 540 pixels high. Following that is a Photoshop transparency where the fields are 33% opaque and the background is white. The next two columns are interlaced. (I set the first to NTSC legal so you could see how a broadcast chain will subdue the cyan). The last column is a pure magenta to cyan transition - which could be done in progressive scan mode. Notice that the interlaced images much more closely resemble the progressive column than does the transparency overlay column. Also, the middle of the progressive column has smooth color blending, while the interlaced columns have very visible banding. Even the overlay column shows some banding resulting from the stretched source (it's also visible in the stretched RG column - just one of the pitfalls of working with 8 bit color).
Comparisons
Other notes: As you look at the frames below, note that the letters A or B were added to the field at normal height. When interlaced, they doubled in height. This illustrates how difficult it is to gauge perspective when you split the fields out. You can also see how black on only one field takes on the color of the other field, while white overpowers the color of the other field.
Sources
For those who wish to download the images: These must be scaled properly when using in your editing system.
Interlaced video
Fields
Frames
Oh, yeah... We have to deal with the copyright issue too.
Contents ©2005
by Robin McCain/Cinematiks.
All rights reserved. Credits must be included in all copies. Limited permission
given for educational use..
