Converting interlaced video (50i/60i) to progressive frames for digital delivery and post-production. Bob method for motion, blend for stills.
You have interlaced footage from playout or an older camera — two fields per frame, time-offset, optimized for CRT televisions. However, modern workflows require progressive full frames. This is where deinterlacing comes in: the software must construct a single, coherent image from these two fields (odd and even). This sounds simple, but it's an algorithmic problem that needs to be solved differently depending on the motion.
The two standard approaches differ fundamentally: Bob deinterlacing doubles the frame rate and interpolates between fields — perfect for static or slow scenes where the phase offset of the fields isn't visible. Blend deinterlacing optically merges the two fields, making fast motion smoother but potentially leading to slight ghosting. You'll immediately notice the difference when playing back action or fast pans: Bob will flicker, Blend will be soft. Professional software like DaVinci, Premiere, or specialized tools offer adaptive methods that switch between both techniques depending on the motion pattern — ideal, but also computationally intensive.
In practice, you primarily need this for: HD cameras shooting in 50i or 60i (still common in broadcast and live events), video tape transfers, or when digitizing old archive footage from television sources. A DVCam tape from the local broadcaster? 50i. A sports broadcast from the archive? 60i. Before you need these clips in a DCP workflow or even just in a 24p project, you must deinterlace. The wrong algorithm will lead to artifacts — especially unusable on a large screen.
Tip on set: If your camera can switch between progressive and interlaced, always choose progressive if the frame rate is suitable. You'll save a complete VFX step later. And when digitizing analog material (film-to-tape transfers), you should ask if the source material has already been deinterlaced — some labs do it beforehand, others give you the original and you do it yourself. Quality check: Always run a test sequence with fast motion through Bob and Blend before processing 10 hours of material.