A Brief History of Interlacing
When broadcast television was invented, engineers faced a problem: transmitting a full video frame fast enough to avoid flicker, without the bandwidth to carry all the data in one go. The solution was interlacing: instead of sending a complete image every frame, the system sent two fields — first all the odd-numbered lines, then all the even-numbered lines. Each field arrived at half the data cost, and the brain stitched them together into smooth motion. It worked brilliantly for decades. Broadcast TV ran on interlaced signals — 50i in Europe, 60i in the USA — right through the analogue era and well into the digital one. Which is why editing software, broadcast monitors and export presets still carry interlaced options today.
The Problem: DCPs Are Progressive
Cinema has always been progressive. Every frame is a complete, whole image. DCPs are progressive too — always. There is no interlaced option. There never will be. So if your source file arrives interlaced, it has to be converted. And if that conversion is not done carefully — or is not spotted at all — you will see the result on screen.
What It Actually Looks Like
The classic tell is combing: on fast-moving subjects, horizontal lines tear across the edges of movement like the teeth of a comb. On diagonal lines — a staircase, an angled wall, a car in motion — you get aliasing: a jagged, stepped edge that should be smooth. It is especially visible on text. If your credits were exported from an interlaced timeline, the fine horizontal strokes of letters will shimmer and strobe. On a laptop screen you might not notice it. On a cinema screen four metres high, it is immediately obvious.
Where It Comes From
Most filmmakers shoot progressive — but they may be editing on a timeline that was set up as interlaced, perhaps matching a broadcast delivery spec or inherited from a template. The footage looks fine on their monitor. The export looks fine in a preview. Automated DCP software will do its best with what it is given — but if the interlacing flags are wrong or inconsistent, the software may deinterlace incorrectly or not at all, and produce a perfectly valid DCP of a visually broken image. Only a human watching the source file critically will catch it.
High frame rate alternative: If your content was shot at 50fps or 60fps and you want to preserve that fluidity in cinema, a 50fps or 60fps DCP is a genuinely viable option supported by most modern cinema equipment. Worth discussing at the start of the process.
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