The letter after the resolution, as in 1080i or 1080p, indicates two very different ways of building the picture. Today almost everything is progressive, but interlaced video is still present in digital television, in broadcasters' archives and above all in old family recordings on VHS, Hi8 or MiniDV. Anyone digitising or reworking this material almost always meets the characteristic comb effect on moving edges, and needs to know how to deal with it.
Progressive: one picture at a time
In progressive video (the «p») every frame is a complete picture, with all lines captured at the same instant. This is how modern cameras, smartphones, computers and web platforms work. A 1080p video at 25 fps shows 25 complete pictures per second.
Interlaced: two interwoven halves
In interlaced video (the «i») every frame is made of two fields:
- the first contains the odd lines;
- the second contains the even lines;
- the two fields are captured at different moments, half a frame apart.
The PAL system transmits 50 fields per second, which form 25 frames. The technique was born with analogue television, to obtain smooth motion without doubling the bandwidth needed: the cathode-ray tube showed the fields one after the other and the persistence of the phosphors and of vision blended them.
Why you see the comb
On a modern screen, which displays progressive pictures, the two fields are shown together. If there is motion in the scene, the even and odd lines depict the object in two different positions and the edge looks jagged, like the teeth of a comb. In static scenes the effect is not noticeable at all, which is why it sometimes goes unnoticed until you watch a pan.
Field order
Every interlaced source has a field order:
- TFF (top field first): the top field comes first; typical of many broadcasts and HD formats;
- BFF (bottom field first): the bottom field comes first; typical of standard-definition DV.
If the order is interpreted the wrong way round, motion jerks back and forth, which is very irritating. Tools such as MediaInfo report the declared order, but it is not always reliable: when in doubt, try both settings on a scene with motion.
How deinterlacing works
Deinterlacing means rebuilding progressive pictures from the fields. The main methods:
| Method | How it works | Result |
|---|---|---|
| blend | mixes the two fields | no comb but double images in motion |
| field discard | uses only one field and doubles the lines | half vertical resolution, jaggies |
| motion-adaptive | interpolates only where there is motion | good compromise |
| frame rate doubling | turns each field into a frame | smooth motion identical to the original |
Keeping the smoothness
The last method is often best for «real» video recordings, such as sport or home movies: a 50-field PAL video becomes a progressive one at 50 frames per second, preserving the smoothness of the original motion. Reducing to 25 fps, by contrast, throws away half the temporal information.
With FFmpeg
The bwdif filter is a good-quality adaptive deinterlacer:
ffmpeg -i tape.avi -vf "bwdif=mode=send_field" -c:v libx264 -crf 18 -c:a aac tape_50p.mp4
The send_field option produces one frame per field, so it doubles the frame rate. With mode=send_frame you get the same frame rate as the original instead. There are also much slower and more sophisticated deinterlacers, such as QTGMC, used by archive restorers.
The telecine case
Not everything that looks interlaced really is. A 24-frame film broadcast in an interlaced format may have been split into fields with no intermediate motion. In this case it should not be deinterlaced but reassembled (inverse telecine or field matching): the right fields are paired up again and the original frames recovered, without loss. With PAL the two fields often belong to the same frame, and the material is effectively progressive.
Digitising tapes: practical advice
- Capture in interlaced form and without scaling: deinterlacing should be done afterwards, carefully, on the digital file.
- Do not change resolution during capture: standard definition should be kept, and any upscaling done at the end.
- Keep the raw capture as a master: deinterlacers improve over time, and you may want to redo the processing.
- Check the aspect ratio: standard-definition formats have non-square pixels, which must be handled correctly.
In short
- Progressive means complete pictures; interlaced means two fields captured at different moments.
- The comb appears when fields are shown together during motion.
- The field order (TFF or BFF) must be set correctly.
- Doubling the frame rate when deinterlacing preserves the original smoothness.
- Films transferred to interlaced formats should be reassembled, not deinterlaced.
