Video mode deinterlacing is employed when the material was originally shot as an interlaced signal. The studio camera used to record the shot shoots at 50Hz (PAL). The odd-numbered fields only containing the odd-numbered line information, and the even-numbered fields the even lines. This is in contrast to film mode deinterlacing where the two fields can be easily combined to form what was just a 25 frame per second sequence.

This represents an immediate problem – the even lines are recorded 1/50 th of a second (or 1/60 th for 60Hz NTSC) after the odd lines were taken. Objects that are moving within the image will not be in the same place in the odd frame as they were in the even one. A simple combining technique (as with film mode processing - top set of circles) cannot be replicated without introducing “combing”, which are the line errors introduced when two non-matching fields are simply overlaid onto each other (bottom set of circles).

With video mode deinterlacing, the processor must compare the two fields of information, recorded 1/50 th second apart, and make its best interpolation from the data to create progressive frames for display. There are more and more complex ways of doing this to create the most accurate rendition of the image.

Non-Motion Adaptive

This is the simplest technique. When the processor is presented two fields of information, it simply discards the second field and creates the entire progressive frame based on the information from the odd-numbered lines. Each even line is an average of the odd line of pixels above and below, this newly created frame is then displayed twice in place of the two interlaced ones.

Since half the information is discarded, the resolution of the image is effectively halved. There are no combing artefacts as with the rudimentary overlaying moving fields atop each other as above, but half the detail is ignored in the process so the final result is not as accurate as the original material. This is most evident away from blocks of solid colour, and in the all-important more detailed area of the image!

Many video processors in place today employ a better technique than this with standard resolution signals. But with 1080i interlaced HD format, where the computational power required to resolve all 1080 lines (and 1,920 pixels per line) is phenomenal, the processor often resorts to just analysing 540 lines i.e. it uses this non-motion adaptive deinterlacing technique.

Motion Adaptive deinterlacing

Motion adaptive deinterlacing involves an analysis step, which ascertains first if there has been any motion between the two frames of information. If no information appears to have changed between each frame, then the process will combine both fields completely. The full line resolution is used to create a progressive frame of the same detail as the original. However, where motion has been detected, the processor “adapts” its approach and employs the interpolative technique above, averaging line information using only half of the resolution of the frame. This is known as field based motion adaptive deinterlacing. This is more efficient, but generally there will always be motion so for moving material the technique produces results only a little better than without motion adaptive detection.

More advanced interpolations use more precise analysis of the data though. The more common advanced technique being “per-pixel motion adaptive deinterlacing”. In this case the two fields are analysed to a pixel-by-pixel level as to whether motion has taken place between them. This is obviously better than basic filed-based motion adaptive deinterlacing since if there is movement, only in that precise area that movement occurred does the processor resort to interpolation. Where motion has occurred, just that area of the image is interpolated. While some detail is bound to be lost in this area of motion, there is more chance of the image being better detailed overall. The results are very true to the original when this is done well.

- within the solid area of colour, the pixels are green in both fields and so the processor can simply combine them immediately
- however the edges will have moved, so the processor retains only the pixels from the first-field. As you can see, this is already a good representation of the original
- finally the processor will fill the blanks by averaging the colour of the pixels above and below, resulting in an almost smooth curve.

Only the more powerful and better-implemented processors will employ per-pixel motion adaptive deinterlacing on standard definition signals. When it comes to using per-pixel motion adaptive deinterlacing on HD 1080i signals, only those high powered processors which use chipsets such as Gennum's VXP or Terranex's HQV enjoy this luxury. Just about all other processors will revert back to the field based motion adaptive approach.

Less common processing implementations will use a region-based technique with HD material, where the processing power available on the chipset rule out a true per-pixel technique over 1080 lines. In an almost compromise of the two techniques above, these processors improve on the field based analysis by breaking down the frames into individual regions for motion analysis. The result isn't as sharp as a full per-pixel analysis technique, but it is much better than a basic field based analysis. The Lumagen VisionHDP and VisionHDPPro processors are an example of processors that employ a region based technique on HD 1080i material.

Diagonal Interpretation of the deinterlaced signal

The more advanced processors all employ a diagonal interpolation step after deinterlacing the two fields together. This filter is employed to the newly deinterlaced frame, to remove artefacts such as “jaggies” created in the areas of the picture where interpolation has had to take place. By analysing the pixels in various diagonal lines, the processor can pick up on different shapes and angles that otherwise may not be picked up by just comparing above and below.