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Web Presenters Video Production Leicester

There are 2 main types of encoding for video. There are Progressive and Interlaced.  Progressive encoding is when each frame represents one frame of video. Interlaced is when each frame represents either all the odd or even rows of pixels in the video. The interlaced frames are called fields and there are both odd and even fields. TV screens traditionally have interlaced displays and these will render the fields one at a time, alternating the fields. A progressive display on the other hand renders all of the frames at once.

Is is best to encode interlaced video when the content is being encoded on an interlaced device such as a TV. Content that is intended to be displayed on a computer display however should be progressive.

In order to encode interlaced video as progressive you need to look at determining the input settings to de interlace the video.

Field Order

Almost all sources of interlaced video will output samples that include both frames interleaved with one another. The outcome is that the even and odd frames of video are slightly shifted left or right in time.   There is no standard as to which fed occurs first in time and this is why users need to specify field order when passing interlaced samples through any application.

Interlacing

One needs to realise that how an  image appears on TV and how the image is actually made up are two completely different things. All videos that are fi,med at full vertical resolution (480 or more across) is likely to be susceptible to interlacing.

Because of this an understanding of interlaced video is critical.

Frames And  Fields

Video is made up of a number of snapshots called frames. Frame rate for the US is called NTSC and there are 29.97 frames for eachsecond. In european countries we use PAL or SECAM that will display 25 frames per second.

It is important to understand that TV does not deal with video in the same way.

TV’s deal with fields which are essentially (half frames).

Since there are 2 fields in each frame then a TV actually updates the display at 50 fields per second for PAL/ SECAm video.

A TV will update each frame of video separately. It is because of this that a TV must update its display twice simultaneously to show a full frame. It first updates the odd field and 1/50th of a second later is updates the even field.

How Do I Remove Interlacing Artifacts?

If you have decided that the output of the video is not  going to be on an interlaced display such as a TV you will want to remove the interlacing artefacts. There are 3 primary methods that are used to do this.

1) Deinterlace The Video

A) Remove one field of the video; either the odd or even lines and then blend (interpolate) the remaining lines. This method isn’t great as you will lose half the frame in your video. This will make the video play less smoothly because the field that you removed represents a unique point in time.

The other issue is that you reduce the resolution of the video because you are removing all of the odd or even lines in that frame. There are some adaptive interlacing plugging that work around this problem where they only affect areas where the interlacing is causing a problem.

B) The other option is to blend together the fields of the video so that the combing effect is not visible. The advantage to this method is that none of the frame are actually removed so you get higher resolution and smoother playing video.

You can, however, get a ghosting effect during motion which is accused by the blending of the two unique fields within the single frame of video.

C) Bob Deinterlacing. In this method each field of video is made into a single a separate frame. Because each field has only half the height of a full frame each field is resized to twice its original height. Bob has the effect of removing interlacing artefacts whilst doubling the amount of frames in the video. PLaying a bob interlaced video on a  progressive display is like lying an interlaced video on an interlaced display because each field is updated separately.

Disadvantages with this are that you have to determine the field order of the video.

It will take more commuter power to play back video with teach as many frames. You will also have to reinterlace the video if you want to properly play it back on a TV.

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