Interlaced vs Progressive

 

Perhaps one of the most difficult concepts with video is the two types of displays and encoding - interlaced and progressive.  There are actually 3 methods of showing video on a screen:  stills, progressive scan, and interlaced scan.

Film is a series of stills, or photographs on a celluloid strip of film, that a projector beams light through and onto a cinema screen:

Progressive scanning attempts to mimic film by scanning the complete image, line after line, consecutively.  However, this takes some time for each frame, and introduces an effect known as flicker.  To get rid of flicker, most progressive scan devices double the frame rate to 60 fps - which is the same as the field rate of interlaced video.  However, to do that if converting interlaced into progressive, means replicating info (see conversion methods below).  The following image is a simplification of a one progressive scanned frame.  The tiny white lines are placed there only to allow you to see the separate scan lines, but are not part of the actual progressive image:

Interlaced scanning doubles the rate of images displayed by first scanning all the odd numbered lines and then scanning all the even numbered lines - or vice-versa depending on the equipment.  Each of these half-images are called fields.  Two fields comprise the original image and are called a frame.  This gets rid of the flicker, but it also does not display frames as clearly and solidly as progressive does.

           

To get 30 fps, for progressive scan you scan 30 frames each second.  With interlaced you scan 30 frame each second, but to do that you must scan 60 fields each second - since 2 fields = 1 frame.

Converting Interlaced to Progressive

Since both scanning methods are popular - both hardware and software may be required to convert from one to the other.  If you have a progressive DVD player and feed an NTSC interlaced video signal into it - then the video must be de-interlaced.

Two popular methods are "weave" and "bob" deinterlacing - and both of these are commonly referred to as line doublers.  The following diagrams explain the methods.  In both cases two fields are used to create two frames.  With interlaced video, the two fields = one frame, at a rate of 30 frames per sec (60 fields per sec).  But with progressive video, there are 60 frames per sec, so we need one frame for every field.

               

Weave Deinterlace                                                               Bob Deinterlace               

The weave method has the advantage that the two fields are averaged in with both of the first two frames - and every line with a frame is unique.  However, the two frames were shot 1/60th of a second apart, and this messes up the timing a bit, since you are showing Field 1 info during frame 1, and you are showing Field 2 info during frame 2.

The bob method retains the timing.  Field 1 is placed into frame 1, so they both display that field at the proper time, and all the fields are shown in the proper sequence.  However, it does have the disadvantage of doubling the lines - so that every set of 2 lines are identical.

Two other conversion methods are Field Adaptive and Motion Predictive - but they require huge amounts of processing.