Interlaced field order for DVD movies

This is a lot more complicated subject than it should be.

1. The only kind of output specified in the DVD-Video specs is interlaced.

2. Film content that's put on an NTSC DVD can either be encoded progressive at 23.976fps with pulldown flags added or encoded interlaced with the right fields duplicated to create a 29.97fps interlaced stream.

2a. Pulldown tells the player to repeat certain fields to compensate for the difference between film frames (24fps - or 23.976fps for pulldown purposes) and NTSC fields (59.58 fields per second). Consider the following sequence of film frames (T denotes the top field during interlaced playback and B denotes the bottom field)

Code:

 1T  2T  3T  4T  5T 
 1B  2B  3B  4B  5B 

When pulldown is applied:

Code:

 1T  1T  2T  3T  4T  5T
 1B  2B  3B  4B  4B  5B 

3. Since the DVD-Video specs don't allow for progressive video it's entirely up to the DVD player and TV manufacturers to determine how progressive playback is acheived. Different players support different types of deinterlacing. The better ones use multiple methods and can adapt to the specific video being played on the fly.

3a. The easiest deinterlacing to recover film frames is to simply read the pulldown flags and use them to reconstruct the original progressive frames. Unfortunately this doesn't work if the pulldown was done before encoding and therefore the video is really interlaced.

3b. Another method is to look for repeating fields in interlaced video to recognize pulldown applied before encoding. If a pattern is detected the player can then attempt to recover the original film frames. This won't give you as good a picture because there are multiple copies of the duplicated fields, and the player has to recognize that they're the same and then pick the best version to reconstruct the frame with.

3c. If the video is interlaced and not originally from a film source (ie real 29.97fps interlaced content) the player can blend the fields in each frame together. This technique by itself will tend to give you the lowest quality progressive picture possible.

3d. Instead of simply blending, some players will bob the video. This involves treating each field as an individual frame and moving every other field up one line to match top and bottom fields to the same spatial point of reference. An entire frame is then made by interpolating (mathematically guessing the position of each pixel in the composite frame.

3e. Motion Adaptive Deinterlacing can be used to decide what to do with a frame. If you have a frame of true interlaced video with no motion between the top and bottom fields there's no need to do anything except combine the fields into a single picture. If there's motion some other method gets used.

3f. The best players use a combination of techniques. If there are pulldown flags they get used first. If there aren't flags, pulldown patterns are searched for. If there isn't any pulldown pattern they will default to whatever they would use for pure video - hopefully motion adaptive deinterlacing.

3g. From what I remember, most progressive scan DVD players (or at least most of the ones around when I was researching this a couple of years ago) still send film content to your TV at 29.97fps. The difference is that entire frames are duplicated instead of individual fields. This is still an improvement for a number of reasons.

4. As long as their is motion in your video field order makes a big difference for actual interlaced video. Not so much for pulldown since, as you can see from my example, field order is always off part of the time. Think of it this way. If an object moves from left to right in the video, and it's in a different position in every field, using an incorrect field order would make the object move left, then right, then left, then right instead of maintaining the same direction.

4a. If your video is progressive with pulldown flags the order doesn't matter as long as it's consistent. The most important thing is that the pattern stays constant.

4b. If your source footage (ie before MPEG encoding) was interlaced then you need to keep the same field order as the original footage. For example, if you started from an NTSC DVD it will almost always be TFF (top field first) but if you started with DV footage it will be BFF (bottom field first). For the reasons stated above the field order for true interlaced material is important, but if you screw it up on your encode it can be changed without re-encoding. This is very similar to adding pulldown flags as you're just telling the decoder to display fields in a different order. There are several programs that can do it. The only one that comes to mind off the top of my head is ReStream.

5. Every time you re-encode a file in any format that's not lossless you'll lose quality. You may not always see the difference on your particular playback equipment, but it always happens.

 

Are you sure the video is actually encoded progressive?

This I understand, however would further quality be lost as a result of changing the field order?[/quote]
Only for interlaced sources.

 

It doesn't sound like it got encoded with pulldown or even true interlaced with a pulldown pattern. You can test this by stepping through the video one frame at a time. If you see 1 frame that looks progressive followed by 3 that look interlaced.