Interlaced or Progressive for SVCD?

[tempo]

hi there,

This is my first post and I would like to ask: Is it correct that a progressive SVCD have less blocks than an interlaced one due to the fewer frames/fields that each second contains? (since SVCD is born to be in a shortage of bitrate)

And, does a progressive SVCD really look terrible on TV?

Thanks for your help!

[adam]

I think you are sort of on the right track but are confusing a few core concepts. Whether its progressive or interlaced does not affect the number of fields/frames. 2 fields equals 1 frame, it doesnt matter whether you store the picture as fields or frames as long as they add up to the same amount. So generally speaking, interlaced it better on an interlaced scan monitor, such as a television, and progressive is better on a progressive scan monitor, such as a pc monitor or an HDTV.

What I think you are referring to is the ability to keep the progressive 24fps source of your film and let the dvd player telecine it to interlaced 29.97fps during playback (soft telecine), as opposed to hard telecining (encoding) it to interlaced 29.97fps. This most certainly does have a significant effect on quality because you now have ~%20 less frames to encode, which means your bitrate is %20 more effective.

No a progressive source does not look bad on a tv because when it is played back it is interlaced. This is exactly what happens when you watch any NTSC DVD on your dvd player.

If you can give me some more information about exactly what you are trying to do then I can be more specific. Most importantly, what is your source? DVD? TV capture? DV? PAL or NTSC?

[tempo]

Thanks for correcting my concept. I do TV captures in SVCD format and I'm in a region of PAL so the NTSC IVTC technique does not work for me.

I've got another question: You said a progressive video would look fine on ordinary televisions, then in what ways an interlaced one can be better?

[adam]

Well with PAL tv captures you are stuck with interlaced. Here is the thing, there is a tradeoff with everything. Your source is interlaced, so you could deinterlace. At low bitrates its sometimes better to do this because it prevents interlacing artifacts. However, by deinterlacing you are losing half of the data, so movement won't be quite as smooth and the picture won't be as sharp. Personally, for tv captures, especially PAL ones, I would always keep it interlaced.

The benefits of progressive video, which I was talking about, are really more prominent with NTSC. Film originates at 24fps progressive, and its much better and easier to keep it this way and let the dvd player do all the work of interlacing it.

So to summarize, if your source is progressive its going to be interlaced at some point before playback on a regular television. Its better to keep it progressive during encoding and let the dvd player do the interlacing, as this lets you keep the original fps which greatly conserves bitrate. If your source is already interlaced (tv capture) then the benefit to keeping it interlaced is that you don't have to deinterlace, which loses data.

[tempo]

Thanks. A few more things I'd like to add and ask...

...Your source is interlaced, so you could deinterlace. At low bitrates its sometimes better to do this because it prevents interlacing artifacts. However, by deinterlacing you are losing half of the data, so movement won't be quite as smooth and the picture won't be as sharp...

Sorry, may I know what's the interlacing artifacts here? (at low bitrate)

So, for a deinterlacing a PAL video, it doesn't conserve more bitrate but sometimes it does (seem to) keep the quality. I always notice that some PAL broadcasts are not really interlaced. They are progressive in a certain sequence like '10' that can be easily made progressive with IVTC tools. For these videos, I think they should be kept as progressive, am I right?

[adam]

More than anything, I think the interlacing artifacts I am talking about are just anomolies in the encoder. Its not as easy for an encoder to encode field based material as it is to encode progressive frames. To describe these artifacts specifically, they are just little discolorations or blocks running horizontally through the picture. They do appear to be more prominent in low bitrate video, but I still think that if your source is interlaced, and IVTC (inverse telecine) isn't possible, then you are best off leaving the footage interlaced.

You can't really IVTC PAL footage, the telecine process works completely differently from NTSC. As you have seen, different broadcast stations will use different telecining patterns. Some may have long series of progressive frames, and you could remove or blend just the interlaced fields if you used an intelligent deinterlacer which could differentiate between the two. This is essentially still just deinterlacing and is prone to the same problems I mentioned before. (blurriness and less smooth playback.) Then again, decomb.dll can really work wonders with interlaced footage so you might want to experiment with that.

I live in the US so I have zero experience with Pal. Maybe someone else who lives in a PAL region can give you some tips on deinterlacing tv captures.

[tempo]

Thanks a lot for all your replies

[DEmberton]

I always notice that some PAL broadcasts are not really interlaced. They are progressive in a certain sequence like '10' that can be easily made progressive with IVTC tools. For these videos, I think they should be kept as progressive, am I right?

For film based source (which includes a lot of TV shows), then usually the progressive 24fps original is just sped up to 25 fps for PAL. So you're correct that these videos aren't really interlaced as such, so you may wish to encode as progressive, although I'm not sure it makes any difference. When they're sped up like this, it's relatively straight forward to convert them back to 24fps NTSCFilm format, which corrects the audio pitch and timing, and due to the lower number of frames to encode and the lower resolution, I find gives better quality SVCDs than keeping it in PAL.

However I've seen a few times a different conversion, that does some kind of field duplication similar to the NTSC pulldown to increase the frame rate, and that is effectively interlaced. It looks bad enough as it is, but trying to convert it to progressive is likely to make a complete mess I don't know of anything that can reverse this process.

If the source is PAL video, then it will be interlaced. Although just to confuse the situation even more, sometimes TV has a film effect applied to it to make it look like a progressive source.

Dave