saving bits

[ulfschack]

I don't know if this has been discussed to death allready, but I can't really find any recent threads about it, so here goes.

Converting PAL to NTSC will save me a lot of bits considering the possibility of going from 25 to 23.976 and from 480x576 to 480x480. Most DVD players will handle both.

I do understand that the length of the video (due to the audio actually) will be affected, but it's not an issue in my case as I edit home videos and can make them whatever length I need. The main thing is to optimize the quality of whatever I put on the disc.

2600 is pretty low for making good DV2mpeg and I cant go above this, but it's also pointless for me to go under 40 mins of length (I gladly would for a quality trade-of tho, were it at all possible), since it'll fit on the CD anyways. BUT distributing those bits to fewer pixels and frames per second should increase quality.

Downsides: 480 streched to fit the TV vs. 576 is bound to be blurrier. I'd probably have to let go of the interlacing too if I don't figure out a way of accurately resizing to 480 .... Actually *thought pops up* it wont be a problem ! I just separate the frames resize them and weave them back together *hah*.

So people, adding this to having borders for overscan it would give me (exaggerating borders to 32 pixels ... which I actually use):

(25*512*480)/(24*480^2)*100= 28% more bits per pixel to play with !!!
(maybe comparable to 3300 kpbs, assuming black borders don't consume bits, which they of course do to some extent)

Surley that will make PALers happy.

Alternativly do a half-way (off standard?) with maintained framerate of 25 and only resize to NTSC which would give me 23% instead. This way there'll be no audio hassle (and probably no compliance to standard either).

PS. Number are a bit on the optimistic side since NTSC probably needs not as wide borders as PAL on the same TV set, at least in the y-direction.

Would anyone care to comment before I set out on my crusade ?

cheers

[adam]

I don't really think converting from pal to ntsc is going to gain you much quality.

The difference in framerate is almost irrelavant. Film is not telecined to pal the same way it is telecined to ntsc. With ntsc additional fields are added, so your bitrate is actually being distributed over more frames. When telecined to pal, film is simply sped up from 24fps to 25fps, with no additional fields added. As far as bitrate distribution is concerned, film is identical to pal because even though it is playing faster, it has a shorter runtime. So in converting pal to ntsc you are actually only saving like .03% of your bitrate (the equivalent of converting film, 24fps to ntscfilm, 23.976fps.)

As far as the smaller resolution, well yeah your gaining some bits per pixel there but I'm not sure that warrants the hassle of it all.

If your concered with the amount of bits per pixel that you get with the svcd format you may want to simply try encoding at 352x576, the pal resolution of CVD. CVDS are compatible with all svcd compatible dvd players and using this smaller resolution will save you alot more bits than you will achieve by converting pal to ntsc. Give this resolution a try, I think you'll like it. I use it whenever I can't achieve an average bitrate of ~2mbits. It comes in handy for squeezing longer movies onto 2 80 min cdrs.

[ulfschack]

Finally a reply! And a good one at that :)

Now I'm pretty much newbie on NTSC so I was just brainstorming. Are you saying that I must have a framereate of 29.97 instead of the 23.976 I was aiming at in the post? SVDC specs states 25 for PAL and 29.xx (~30) for NTSC, but in seeing that DVD2SVDC ran TmpgEnc at a rate of 23.976 made me wonder if I couldn't reduce my PAL to that. Evidently as I looked closer in TmpgEnc it seems as if this is some sort of half-truth, where "internal" fps (or something to that effect) still is 29.97.

I tried authoring an SCVD anyway but it played only for a few seconds. I get the same behaviour when I try the alternate (352x576) resolution.

There still are a number of possibilities left fro me to try. But it seems I have to stick with the SVDC standard for now ... pity.

cheers

[Kedirekin]

adam is right; 480x480 at 29.97fps wouldn't gain you anything over 480x576 at 25 fps. In fact, they're effectively the same.

25*480*576 = 6912000
30*480*480 = 6912000

However, mpg gives you a way to cheat - a 23.97 fps encode with RFF/TFF flags is recognized as 29.97 fps, but only eats the bits for 23.97 fps. Achieving a 23.97 fps encode is kind of a grail for us. That's why we jump through so many hoops (like IVTC).

Once you have an encode at 23.97 fps, just run pulldown.exe on it and you're good to go.

With the 25->23.97 change in framerate, you'll almost certainly have to adjust your audio length to match, but that has been discussed here at length, so you shouldn't have trouble finding info about it (I believe doom9 even has a guide for it).

[adam]

http://www.doom9.org/synch.htm.

First off, this link will probably explain everything you need to know but anyway...

Film originates at 24fps. The ntsc standard calls for 29.97fps. The way this conversion is achieved is by first slightly slowing down the film to 23.976fps, which is known as ntscfilm. Then this is split into fields and some of these fields are repeated to bring the framerate up to 29.97fps. The problem is that you now have %20 more frames and that uses %20 more bitrate. This is something that you have to live with on many forms of media ie: broadcast tv, vhs. Fortunately dvds, vcds, and svcds have their own method of getting around this limitation by automating the telecining process in real time as the disk plays. Dvds use RFF/TFF flags to do this and svcds use what is called a pulldown flag, vcds are supposed to just be autotelecined without any type of flag. What this does is allow you to take the original 23.976fps encoded film, and encode only those frames and have the hardware or software dvd player do the telecine in real time. This saves you %20 of your bitrate. So when making an ntsc svcd you can actually encode at either 29.97fps or 23.976fps with the pulldown, its your choice.

Now, like I said before, when film is converted to pal it is simply sped up to 25fps, no frames are added. You still have the same number of total frames as you did when you were at 24fps. So if you were to convert pal to ntsc then all you would be doing is slowing your 25fps material down to 23.976fps (ntscfilm) and using the 3:2 pulldown flag to telecine it to 29.97fps (ntsc) as it plays. Now that I think about it, the conversion from pal to ntscfilm doesn't even remove any frames either so you are not gaining any extra bits at all.

Basically what I'm saying is that the higher framerate of pal is offset by the shorter playtime it has so that, as far as bitrate allocation is concerned, it is identical to film. Maybe my description isn't very clear but just trust me on this one, as far as framerate is concerned you get no benefit from converting pal to ntsc. The only exception I can think of is if you are making a vcd where the bitrate is non-negotiable...the shorter playtime of pal would just make your file smaller, which doesn't help much in most cases.

I'm not sure why your svcds are ending abruptly but I doubt it has to do with the resolution, especially since it occured when you used a svcd compliant resolution as well. Keep experimenting but I would still encourage you to give the CVD (352x576 @25fps)format another try. As long as your dvd player supports svcd than I can guarantee that a CVD will work also, and like I said, this will save you significantly more bits than converting from pal to ntsc, and that seems to be your primary focus.

Good luck.

[ulfschack]

Thanks Guys. You really go beyond the call of duty :)

I'm more educated now. Funny name "pulldown" for something that ADDS frames in a second. I surely have to experiment more in this area.

I do however think that in some way I was missunderstood by both of you. See, I make my own films. I don't have to have as many frames on a cd (PAL or NTSC)if I dont wanna. So reducing the number of frames will surely give more bits per second (thusly per frame). Quality is the main thing here (...so I'll edit out a minute from gramdma's speach ) . Also the smaller frames 480^2 gives single pixels more attention (albeit maybe for the price of a more blurred picture ... has to be).

Again thanks a bundle. I'll probably report back and post about my success or failure wheather you like it or not:)

Gonna give that CVD res another go too.

Cheers