Tryin' to understand all this 24p stuff

yirm wrote on 4/7/2003, 7:54 PM
I'll make it easy. I have a regular DV camcorder (Digital 8). I am making DVDs for regular TVs. Does the new 24p stuff help me? I noticed something about shorter rendering times and less disk space. That's nice. But does it require progressive DVD players, or special televisions for playback? What's the downside for standard TVs and DVD players?

Pardon my ignorance, and thanks.

-Jeremy

Comments

sms wrote on 4/8/2003, 10:35 AM
There is an article in the latest edition of Computer Videomaker on 24p. The article is really a review of a Panasonic miniDV camcorder but since the camera can shoot in 24p mode they also explain some of the mystery. Probably won't answer all your questions but if you know nothing about this you are bound to learn something.

By the way, it appears that the reviewers were split as to the usefulness of shooting in 24p.
gold wrote on 4/8/2003, 11:34 AM
Good luck; movie film has nearly infinite resolution and the projector flashes each frame twice to give effectively 48 frames/sec to reduce flicker due to limited human eye persistance time. Don't think that low res digital is going to look the same.
CrazyRussian wrote on 4/9/2003, 10:33 AM
First of all, I'm not an expert in this, but I think i know something. When i started playing with video editing I read some info on progressive vs. interlaced. Now, I'm not sure, but based on this: "DVD Architect now supports 24 fps, progressive-scan video." (readme file from DVDA 1.0b) I would assume that essentialy 24 fps is progressive. If this assumption is wrong, then all info below applys only to "Progressive Scan", if my assumpsion is right, then you can apply it to 24fps.
Firts: if you dont have digital TV, or TV capable of at least SDTV resolution progressive scan will not do you any good because even if you output progressive to your old TV it will break it back down to 60 fields, effectevely downgrading your video back to interlaced
Interlaced video has 60 FIELDES, not frames, FIELDS. Field is half of the frame, so interlaced video is projected as follow:
out of 480 visible scan lines (there is 525 scan lines total, but rest is not shown: "out of safe area") odd lines displayed for 1/60 of a sec, then even lines displayed for another 1/60 of a second, making it 30 fps (i cant remember why they drop frames making it 29.97 fps)
Porgressive output combines these 2 fields to make a frame and outputs entire frame on all 480 scan lines, making image brighter, more solid, fuller and more realistic.
From what i can remember upgading video from interlaced to progressive is not a problem, many artifacts introduced when film (24fps) is downleveled to interlaced, and then to progressive, that is because when 24 frames split into 60 fields, they are not neseserly assembled back to 24 in the same order, introducing combing or feathering.
I hope this helps on this subject
if you need more info just do a search on google for "progressive explained" or "interlaced vs progressive"
yirm wrote on 4/9/2003, 3:06 PM
Cool. My question is when a standard DVD player and TV play back your 24p encoded video (you are correct, the p is progressive), will it look worse than if you used the standard NTSC settings?

-Jeremy
CrazyRussian wrote on 4/9/2003, 3:58 PM
Again, theoretical answer :), as I never did practical tests for this, it should look the same when you play 24p on non progressive scan dvd. I think more problem will arise playing interlaced 29.97 fps on progressive DVD if it set to ouput porgressive out. 24p->29.97 will greatly depend on denterlacer used in DVD. Also many vide materials dont follow mpeg frame sequince rules and when it gets out of sequence, it will be up to deinterlacer to decide what to do. It best to play interlaced DVD as interlaced and not to setup DVD to upgade it to progressive
gold wrote on 4/9/2003, 5:08 PM
http://www.in-sync.com/index.php/fuseaction/support_documentation_24p_editing
is a link that claims 24p is superior to 30 frames/sec interlaced. The example they show is not totally fair to a quality tv system as quality tv's have line doublers or quad filters that remove the time displacement between even and odd lines by (1)for even lines averaging the two adjacent odd lines and (2) for odd lines averaging the two adjacent even lines so that all lines in a frame are at the same time increment. So actually I think the quality of the tv has more to do with the quality of the image on interlaced. Progressive allows a "cheaper" tv to produce similar quality images since no digital capture and averaging is required. The progressive concept should always give a better image but 29.97 should be superior to 24 if both are progressive due to the reduced flicker (note: most folks prefer 70 Hz vertical to 60 Hz vertical on computer monitors as it is less tiring). But this is also "theoretical" and subject to error. I personally feel that trying to get 24 frames/sec to emulate film is somewhat akin to the folks that buy tube model amplifiers rather than transistor amplifiers because they think the distortion adds quality to the sound; 24/sec vs. 30/sec (with some rounding) which do you think would better capture an analog (continuous time) real world?
SonySDB wrote on 4/10/2003, 8:48 AM
I will not be able to address all the questions on this thread, however, I would like to provide some clarification.

For 24p, the video data is encoded as progressive frames at 23.976 fps with information to instruction the player to perform 2-3 pulldown. What is 2-3 pulldown? 2-3 pulldown is the process that is normally used to convert film to television. 2-3 pulldown means that 2 fields will be displayed for the first frame and 3 fields will be displayed for the second frame. The effective frame rate becomes 29.97 fps because (2 fields + 3 fields) / (2 fields + 2 fields) * 23.976 = 29.97 fps.

That's rather technical and it's ok if you don't understand that; all you need to know is that 24p with 2-3 pulldown is essential the same as your video being 29.97 fps. It just stores the video data in a different way. So, this should play like any other DVD on your DVD player.

The most logical use for this is when your source footage is shot in 24p. However, some may also employ this for compression advantages (at the loss of temporal resolution) or to achieve a certain look. (People may subconsciously associate this look with high budget productions.)
yirm wrote on 4/10/2003, 9:04 AM
> The most logical use for this is when your source footage is shot in 24p. However, some may also employ this for compression advantages (at the loss of temporal resolution) or to achieve a certain look. (People may subconsciously associate this look with high budget productions.)

Thanks, SBD, for clarifying. Temporal resolution being related to color? Anyway, from what you are saying, it sounds to me that there will be no compatibility issues by using the 24p templates. I will have to A/B some footage to see how it looks to me in both scenarios. But I have already tested and seen smaller file sizes and substantially shorter render times. I have a feeling that after A/B'ing I'm probably going to end up using the 24p templates. Regardless, kudos to SoFo for adding this functionality. Seems like a great option even for those of us who don't can't even come close to affording that Panasonic camera.

-Jeremy

gold wrote on 4/10/2003, 2:26 PM
Temporal resolution is time 1/24 vs. 1/30 of a second. Search on the term aliasing to see what effect the sample rate has on the image. Basically Nyquist and Shannon claim that you must sample at twice the highest frequency component in the signal or else information folds into lower frequency components. In this case frequency would be related in relative motion of objects in the image from one frame to the next. I'm not sure what this really means in mpeg2 compression since there is so much quality loss due to compression (the truncation part following the discrete cosine transform) that it may "wash out" the effects of aliasing.
gold wrote on 4/10/2003, 4:54 PM
O.K. I found a link myself
http://van.hep.uiuc.edu/van/qa/section/Everything_Else/Gizmos_and_Gadgets/20020326232750.htm
according to this uiuc page one must "blow a raspberry" to see the difference between 24 and 30? [to read it is to believe-strange but true/] Oh, well aliasing and artifacts are closely related
cdruiz wrote on 4/11/2003, 12:09 PM
Jeremy,

Be sure to let us know what your a/b comparison yields. I haven't been able to do an A/B comp, but I too see smaller file sizes and shorter render times. The use of 24p (or just the option to use it) is just one more reason I am happy I went with the big boys and left Studio 8 behind. Thanks Sonic Foundry.
cdruiz wrote on 4/11/2003, 3:29 PM
Sonic Dennis on the Vegas forum says we shouldn't use Vegas to convert 60i to 24p.
:-(
yirm wrote on 4/12/2003, 11:43 AM
I'm hoping for further clarifications of their views.
mbo wrote on 4/12/2003, 9:44 PM
read here:
http://www.thedigitalbits.com/officialfaq.html#3.8
serch page for "progressive"
Michal
gold wrote on 4/15/2003, 7:45 AM
Actually the link posted in the previous post presents some technical fuzziness. Phosphor persistance was considered in the original spec but phosphors have changed during the years so it has little if any meaning today. Here is another link that discusses line doubler types and 24P. To really understand the subject you need to start with human anatomy 101 as everything involving video and filming is based on the limitations of human vision and the ways it works. Anyway here's the other link, but do the search--you'll find out how birds of prey have 2 foveas and a lot of other fascinating vision facts such as quadchromate mothers usually have color blind boy children--lots of neat stuff to learn--when I taught a graduate course in multimedia computing we covered a lot of this kind of information as a basis for understanding compression codecs. With no futher ado, another link
http://members.aol.com/ajaynejr/viddoubl.htm
DDogg wrote on 4/17/2003, 3:29 PM
The original poster said:
"I have a regular DV camcorder (Digital 8). I am making DVDs for regular TVs. Does the new 24p stuff help me?"

I read this whole thread looking for a couple of simple things relevant to the question. Maybe I missed them:

1> If the original source is interlaced then Vegas must convert to non-interlaced for 24p which is normally destructive I think? > So, what method is used internally in Vegas to de-interlace? Is it a blend? Taking every other field? I would not expect Vegas is doing hybrid MoComp or something that complex?

It seems the answer to the usefulness of 24p would be dependant on whether it started out as 24p which I think seems to be what Vegas designed for, or whether the source is 60i converted to progressive. In which case the de-interlace operation might reduce the quality of the source. @SonicSDB, could you comment please.

SonySDB wrote on 4/21/2003, 7:41 AM
In Vegas, the deinterlace method is set on the Video page of the Project properties dialog (File | Properties...). It is blend by default. The deinterlace operation will decrease the quality of the source.
cdruiz wrote on 4/21/2003, 3:50 PM
The following is from the Vegas board. It tells us a little more.
I think that I will abandon 24p and stick with my GL2's 30p. It has
a subtle film look and I won't lose any more resolution.

Having said that I have rendered some 60i stuff to 24p Quicktime movies
to keep the size down and they came out looking great!

Let me know what you end up doing "yirm."

I wrote:"What if I shoot in frame mode on my Canon GL2?"

Sonnic Dennis replied:
Then create a 30p project template and AVI render template by starting with the standard DV templates and changing the field order to "none (progressive)".

If you are trying to create 24p footage from 30p or 60i, converting 30p to 24p has very visible motion judder. You might be better off converting 60i to 24p; the motion will be smoother with some loss of vertical resolution. But either are only an approximation for shooting in 24p to start with.

///d@
DDogg wrote on 4/22/2003, 7:47 AM
"The deinterlace operation will decrease the quality of the source"

Thanks SonicSDB, that was what I was looking for. It may well be better to do these deinterlace operations with the less destructive and more advanced deinterlace filters in avisynth and then serve the video stream to Vegas. We can do this now via VFAPI but it is a bit clumsy for a lot of folks. This is one of the reasons I am trying to get a definitive answer on whether Vegas will ever be able to read native AVS files in this thread http://www.sonicfoundry.com/forums/ShowMessage.asp?MessageID=176057&Page=0

So far no answer. Because Avisynth can also be used for "ripping" I am concerned that SoFo may not understand the full value of Avisynth to this community for uses such as these.