Understanding DVD technology

[clovejoy@plexusengineerin]

All,

I'm attempting to try and understand the DVD format for authoring and editing purposes. I have started to read the guides and have several questions to ask to help with my general understanding. I started with the video basics guide and thus have questions regarding that guide. If anyone would be willing to share their insight I would greatly appreciate it. Here goes:

---Now, at the time when TVs first came to the market the technology to write 525 or 625 lines 60 respectively 50 times per second was prohibitively expensive and not suited for the mass market.

1. What does "to write 525 or 625 lines 60 respectively 50" in this sentence mean?

---Reducing the refresh ratio would have required more complicated circuits and wasn't an option either - plus the human mind has a lower limit as to what it accepts as fluent motion.

1. I'm not sure I understand the "human mind" portion of why lower refresh rates weren't used. Is this because the "phosphor" loses it's illumination before it's re-freshed?

---But the TV engineers had an idea: What if we only wrote every second line of the picture during a sweep, and wrote the other half during the next sweep? Doing that we only need 25 respectively 30 pictures per second (meaning less bandwidth used meaning more TV stations in the same frequency band.

1. I understand using two sweeps but I don't understand the rest of the paragraph. What does the 25 respectively 30 part mean? Why would you push more stations through when the TV is only capable of displaying one station at a time (disregarding the later invented PnP TV's)?

---In the NTSC world the switch to color required another change: The refresh rate had to be slightly lowered from 60 Hz to 59.97 Hz (resulting in 29.97 pictures per second) to accommodate the colors - that's why we have this strange framerates in the NTSC world today.

1. I don't understand why the switching to colors would require a change in the refresh rate?

2. How does the refresh rate affect FPS?

---Early PC monitors still supported interlaced modes but the higher contrast and bright backgrounds gave us such a headache that these days we are fortunate enough that most screens don't even support interlaced mode of operation anymore.

1. I don't understand why "higher contrast and bright backgrounds gave us a headache" when interlaced mode is used???

---Recently there have been TV screens which support a progressive scanning mode.

1. Why do only PC monitors use progressive scan???
2. Can't modern technology duplicate this functionality on TV's?

---TV channels transmit text pages in these lines, they can contain signals that screws the automatic gain controller of your VCR (the Macrovision analogue copy protection system), etc.

1. Does anyone know what these "text pages" say?

---PAL screens require 25 pictures per second and each picture has to be split into 2 fields.

1. Why does each picture have to be split?
2. How is it split?

---But as 25 isn't so much higher than 24 what we commonly do in PAL countries is that we take the original 24 fps (frames per second) movie and speed it up to 25 fps. This means that voices and music has a higher pitch and that the movie is somewhat shorter but unless you do an one to one comparison hardly anybody notices.

1. I understand this affects video in that the movie will be slightly shorter but why is audio affected?

---From the 10 fields you put the first two fields together to reconstruct frame 1, then the 3rd and 4th field to reconstruct frame 2. But then if you put the 5th and 6th field together you get neither frame 2 nor frame 3.

1. Isn't this different to what Robshot explains in his "Video and Audio Syncing Problem: Why and How" article?
2. Shouldn't fields 5&6, 7&8 and 9&10 equate to frames 3,4 and 5 respectively?
3. Why would this article say "if you put the 5th and 6th field together you get neither frame 2 nor frame 3" when the article just explained that putting fields 1&2 and 3&4 together result in Frame 1 and 2?

I'm truly sorry for the long post with many questions but I'm not about to try and learn how to author DVD's without a COMPLETE understanding of the basic. I appreciate all the help I can get.

Regards,
Carl.

[Matthew]

Originally posted by clovejoy@plexusengineerin
I'm truly sorry for the long post with many questions but I'm not about to try and learn how to author DVD's without a COMPLETE understanding of the basic.

Can can reauthor DVDs just fine and don't know the answers to any of your questions :P

Well, except one :P

"1. I understand this affects video in that the movie will be slightly shorter but why is audio affected?"

Lets say a movie is 1000 frames (short movie I know :P). At 24 FPS it will run for 1000/24=41.667 seconds. At 25 FPS is will run for 1000/24=40 seconds. As the movie is of a different length, if the audio is not adjusted, you'll end up with an audio track that is longer than the video. Which means progressive desync.

NTSC is slowed down to 23.976, so the audio is ever so slightly longer and the pitch ever so slightly lower.

As an aside, the audio is sometimes pitch corrected on PAL releases, but if botched it can introduce artifacts (pops, etc).

[MvB]

What does "to write 525 or 625 lines 60 respectively 50" in this sentence mean?

that means, as i understand, that NTSC progressive with 525 lines and 60 Hz or PAL progressive with 625 lines and 50 Hz were probably to expensive.

I'm not sure I understand the "human mind" portion of why lower refresh rates weren't used. Is this because the "phosphor" loses it's illumination before it's re-freshed?

I seems so. 25 Hz would flicker so much that it would produce headache. So they went for 50 half frames. There is another reason for that as a friend of mine told me:
in these times there where no high resolution CRTs or those CRTs were too expensive. So they built a CRT with half the resolution of the picture they wanted to show and split the picture into 2 half frames, shown with a timeframe of 1/60 respective 1/50 s thus creating the illusion for the human eye that to see a picture consisting of 525 | 625 lines with less flickering, displayed on a tube not capable of displaying this picture in progressive mode.

oh, i have to go to work now :(

[manono]

OK, I'll take a crack at it. But my knowledge is incomplete, so if I say anything wrong or stupid, someone please correct me.

What does "to write 525 or 625 lines 60 respectively 50" in this sentence mean?

That sentence is badly written. It means 525 lines at 60 Hz for NTSC, and 625 lines at 50 Hz for PAL.

I'm not sure I understand the "human mind" portion of why lower refresh rates weren't used. Is this because the "phosphor" loses it's illumination before it's re-freshed?

Dunno. Maybe the great man himself (Doom9) can explain it.

I understand using two sweeps but I don't understand the rest of the paragraph. What does the 25 respectively 30 part mean? Why would you push more stations through when the TV is only capable of displaying one station at a time (disregarding the later invented PnP TV's)?

If you're writing 60 fields per second (30 frames, for NTSC), instead of 60 frames per second, or 50 fields per second (25 frames, for PAL), rather than 50 full frames per second, you're using half the bandwidth, thus allowing twice the number of stations in the frequency band. Or something like that.

I don't understand why the switching to colors would require a change in the refresh rate?

Me neither.

How does the refresh rate affect FPS?

If the refresh rate is 59.94 Hz (he incorrectly says 59.97), and you're outputting fields, then that's 59.94 fields per second, or the equivalent of 29.97 frames per second. And at what framerate are NTSC DVDs? 29.97fps.

I don't understand why "higher contrast and bright backgrounds gave us a headache" when interlaced mode is used???

Me neither, but I came to computers just after interlaced monitors disappeared, and have never used one. Screensavers are an anachronistic holdover from those days when you had to keep the screen changing, or face burn-in from the phosphors.

Why do only PC monitors use progressive scan???
Can't modern technology duplicate this functionality on TV's?

I may be misunderstanding you here, because you had just quoted Doom9 saying just that. If you have a DLP TV, LCD TV, plasma screen TV, projector, (and other kinds, I think) then you have a progressive display. They're becoming more and more common.

Does anyone know what these "text pages" say?

Ever watched the subtitles for the deaf, or foreign language subtitles on a TV? That stuff. Not the subs for a foreign language movie, but for regular TV shows. I think it's sometimes called teletext or closed captioning.

Why does each picture have to be split?
How is it split?

Back to the stuff from earlier in the article about saving bandwidth and doing something possible, given the constraints of the earlier technology, by transmitting in fields instead of by full frames. So PAL TVs transmit at 50 fields per second or 25 frames per second. And PAL DVDs are 25fps.

I understand this affects video in that the movie will be slightly shorter but why is audio affected?

Finally an easy one. If you speed up the video from 24 to 25fps, to keep the audio the same length, it also gets sped (speeded?) up ( raised by half a tone, I think). Some people can notice it, and a few are really annoyed by it. Me, I can't tell.

Isn't this different to what Robshot explains in his "Video and Audio Syncing Problem: Why and How" article?

No. Look at the diagram in Doom9's article again. The 5th field is the top field of the original frame 2, and the 6th field is the bottom field of the original frame 3. They don't match up. There is, however, one difference between the 2 diagrams in the 2 articles. Robshot's illustrates 3:2 pulldown, and Doom9's illustrates 2:3 pulldown (much the more common way to telecine). But although different frame numbers are interlaced, depending on which method you use, the net effect is the same. When telecining from 4 frames to 5 frames, 3 frames remain progressive, and 2 wind up interlaced.

Shouldn't fields 5&6, 7&8 and 9&10 equate to frames 3,4 and 5 respectively?

They equate to the 3rd, 4th, and 5th frames of the 5 frames created from the original 4 frames. You're starting with 4 frames or 8 fields, and you're turning it into 5 frames or 10 fields. That's what telecining is all about. On an interlaced display (a standard interlaced TV set) you can't tell. 2 of those 5 frames are interlaced. The 3rd and 4th frames were each created from 2 different frames of the original 4.

Why would this article say "if you put the 5th and 6th field together you get " when the article just explained that putting fields 1&2 and 3&4 together result in Frame 1 and 2?

Because after the telecine process, the 4 progressive frames are converted to 5 frames, and from that diagram, frame numbers 1, 2, and 5 remain progressive, but frames 3 and 4 are interlaced. That's what he means by "neither frame 2 nor frame 3" when talking about the new frame 3. When you combine fields from 2 different frames, the result is an interlaced frame.

I hope that helps, and I hope I wasn't misleading. There are people around here that know much more about the earlier questions than I.

Wanna do some more reading? This is related (some of it anyway):

http://www.doom9.org/ivtc-tut.htm

And this one's a gold mine of information (for NTSC):

http://www.hometheaterhifi.com/volume_7_4/dvd-benchmark-part-5-progressive-10-2000.html