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720p to DVD – Interlaced or Progressive?
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Not to confuse things anymore but there is no such thing as 23.98 in the DVD spec…..ONLY 29.97 OR 25 FPS. Remember, DVD is 20 year old technology so things like progressive video weren’t a thought! Also the article is incorrect to say you can squeeze higher bitrates into a encode from which inverse Telecine has been applied. It only throws out duplicate frame whICH saves space not allow you to increase bitrates.
Froam the spec:
A disc has one track (stream) of MPEG-2 constant bit rate (CBR) or variable bit rate (VBR) compressed digital video. A restricted version of MPEG-2 Main Profile at Main Level (MP@ML) is used. SP@ML is also supported. MPEG-1 CBR and VBR video is also allowed. 525/60 (NTSC, 29.97 interlaced frames/sec) and 625/50 (PAL/SECAM, 25 interlaced frames/sec) video display systems are expressly supported. Coded frame rates of 24 fps progressive from film, 25 fps interlaced from PAL video, and 29.97 fps interlaced from NTSC video are typical. MPEG-2 progressive_sequence is not allowed, but interlaced sequences can contain progressive pictures and progressive macroblocks. In the case of 24 fps source, the encoder embeds MPEG-2 repeat_first_field flags into the video stream to make the decoder either perform 2-3 pulldown for 60Hz NTSC displays (actually 59.94Hz) or 2-2 pulldown (with resulting 4% speedup) for 50Hz PAL/SECAM displays. In other words, the player doesn’t “know” what the encoded rate is, it simply follows the MPEG-2 encoder’s instructions to produce the predetermined display rate of 25 fps or 29.97 fps. This is one of the main reasons there are two kinds of discs, one for NTSC and one for PAL. (Very few players convert from PAL to NTSC or NTSC to PAL. See 1.19.)
Because film transfers for NTSC and PAL usually use the same coded picture rate (24 fps) but PAL resolution is higher, the PAL version takes more space on the disc. The raw increase before encoding is 20% (480 to 576), but the final result is closer to 15%, depending on encoder efficiency. This translates to an increase of 600 to 700 megabytes on PAL discs compared to NTSC discs.
It’s interesting to note that even interlaced source video can be rendered as progressive-structured MPEG pictures by a good encoder, with interlaced field-encoded macroblocks used only when needed for motion. Most film sources are encoded at 24 frames per second (the inverse telecine process during encoding removes duplicate 2-3 pulldown fields from the videotape source, and the remaining field pairs, although technically in interlaced form, can be re-interleaved by a progressive player). Most video sources are encoded at 25 or 30 interlaced frames per second. These may be mixed on the same disc, such as an interlaced-source logo followed by a progressive-source movie.
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Took me forever to find this post….Roger does a fine job explaining it here:
https://forums.creativecow.net/thread/155/870715#870746
1) NTSC DVD’s may legally contain 29.97fps mpeg streams.
2) Compressor supports encoding 29.97fps MPEG files from 24fps source material, using 2:3 pulldown flags.
3) DVD Studio Pro supports writing 29.97fps DVDs, but not 24fps DVDs, because such cannot not exist.
4) If I import a 24fps asset into DVD Studio Pro it will show as 29.97 fps in the Asset tab, AND that’s also what will be written in the MPEG-2 file. Quicktime shows 24fps if I open the same mpeg file as what was dropped into DVDSP, because it isn’t connected to an interlaced monitor. Therefore it cannot do anything but ignore all the 12 repeated fields per second (otherwise indicated by the pulldown flags). This means that only 60 – 12 = 48 fields (or 24 frames) per second will be displayed on the computer screen. That’s no problem, because all the encoded pictures will be displayed in a timely manner. There is no need to display at the broadcast frame rate, 29.97, so the pulldown mechanism isn’t useful here.You can use “MPEG Streamclip”, drop a file, m2v, VOB et.c., onto it, then hit Command+I, to Show Stream Info. Among other things it will tell you which frame rate is stamped into the picture headers of the MPEG-2 video file.
If an NTSC video file is playable on a DVD it will tell a frame rate of 29.97 fps. That is regardless whether the source movie was 23.976, 24 or 29.97fps, before encoding it to MPEG-2.
If MPEG Streamclip says that the encoded video is 23.976 or 24fps it will not be accepted by e.g., DVD SP and cannot be played by a DVD Player either.
If for example QT player tells you that a movie from a DVD is a 24 fps MPEG-2 movie it is not telling you the truth about the .m2v file, it is just telling you how it will be displayed on your computer (progressive) monitor.
QT Player doesn’t tell you the correct picture size either, when playing an .m2v file.
I don’t think you could get it to show the correct picture size, 720×480 (NTSC) or 720×576 (PAL), which are the most common sizes for SD DVD. Instead it may report sizes like 640×480, or something even funnier ones like 720×560 or 720×404, which aren’t even possible to fit into an MPEG-2 file. So, you can be sure that these number do not come from the MPEG file. They are just the window sizes that QT Player chooses to scale to, for display on computer monitors, because unlike NTSC TV monitors they can only display square shaped pixels. A TV monitor never shows square pixels (or Pels, Picture elements). There the height of a Pel is always the same as the height of a scan-line, but the width of a Pel is either narrower or wider than that. For example, when a 720×480 picture is displayed on a picture aspect ratio of 16:9 the Pels become wider than if the same picture size is displayed on a 4:3 TV screen.MPEG Streamclip will read the picture sizes from the .m2v file and report them as they are written in the file. You should never expect QT player to do that. I have never seen that happen.
QT Player is a handy tool, but you cannot trust it for any serious measurements. You should understand that it was never meant to be a true MPEG-2 compliant decoder/player.
If I wasn’t so tired I would have explained about 2:3 pulldown and why that name is meaningful. Many people, without video knowledge from before the computer age, seem to get this all backwards;-)
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Roger helped me through this last summer (he’s incredibly responsive-thank you). I had used BitVice to make a true 24P (23.976) mv2 from a HD 23.976 progressive source thinking (incorrectly) that was the way to achieve a “Hollywood-type” DVD. When I tried to import that file into DVDSP it said it was an unsupported format. I emailed Roger asking what’s up with my favorite encoder and he explained. Problem solved!
John
MacPro 8-core 2.8GHz 8 GB RAM OS 10.5.5 QT7.5.5 Kona 3 Dual Cinema 23 ATI Radeon HD 3870
ATTO ExpressSAS R380 RAID Adapter, PDE Enclosure with 8-drive 6TB RAID 5
24″ TV-Logic Monitor
Final Cut Studio 2 (up to date)Pro Tools HD w SYNC IO, Yamaha DM1000, Millennia Media HV-3C, Neumann U87, Schoeps Mk41 mics, Genelec Monitors, PrimaLT ISDN
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What was the issue and how was it resolved?
E
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I had created a custom 23.976 preset for BitVice that inadvertently removed the pulldown flags from the mv2 so Compressor wouldn’t import it. I used the BitVice preset and all was well.
John
MacPro 8-core 2.8GHz 8 GB RAM OS 10.5.5 QT7.5.5 Kona 3 Dual Cinema 23 ATI Radeon HD 3870
ATTO ExpressSAS R380 RAID Adapter, PDE Enclosure with 8-drive 6TB RAID 5
24″ TV-Logic Monitor
Final Cut Studio 2 (up to date)Pro Tools HD w SYNC IO, Yamaha DM1000, Millennia Media HV-3C, Neumann U87, Schoeps Mk41 mics, Genelec Monitors, PrimaLT ISDN
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Did you ever do that test Dave?
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