Posted: Mon 28 Feb 2005, 21:24 Post subject: GOP frame advise for future editing flexibility Tmpg Xpress
I'm converting my Hi8 tapes playing them on a Hi8 camcorder via a DV camcorder (svideo coupled) to a (DV) capture card. I borrow the Hi8 camera so I must act fast to capture and convert them to MPEG2.
After all the tapes are captured and converted to mpg2 I put them on a DVD not as FOB files but as one mpg file just for future use. Because when I bring back the Hi8 recorder it is my plan to edit all those files to some interesting movies like "vacation 1", "granddaddy" etc...
What is the best way to set the variables in Tmpg Xpress to maximize the flexibility and quality for future editing?
I did read a lot abouth IBP frames GOP etc.
But that really didn't answer my question completely.
I already poses "MPEG Video Wizard" because I found out that Premiere doesn't understand the mpg2 files (but they can handle my DV captured files before I (re)compress tem in Tmpg.
My footage is 90 minutes (standard Hi8 length).
A: Shall I use CBR 6300 kbit/s (than a 90 min mpg will fit on a DVD)
B: Shall I use VBR than the bitrate will be 9250, 6300, 2500 (max, avg,min) and will fit on a DVD
C: It seems good (in my case) to use only I and P frames (no B).
D: Will a audio stream MPG1 layer2 128 of 192 kbit/s harmfull for later editing?
Can anyone say something about those:
1. MPEG system (combined Video + audio) in one MPG stream
2. 720x576 (I use PAL)
4. 25 fps
6. DC component precision: 10 bit
8. Bottom field first
9. Motion search precision: highest (with error correction)
10: GOP: IPPPPPPPPPPPPPP (1xI, 14xP so no B frame)
11: what figure should I use for the GOP limit (15 is suggested for PAL)
15. Include GOP sequence header
16. Output bit stream for edition (closed GOP) seems to be very important
17. Detect scene change
18. Optimize half/full pixel motion search
19. Use key frames as I picture
Any other suggestion for conversion to MPG2 for later (editing) use?
Joined: 04 Feb 2003 Posts: 587 Location: Lisboa, Portugal
Posted: Tue 1 Mar 2005, 3:30 Post subject:
Editing in MPEG-2 is generally a bad idea, because parts of the video will need to be recompressed even if all you do is a simple cut (all formats need to be recompressed if you add effects, titles, etc., but most formats do not need to be recompressed due to simple cuts).
If the DV camcorder is yours, my advice is that you copy all the Hi8 tapes to DV and keep them until you are ready to edit (and, in fact, keep them after that - DV tapes last longer than recordable DVDs, if you store them properly). Alternatively, if you have a big hard drive or two, you can keep the video as DV AVI or DV QuickTime files.
Editing in DV avoids most of the recompression and, even in the parts that do need recompression (due to titles, etc.), will give you much better quality than editing in low- or medium-bitrate MPEG-2.
There really is no magical way out of this. To get MPEG-2 files that can be edited with minimal recompression and no apparent loss in quality, you'd need to capture / create them as I-picture only, with a bitrate of 25 Mb/s or more. But that just happens to be the same bitrate of DV files, so you might as well leave them in DV, which is more easily accepted by editing programs. The downside of this is, of course, that 25 Mb/s files are going to be pretty big (about 13 GB per hour).
Storing the files in DVD-compliant format (i.e., under 9.8 Mb/s) will save space but will produce a noticeable loss of quality when you edit them later. DVD-compliant MPEG-2 is a "distribution" format (and even there, only really meant to compete with VHS), it's not a mastering / editing format.
Store it on disk: I need 56*90*60*3,6 Mbyte (datastream of DV per second)=1088640 MB or (1,09 TB).
Hmmm.... I'm didn't clean up my HD the other day so that won't be a option for the next year ;o)
Store it on DV casettes: of course that is the best idea but I need 56*1,5=84 tapes (DV tapes store only 60 minutes). That will be a lot of money. That will come close to a second hand Hi8 recorder (which probably the best solution ti solve this problem).
I thought that a program like "MPEG video wizard" will only recompress parts that are altered and won't touch (recompress) the unaltered frames. Tell me if that isn't the case.
Joined: 04 Feb 2003 Posts: 587 Location: Lisboa, Portugal
Posted: Tue 1 Mar 2005, 19:05 Post subject:
MPEG can't be edited on a "per frame" basis, because frames depend on each other. It has to be edited on a "per GOP" basis. If you change one frame, every frame in that GOP will usually have to be recompressed (and it will be recompressed by the editing program, which is likely to be a bit worse than TMPGEnc).
Naturally, you can use GOPs that are a single frame long, but that means you must use only I-pictures, and to get good quality using only I-pictures you need about 25 Mb/s (same bitrate as DV). Also, even in that situation, if you add titles, adjust the contrast, colour balance, etc., the frames will need to be recompressed (by the editing program).
So, as I wrote above, there's really no magical solution for this. If you can't store it in DV (or very-high-bitrate MPEG), you'll have to deal with some loss of quality when you edit it. In that case, your settings are fine, but for compatibility reasons I would advise you to set the bitrate limit to 8000 (some older players aren''t expecting it to go beyond that, although it would be within spec). Also, using I=1 P=7 will give you quality almost identical to P=14 while reducing the size of the GOPs, so when you cut the clip, less frames will need to be recompressed.
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