Posted: Sat 2 Sep 2006, 6:10 Post subject: Proper MPEG2 conversion bitrate / resolution for old footage
I would be very grateful for your help with a confusing issue.
I have a Sony Digital8 camcorder, and I was shooting DV for a number of years. I usually transfer the footage to a PC, split the resulting AVI file into clips, then encode them to MPEG2 using TMPGEnc at a constant bit rate of 8000 kbps and author a DVD. The video from one Sony D8 tape (60 min in SP mode) more than fits onto a standard single-layer DVD.
Now I am finally ready to undertake a long-due project and start converting my old analog videos to the digital format, in order to show my grandchildren what their parents were like . These archive videos were taken in Hi8 format by my previous-generation Sony Hi8 camcorder. I verified that these tapes play nicely in my Digital8 unit (as they should), which generates DV that I upload to the computer. Thus, analog->digital conversion is handled by the camcorder automatically, and I am left with an about 24.6 Gb AVI file of DV for a standard Sony 120-min Hi8 tape. I would like to preserve this footage with the minimum loss of quality.
My question is: how to choose an MPEG2 conversion bitrate and resolution for the footage that was thusly digitized from a Hi8 tape? If I stick to the 8000 kbps CBR, the whole set of resulting files will obviously not fit on a single DVD. - And for a variety of reasons, including my desire to have one-to-one correspondence between archival tape numbers and future DVD numbers, I would much prefer to have them together on one disk - if it does not involve loss of quality. The calculated bitrate to encode 120 min on a 4.37 Gb DVD would be around 5600 kbps.
I reason that since my Hi8 original had lower resolution than a DV format (something like 510 horizontal lines??), the process of converting Hi8 to DV just extrapolated the data in each frame to achieve DV format size, so if I choose the 8000 kbps CBR as the highest possible rate I would just preserve conversion artefacts, while going down to some (which?) rate would just bring a resolution back to quality similar to the Hi8 original.
The analogy I am making is the following: If someone had blown up a digital image 4x by extrapolating one pixel in both horizontal and vertical directions, then reducing a size again would bring quality on par with the original.
Or should I just encode the AVI clips to MPEG2 in "half-resolution" (352x480), instead of 720x480 ?
Is my thinking wrong? I was not able to find any reliable data on the subject. My goal is to maintain the best quality without artificially improving what cannot become better that the source.
Joined: 04 Feb 2003 Posts: 587 Location: Lisboa, Portugal
Posted: Mon 4 Sep 2006, 1:58 Post subject:
Thus, analog->digital conversion is handled by the camcorder automatically, and I am left with an about 24.6 Gb AVI file of DV for a standard Sony 120-min Hi8 tape. I would like to preserve this footage with the minimum loss of quality.
Record it to a DV (or D8) tape in digital mode, then. That's the best quality you can get (at least without investing in high-end analogue capture equipment), and it'll probably last a lot longer than any recordable optical media.
My question is: how to choose an MPEG2 conversion bitrate and resolution for the footage that was thusly digitized from a Hi8 tape?
Well, as with any other source material, it's a tradeoff between encoding time, quality and space. If 8000 CBR lets you fit enough material into the number of discs you want to use, go for it. If not, then 2-pass VBR (with an average of 5000-6000) will give you about 30% more compression with very similar quality. If that's still not enough, you can increase the compression (i.e., lower the average), at the cost of (noticeable) quality loss, or you can use more discs.
No magic solution.
But if I were you, I'd keep the meterial in DV / D8. In a few years DVDs will be history, and you'll have optical and holographic media that can easily store 25 GB of data. In fact, you already do (Blu Ray), but it's not cost-effective yet.
I reason that since my Hi8 original had lower resolution than a DV format (something like 510 horizontal lines??),
No, all video has the same vertical resolution (number of lines). Specifically, 480 visible lines for NTSC and 576 for PAL. Horizontal resolution is a vague concept in analogue, but officially the resolution is always 720 samples / pixels per line (704 plus overscan). These are the values for the Y (luminance) channel. Chroma resolution is usually between half and one quarter.
while going down to some (which?) rate would just bring a resolution back to quality similar to the Hi8 original.
No, doesn't work that way. In fact, while VHS is obviously a lot worse than 35 mm film, you need a higher bitrate to store a reliable representation of a VHS tape than you do for a DVD made from film. Commercial DVDs (made from digitised film, or digitised Beta which in turn was made from film) can get away with average bitrates under 4000 and still look great. Try the same with a VHS capture and it's dancing block hell.
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