03-10-2019, 03:02 PM
colony colony is offline
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Hello All,

When VCR/Hi-8 tapes are captured/archived at MPEG-2 broadcast rate (10-20 mbps), can those files later be reasonably compressed for DVDs?
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03-11-2019, 09:17 AM
jwillis84 jwillis84 is offline
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I am pretty sure that they can, but your likely to suffer a generation loss of detail.

I am very bad at this, but I believe MPEG2 works by "smoothing" over an image first with a DCT - Discrete Cosine Transform to filter out high frequency changes that are mostly "noise" in the image. (Think of the DCT as "compression" it throws away the useless part of the image.) That sort of makes it more "blocky" but at a very subtle layer, then it applies a course grid and spreads a limited amount of bandwidth over each grid "block" to share the available bit rate over the entire image (This is not so much "compression" as triage, making the best picture from the limited amount of resources available in the data stream.. pitching things on a moving train.. deciding what you can keep, and what you can't afford.)

If the image "still" has too much variability.. then individual "blocks" look "wrong" and thats the artifacting we see in MPEG2 with too low a bit rate. Its (like) interlace combs.. only turned into squares across the entire image.. for basically the same reasons.. too little bitrate for the fast changing part of the image in that specific location.

In this case any existing "blocky" artifacts would be spread across adjacent grid blocks during the second pass and look "chewed up" or not as square.. spreading the "disease" around the image. But you have a very high bit rate.. so that spreading should be minimized.. its hard to predict.. it will depend on the original video footage.

Basically you have an experiment on your hands.. but I would not recommend it as an ongoing procedure. If at all possible try to avoid doing that.

And like I said.. I'm not an expert in "twice chewed" video sphagetti... sounds awful.

You might consider storing the higher bit rate MPEG2 files on a standard Blu-ray disc which has at a minimum x5 times the space, and up to x8 times the space without further compression. MPEG2 is an option on standard Blu-ray discs, its part of the standard for SD resolution video.. its just not often used. And at a minimum you could use Blu-ray for storing your "master" copy of the footage (as a data file Blu-ray) and make DVDs from that.. until technology catches up to the point where you have an option other than MPEG2-MPEG2 double pass.. perhaps some kind of MPEG2-Synthetic3D frame modeling in 2030.

Last edited by jwillis84; 03-11-2019 at 09:36 AM.
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03-11-2019, 11:23 AM
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lordsmurf lordsmurf is online now
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DVD-Video is max 9.8mbps (10.80mbps with some audio streams). So no.
But the 15mbps broadcast MPEG bitrate is Blu-ray compliant.

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03-11-2019, 04:52 PM
colony colony is offline
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Thank you jwillis84, for your thoughtful explanation including what I can expect. Thank you also, lordsmurf, for the botoom line concerning max DVD usage.
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broadcast rate, mpeg-2

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