I've been making DVD's of my travel videos for years but am only an amateur when it comes to using the various video editing software. I've had HD camcorders for several years now but now that I have a blu-ray burner there are some technical issues that have come up that have me wondering as to what I should do to get the best possible results. Specifically, I have been using the Sony CX-7 camcorder lately. Apparently the .MTS files it produces are 1440x1080 (16/9) at 29.970fps, 60i,  AVC (Main@L4.0), CABAC/ 2 ref frames, and 13.9Mbps. Up to now I burned my video to dual layer ordinary DVDs which gives me about an hour's worth of video on each. I am now trying to create blu-ray discs. For one thing, they hold 25GB rather than the 8.4GB for the DVDs. As my first test I am working on a 2 1/2 hour film (which I had previously already burned to several DVDs). For the heck of it I thought I would try to create a 24fps file rendered at 15-18-20 Mbps (min-av-max) variable frame rate, 2 pass, MPEG-2 (Version 2)(Main@High). This produced a 19GB (video only) 17.2 Mbps file. However, I thought the quality of this was worse than that of my DVDs. I just finished another burning where I set things to 60i, 22-20-18 Mbps (the "standard" 25 Mbps for blu-ray produces too large a file). I got a 21.2GB file (video only) rated at 19.1 Mbps. I haven't had time to burn this to a blu-ray disc yet.

My question is this: What exactly happens to the encoding when one changes the bit rates? Since the native video is 13.9 Mbps, do I really get better quality by encoding this at a higher bit rate? Since the bits are streamed faster for the same content, is there some interpolation going on? I don't want to waste time on encoding if it does not, at least in theory, produce better results.

mka:

My question is this: What exactly happens to the encoding when one changes the bit rates? Since the native video is 13.9 Mbps, do I really get better quality by encoding this at a higher bit rate? Since the bits are streamed faster for the same content, is there some interpolation going on? I don't want to waste time on encoding if it does not, at least in theory, produce better results.

In theory, a higher bitrate is better.  But a person also wants to use expensive BD space to the max and efficiently.  If the native bitrate is 14 mbps, raising the disc max bitrate to a higher rate will not make the video look better than the raw original, but might reduce the compression artifacts that might creep into the output as a result of recoding during disc image stream file creation.  This issue needs some testing.  Ideally, though, it could be skirted altogether by introduction of "smart rendering," which would mean no loss whatever and a much faster render time.

The honest truth is that the only way to know the quality impact is to burn a project at different bitrates and let your eye judge the difference when viewing the separate versions played back on a large screen.  The obvious constraint is that a 21.8 GB BD takes just too plum long to render twice, and no fool will spend $10 for a 25-BD to burn a short project just to test the difference.  The answer is to get two BD-REs and burn a short work at different bitrates on each, or else burn a project to two AVCHD-DVDs at different rates.  To compare two m2ts clips in a stream image on a hard drive, at different bitrates, might be misleading, unless you use precisely the same player software as your disc player.

I recently burned a 4.5 hour AVC-BD at 15 MBS.  I originally tried 19 mbps, but the 24.8 GB disc image was too big to fit on a BD-SL.  The native files were AVCHD 1440x1080 60i, some at 9 mbps and some at 13 mbps.  I compared the 15 mpbs version to a truncated version of the 19 mbps one.  I saw no differences, although there were few action sequences in optimum light to afford an ideal stress test.  I suspect the loss is negligible if one is working with video with limited motion, stills, or low light.  The loss to any HD video may also be modest (or not noticed at all by lay viewers accustomed to SD) so long as the bitrate is at least as high as the native or above 13 mbps.  Sports or action video shot in proper light should probably be exported at the maximum bitrate your disc space allows.

Sooner or later, someone should do a rigorous side-by-side test of the AVC and MPEG versions of BD projects at high, medium, and lower bitrates.  The test material would best include varied video material presenting typical distinct lighting or motion situations.  The trouble is, of course, that no one but a tycoon sets out to do anything short in BD. 

I always wondered why creation of 24p output from 60i source would do any good at all.  Your answer puts that to rest.  What I still don't understand is whether 24p source video offers any distinctive charm, except perhaps better low light quality.  Also unknown: whether 24p degrades more than 60i when recoded during disc image creation.

Thanks for your explanation JKoch although you did not completely explain what technically happens with a higher bit rate.

I've now looked at my second blu-ray disc version (60i, 19.1Mbps) and I'm quite happy with it. It seems to be at least as good as my AVCHD DVD version on my 100" screen. I thought that people would say one should let one's eyes be the judge as to what is best for rendering. The problem with that is that this is useful when there are gross differences but when the differences get subtle it is not easy to differentiate since I cannot look at the pictures side-by-side.

By the way, the reason I tried 24fps is that I read that this can produce a more film-like quality.

A higher bitrate means more information per frame.  The effect of a higher bitrate depends, of course, on the amount of new visual information to convey.  It's not much if the image is stationary or changes little.  It can be a lot if there is complex motion with lots of details: a crowd of thousands stampeding, a rough ocean, falling snow, a pan of a building, playing field, or flower garden.  If the motion and information exceeds the capture bitrate, you get a blur, pixel blocks, or stuttered motion.

Some videocams offer capture at variable bitrates, which saves memory when motion is light, with the option to capture at a fixed rate, which would be HQ.

When Studio recodes to create discs, the process evidently reconstitutes the video in a way that simplifies some of the information and throws out bits.  Therefore, to raise the output bitrate (which is only the maximum, not the average) may reduce the loss.  This is only the theory, as proposed by poster Bittmann.  I sure wish a Pinnacle employee wise on matters of AVCHD (perhaps M. Pingel?) would weigh in and impart the real story, plus reveal why smart-rendering (which is really dumb-rendering) should not be relatively easy to include in Studio.  My dumb guess is that AVID / Pinnacle relies on outside contractors on such matters and that financial hardships preclude investments in anything more than things like Montage, which promised at least some "bang for the buck" to perk up scales.

I recently took a close look at the image quality of the BBC Planet Earth BD set, which packs a bit over 3 hours of 1920x1080 video onto each 25 GB disc.  My Sylvania BD player does not give a reading on the bitrate, or maybe I've just not learned how to get it to display, but based on the amount of video on each disc, I'd guess the average could not exeed 20 mbps.  In fact, I'd bet the average was below 15 mbps.  Most of the images are quite clear.  Some sequences of 100k flying snow geese or caribou were a bit blurry, but atmospheric conditions or the nature of the original capture might have been a constraint: there is almost no such thing as air devoid of haze or dust, light is often an enemy, and great events often happen when light is not.  If the Planet Earth videos are an HD showcase, then I suppose that means that the ordinary Joe or Jane who creates projects at that rate should obtain reasonable results.  I understand that the Nero player and possibly Power DVD display the bitrates of a video, and I'd be curious what those are for a commercial BD with lots of action.