CBR vs 2-pass VBR comparison (really 3)

Quantum · Member Joined: 11 Dec 2003

Hello,

I am converting some personal camcorder footage of mine (wedding) to DVD and have been performing some experiments to find the best quality conversion. All footage is less than 60 minutes and I am using 1 AC3 2-channel audio track. TMPGEnc Xpress 3.0 is my encoder using 10 bit DC Component Precision and Highest (with error correction) Motion Search Precision.

1. My first thought that since space was not an issue (<60 minutes per DVD) that constant bit rate (CBR) would be quick and appropriate using a bitrate of 9200 kbits/sec. So I encoded a video that was 34 minutes and 34 secs long with a 448 kbits/sec AC3 audio track using TMPGEnc Xpress 3.0. When you select CBR in TMPGEnc Xpress the Average/Minimum/Maximum bitrates all get set to 9200 kbits/sec. The integrated bitrate calculator said the compliation would be 2428.65 MB. The resulting output file was 2,534,987,780 bytes.

2. Then I wondered if using the Varible Bit Rate (VBR) mode with 2 passes would optimize GOP's, I-, or B- frames. I don't really understand these that well but I thought what the heck, perhaps 2 passes would be better even if the bitrate was still 9200 kbits/sec like the CBR example above. So I set TMPGEnc Xpress 3.0 up in VBR 2 pass mode with the Average and Maximum bitrates set to 9200 kbits/sec and Minimum set to 0. The integrated bitrate calculator said the compilation would again be 2428.65MB. The resulting output file was 2,384,832,516 bytes.

3. Having seen the discrepency in resulting file sizes above I suspected it may have been due to setting the the minimum bitrate to 0 and only setting the average and maximum bitrate to 9200 kbits/sec. So I set the Average, Maximum, and Minimum bitrates to 9200 kbits/sec and ran TMPGEnc Xpress 3.0 again in VBR 2 pass mode. The integrated bitrate calculator said the compilation would be 2428.65MB for the third time. I expected to have a resulting encoded file the same size as the CBR example #1 because all the bitrates were now set the same as well as all other settings. The resulting output file was different coming in at 2,534,973,444. Granted pretty darn close to #1 but still different.

So the question I'm trying to answer ultimately is which mode is best for maintaining the best possible quality between my DV source and MPEG2 output for DVD?

To get that answer I'm wondering:

1. Is there any advantage to using 2 pass VBR mode with the same bitrate settings as CBR? (Does 2 passes optimize picture quality through different GOP, I-, B- frames settings, ect.)

2. If average and maximum bitrates were both set to 9200 kbits/sec in example #2 with VBR 2-pass, why did the resulting file come in smaller than example #1 with 9200 kbits/sec CBR?

For the sake of argument in this discussion let's say that time encoding and size of files don't matter.

Thank you very much for any insight you can provide.

daamon · Cockney Pommy Joined: 10 Jun 2003 Location: Melbourne, Oz

Wow...!!!

<- That's how my eyes are now...

OK, here goes... I'm gonna pick through and address points and questions - there's quite a few hidden in there!

1st Attempt: All fine there...

2nd Attempt:

Quantum · Member Joined: 11 Dec 2003

Thank you very much for the lengthy and informative response. I've been doing some additional research on I, B, P frames as well as analyzing my three encodes by frames. What I found:

1. Each GOP TMPGEnc Xpress 3.0 encodes for my NTSC project for both CBR and VBR are as follows:

IBBPBBPBBPBBPBBP, IBBPBBPBBPBBPBBP, ect. so I guess my NTSC GOP is set for 16 if you count the I frame.

2. I have "detect scene change" checked in TMPGEnc Xpress and I've noticed in at least one spot in both VBR encodes that TMPGEnc Xpress ended the GOP early (before 16) and started another I frame at the scene change. The CBR encode keeps the GOP at 16 before the scene change and then starts a new I frame at the scene change (after the last's GOP's 16).

Once of my original questions was why is the file size smaller even though I selected 9200 for both average bit rate and maximum bit rate while 0 for minimum bitrate in example #2? I've been thinking and to get an average of 9200 kbits/sec then the maximum and minimum would have to both be held to 9200 kbits/sec. It appears that if you set the minimum to 0 and average/max both to 9200 kbits/sec than TMPGEnc Xpress is doing a VBR encode that uses a minimum less than 9200 kbits/sec and therefore isn't really following a 9200 kbits/sec average.

I also burnt two DVD+RW's, one with the CBR 9200 kbits/sec encode (example #1) and VBR encode with min/max/average set to 9200 kbits/sec (example #3) and I really couldn't tell a difference subjectively on my 27" TV.

I'll continue to look for an objective difference between the two above and hope if someone knows of a nuts and bolts difference (different algorithms?) for those unique situations they can explain it for us.

Thanks again.

Shadowmistress · Posted: Dec 31, 2004 05:56

You're missing a fundamental understanding of what CBR/VBR is. This can become a very complicated answer, but I'll try and simplify it as much as I can.

CBR= Constant biterate. The disk in your dvd player will spin at the same speed throught the entire playing of the movie.

VBR= Variable biterate. Your dvd player will either speed up or slow down the spin speed of your disk while playing the movie, depending on how much information it requires to play each scene.

Each individual frame of a movie has a different bitesize. Each frame is a sum total of light, color and motion variances. (Lets forget about GOP for a second cause that's just another calculation in the mix and has nothing to do with it anyway.)

CBR. If you choose a constant biterate that is too low, you loose quality in your picture as some information gets "dumped". If you choose a constant biterate that's too high, you are overinflating your movie and wasting disk space. Most people don't use CBR.

VBR is actually CBR evolved. A variable biterate will try to adjust for space while keeping quality. It asks you what average biterate you would like (you feed it what the calculater states will fit into your dvd), it asks you how high can it go to accomodate high motion scenes that require more biterate to keep the quality of the picture, and it asks you how low it can go to save space on your disk.
This results in an effect that most people (inaccurately) call borrowing bites from low motion scenes to make high motion scenes look better. You end up with improved overall picture quality with saved disk space.

When you input the exact same numbers in your ave/max/min values in vbr, you are essentially telling vbr that it cannot deviate from that biterate. This is exactly the same as using CBR.

When you use 0 in the min field you are allowing vbr to compress the parts it can to save on space, but since you've given identical ave/max biterates, vbr will not compress those parts very much in order to keep up with the overall quality of the rest of the movie.

The proper way to use vbr is to set the max way higher than the average and the min way lower and let vbr do the adjusting for you.

I can't tell you what those numbers should be, because I use CQ rather than vbr, but that's for another post to try to explain.

To illistrate how infuriating and complicated the process is, isolate exactly 50 frames of your movie with a shot of a person standing still and encode it using CBR at 01 kbps. Then isolate 50 frames of a pan shot from side to side and encode it using CBR at 01 kbps. Even though you have 50 frames of each, the pan shot will have a much higher filesize than the still. (Yes it will look like shit, but this is just for curiosity's sake.)
By coding at 01 kbps you are allowing the encoder to just use the absolutely necessary bites required to make each frame.

monzie · Member Joined: 16 Nov 2003 Location: The Village

[quote="Shadowmistress"]You're missing a fundamental understanding of what CBR/VBR is. This can become a very complicated answer, but I'll try and simplify it as much as I can.

CBR= Constant biterate. The disk in your dvd player will spin at the same speed throught the entire playing of the movie.

VBR= Variable biterate. Your dvd player will either speed up or slow down the spin speed of your disk while playing the movie, depending on how much information it requires to play each scene.

quote]

No it doesnt.

The DVD does not spin at a different rate at all.

daamon · Cockney Pommy Joined: 10 Jun 2003 Location: Melbourne, Oz

adam · Member Joined: 13 Sep 2000 Location: United States

The speed the DVD spins will slow down progressively as it plays but in no case will bitrate have any effect on it.

http://electronics.howstuffworks.com/dvd12.htm

CBR actually does not mean constant bitrate, it means constant buffer rate. The bits per second still fluctuate per scene just not by as much so that the buffer size remains constant.

Shadowmistress · Posted: Jan 01, 2005 02:54

Adam, this is what I was going on:
http://www.iis.ee.ic.ac.uk/~frank/surp99/report/kht97/work.html

Quantum · Member Joined: 11 Dec 2003

I've been doing some more research on encoding methods and ran across a CCE manual on google. According to what I've read, CCE has a mode called multipass CBR that allows you to set one bitrate value with multiple passes. So that natually prompted me to ask what is going on during the multiple passes in a CBR encode? Can it be duplicated somewhat in TMPGEnc by running it in VBR mode with min/max/average bitrate settings of 9200 kbits/sec?

If anyone has information on multipass CBR encoding I would be interested to hear it and probably others would be too.

My next step is to get a trial version of CCE and do one of these encodes and compare it subjectively to my TMPGEnc Xpress encodes.

adam · Member Joined: 13 Sep 2000 Location: United States

You could accomplish roughly the same thing using the same min, max, avg settings in TMPGEnc, yes.

As stated before, CBR just means constant buffer rate. There is still some fluctuation at the GOP level. Mpeg encoding is all about finding redundancy in a series of frames and storing that repeated information in one frame, and then recalling it for all others that use it. The more passes you use to analyze the clip, the more redundancy you find, and thus the better compression you achieve....up to a point of course. This holds true even for CBR encodes, obviously not nearly to the same degree as VBR encodes though.

SatStorm · Posted: Jan 04, 2005 04:18

IMO, with TMPGenc, just use 2 PASS VBR with min / aver / max the maximum avalaible bitrate for DVDs (9200 kbits/sec that is)

That way you emulate something called 2 pass CBR which in theory can be better a simply pass CBR.

You can't go higher than this.
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SaSi · Very Confused Joined: 10 Jan 2003 Location: Hellas

I agree that 2 pass CBR can produce better encoding compared to single pass. However, if bitrate is set high enough (as originally stated at 9200kbps), isn't a single pass encoding "equal enough" to a 2 pass one?

In the same fashion, when someone encodes at 9Mbps, I think that things like motion search become less important as there is plenty of bitrate to spare.

There has to be an optimum encoding strategy for low bitrate and a different one for high bitrate.
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video_sync · Member Joined: 11 Nov 2007 Location: Sweden

guns1inger · So Very Tired Joined: 01 Apr 2004 Location: Miskatonic U

You have waited over three years to post this ?
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SatStorm · Posted: Apr 10, 2008 06:17

@video_sync: 9200+audio with TMPEGenc 2.5. That encoder can't go higher without issues. Also can't go lower 3.000kb/s without issues.
But that's TMPGenc-ology, a long forgotten knowledge of the ancient ones

jagabo · Member Joined: 09 Dec 2005 Location: none

The only people who could tell you what the difference between 2-pass VBR encoding with minimum, average, and maximum bitrates as 9200 vs. a CBR encoding at 9200, with TMPGEnc, are the programmers at TMPGEnc.

But here's a little thought experiment for you:

Write down three numbers. The average of the three numbers must be 9200. The biggest of the three numbers cannot be more than 9200.

I know what three numbers you wrote down without you telling me. In fact, the experiment can use any number of numbers. I still know what they all are.

jman98 · Member Joined: 08 Oct 2004 Location: Freedonia

SatStorm · Posted: Apr 10, 2008 13:10

jman98, we don't talking here about VCDs and Mpeg 1.
We talking about DVDs and Mpeg 2.

TMPGenc is a good encoder but it has issues on static scenes when you go below 3000kb/s and you encode to mpeg 2. It create macroblocks on the dark areas. FYI, the popular TMPGenc 2.5 pro encoder, also creates problematic mpeg 2 for DVDs overall. Minor things that nobody gives attention. But those issues shows when you use more than 9200KB/s for video.

The point of 2 pass CBR is the better pixel allocation of the video data during frames. Some encoders offer 2 Pass CBR mode.
Yes, 2 Pass CBR helps with mpeg 2 and specifically with TMPEGenc. Of course you must be mad with the picture quality to use it, only the very few can tell the difference.

jagabo, if you have a full HD TV and a DVD player that upscales Full D1 material created by TMPGenc to 1080i, do a test yourself: Find a dark scene from a movie you captured from TV and encode it as usual, with low bitrate around 3000kb/s, CBR@9200 and 2Pass VBR all set to 9200. You be amazed with the results. Watch this upscaled to 1080i.

themaster1 · Member Joined: 16 Nov 2006 Location: France

Hmm 2 pass CBR? I have never tested this but i'm gonna

My opinion about VBR is >>never use this<< unless it's necessary: length of the video content too long.
Slowing / Speeding the disc is not something i like much

If at some point you fast forward or rewind, your dvd player will suffer and the disc as well.That's how i see/understand things.

edit:
Which mpeg encoder can encode in 2pass Cbr? (procoder 2 don't)

SLK001 · Member Joined: 24 May 2001 Location: United States

SatStorm · Posted: Apr 10, 2008 18:57

CCE had 2PASS CBR once upon the time from what I recall.

Basically. as I said 3 years ago, you can emulate 2 Pass CBR by setting 2 Pass VBR with the same value for low,average,high bitrate. You can do this with any encoder that allows 2pass Vbr

Back in the old days, we use to do whatever possible to create the best VCDs, CVDs and SVCDs. 2 Pass CBR used frequently on commercial VCDs, it was industry's "secret" to have slightly better picture quality on their movies. Better pixel allocation per frame, was the only thing they could do, especially on VCDs, which the specifics are not flexible at all. When our PCs was Duron 700 and Celerons, 2 Pass was something for the very advanced enthusiasts! I remember myself waiting 36 hours for a 2 hour conversion using 2 Pass VBR back in 1999, using TMPGenc 0.12 I think, the last freeware version of this encoder....