MainConcept Questions.

DwSw wrote on 12/1/2002, 10:31 PM
Hi Everyone,

I am just now coming up to speed with Vegas Video and the MainConcept encoder. I have run a few experiments to determine the best settings in MainConcept.
My setup:

1. Editor: VV 3.0
2. MPEG2 encoder: MainConcept 1.0
3. Burner software: MyDVD 3.5
4. OS: Win2K
5. Test videos: 30 seconds of panning, 30 seconds of still, 60 seconds combined panning and still.
6. Video: from Cannon Optura (native DV, not originally analog).
7. DVD Player: Toshiba (with bit rate display).

Experiment:

* Vary MainConcept (CBR and VBR) bit rate parameters and compare quality (somewhat subjective)and note average bit rate during play back and bit rate max/min swing.
* Compare bit rate of burned videos with Men In Black video noting average bit rate and bit rate max/min swing.

Questions:

1. When the encode type is set to VBR, does MainConcept do a prescan of the file to determine the bit rate? It doesn't appear to do this. Even when I run batch mode. Most VBR programs make two passes: first pass, to compute where to use the low to high bit rates; second pass, encode using previous computations.

2. When comparing the bit rate with a "professional" production, this example Men In Black, and encoding my video in VBR, I noticed the following:

MIB: the average bit rate was approximately 5Mb/s with a swing from 3Mb/s to 8Mb/s. A swing of 5Mb/s.
My production: VBR settings -- target 5Mb/s, max 8.2Mb/s, min 2.5Mb/s -- measured average bit rate was approximately 5Mb/s with a swing of 4.8Mb/s (min) to 5.1Mb/s (max). A swing of 0.3Mb/s. This was consistent with all types of video: panning, still, and combo. I would expect the combo (which was half panning and half still) to be a higher bit rate during panning and less during the still to average out at 5Mb/s. The bit rate is roughly the same through out both segments. I also encoded using CBR (5Mb/s) and noted approximately the same target and swing range. Variable bit encoding doesn't seem to be very variable. Might as well encode at CBR. Note: I am pretty sure that MyDVD is not re-encoding the videos (don't see any indications and the set target/average bit rate always matches the measured average bit rate). However, in the end it could be re-encoding the files.

Please help.

Thanks,

Dwayne


Comments

vonhosen wrote on 12/2/2002, 2:26 AM
1.
MainConcept doesn't do 2 pass VBR only 1 pass. You will not get the variation in bitrate that you experience with MIB in a single pass MainConcept VBR encode.
Other encoders like Canopus Procoder will do 1 or 2 pass VBR depending on what you want.

If you are only looking to put about an hour on disc having CBR at 8000kbs isn't going to look any inferior to a VBR encoded file.
Here are the main differences between the types of encode (forgive the plageurism please):-

CBR - Constant Bitrate
Goal: Constant bitrate through encoding
The bitrate does not vary up or down - it remains constant.
This produces constant size but variable quality.

1-pass VBR - Single pass Variable Bitrate
Goal: Constant quality through encoding
The bitrate varies from the base video bitrate up to the defined max video bitrate.
When the complexity of the video increases, the bitrate will be increased up to the max video bitrate.
This produces constant quality but variable size (assuming the max bitrate is sufficient to reproduce the complex video sections).

2-pass VBR - Two pass Variable Bitrate
Goal: Set average bitrate with variable instantaneous bitrate
The bitrate varies up to the defined max video bitrate, with the overall average maintaining the defined base video bitrate.
When the complexity of the video increases, the bitrate will be increased up to the max video bitrate but bitrate in areas of lower complexity will be decreased in order to "free" bits for the complex sections.
This produces constant average quality and constant size (assuming the max bitrate is sufficient to reproduce the complex video sections and the defined average bitrate is not so low that bitrate peaks create poor quality in lower-bitrate sections).

Very important points:
1) CBR is fastest, 2-pass VBR is slowest (assuming same Speed/Quality settings)
2) 2-pass VBR is not necessarilly better than 1-pass VBR - especially if predictable size is not a concern.

An easier way to think about it:
CBR is like a straight cylinder. Its size is constant throughout.
1-pass VBR is like a balloon - it can increase as necessary but won't get bigger than the max you set.
2-pass VBR is like a bean bag - some parts can have more beans, other parts can have little or no beans, but overall - it's always a set number of beans.

2.
When Hollywood encode they will do whatever is required to get the best results with the most economical bitrate so they can fit the movie & all those extras on the disc. They will pay a skilled operator with great equipment to tweek the encode, often breaking the movie into small parts (or even frame by frame) to optimise the bitrate for each particular scene. Put that together with the fact that they have the best lighting, cameras & skilled staff to give them fantastic clean footage to start with. You can definitely see where the money is spent in the end product.
DwSw wrote on 12/2/2002, 8:39 AM
Vonhosen,

Thanks.

I am considering putting two hours on a disk. However, I have concluded that 5Mb/sec (equates to approximately 1.5 hours) is optimal for quality versus quantity.

However, theoretically I could get more time on a disk if the bit rate expended during less complex scenes, which appeared to be wasted, could be applied to the bit rate during more complex scenes.

I saw what appeared to be a fixed, "constant," bit rate even though I selected variable.

Since I set the max to 8.2Mb/s should I see occasional spikes towards 8.2Mb/s and corresponding spikes towards 2.5Mb/s? I didn't see this. However, it is possible that my DVD player may not be very good at measuring transients. However, I did see a much wider variability in the Men In Black DVD. So, my DVD player has some ability to measure transients.

Dwayne
nolonemo wrote on 12/2/2002, 10:32 AM
I'm just guessing, but I suspect that you can only maximize the use of the full bitrange specified in VBR by using 2-pass; i.e., unless the encoder knows what's coming up from having done a pre-scan, it will have to allocate bitrate conservatively.
vonhosen wrote on 12/2/2002, 11:27 AM
I don't think with the MainConcept encode you are going to see the kind of variations you want as it is just the 1 pass.
The other avenue you can look at is the audio you are using to help keep the bitrate up to an acceptable level.

For 2 hours you can have a combined bitrate of 5.0Mbs avg.
If you are using PCM .wav audio then that will only leave you about 3.4Mbs for video.
If you use ac-3 or mpeg audio at about 192kbs you'll get 4.8Mbs available for video.

It depends on your target audience if you can use mpeg audio. NTSC doesn't officially support it but growing numbers of NTSC players will play it.
Check at www.vcdhelp.com for what players support mpeg audio.

JJKizak wrote on 12/2/2002, 12:18 PM
My expierience as a newbie is the VBR will get you the most on the disc
with Dolby-AC3. The best quality I have obtained is 7.3 megs CBR, 10 bit,
Best quality, Lower frame first, 48kc sample, dual file output (separate
audio and video files), with interlace flicker and resample switches activated
and the rest in NTSC DVD default. Unfortunately, about 90 minutes is all
you can get on the disc. My experience with VBR is it is not fast enough (Inept
terminology)to follow the variations in flowing water or blowing grass or 24 fps film converted to tape and there is a slight color loss. That's why I use the CBR as it seems to blow right through all of this with little effort.(and very little
color loss). I also set the REEL DVD bitrate at 9.8megs, Dolby AC-3, 192KC sample and NTSC DVD.You absolutly must view the final product on the TV or computer monitor depending on where it is going to be viewed.

James J. Kizak
owlsroost wrote on 12/2/2002, 1:25 PM
If you want to look at the bitrate variation of an MPEG file, get Bitrate Viewer - http://www.tecoltd.com/bitratev.htm - it plots a nice graph of bitrate (and quantisation) against time.

I almost always load MPEG files after encoding into this just to check everthing is as I expect.

(and I've always found the MC encoder to be fairly 'conservative' about VBR bitrate variation too)

Tony
DwSw wrote on 12/2/2002, 8:09 PM
Agreed. The fact that it is single pass probabaly explains (at least partially) the measured max and min swing.

Unfortunately, my burner, MYDVD, requires PCM and will convert to PCM if encoded in a differnt format. Probably makes the software cheaper to support only one audio format. Also, it will keep newbies like me out of trouble.

Christmas is coming up. Any encoder recommendations?
DwSw wrote on 12/2/2002, 8:11 PM
Cool! Thanks. I will run my experiment again with this software.