Is VBR 2 Pass that much better than VBR 1 Pass or CBR?

[Walter Biscardi]

2 pass is always better. The first pass is analyzing your project and the second pass actually executes the compression. You will always get better results going 2 pass.

In most cases the difference is most prominent in areas of motion or quick changes in light to dark, such as when flashes go off in your case. Generally when it’s one pass you’ll see much more blockiness in those situations then when you go 2 pass.

Especially for a wedding project, I would recommend 2 pass as I’m sure you deal with a lot of lighting variations and lots of motion from time to time.

Walter Biscardi, Jr.
Editor, Colorist, Director, Writer, Consultant, Author, Chef.
HD Post and Production
Biscardi Creative Media

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[Jeff Pulera]

Sorry, double-posted by accident

Jeff

[Jeff Pulera]

Hi Adam,

We’ve been going back and forth on “the other forum” for days about this.

Why don’t you create a sample and see? I have done this myself – take several short segments from the wedding to represent different types of footage – some outdoor, some ceremony, some dancing with flashing lights, a good cross-section of all your footage. Toss some bits together on the timeline, maybe 5 minutes worth, and then encode for H.264 Blu-ray. In fact, encode twice, once with the 2-pass VBR, and once with CBR, at the data rate you intend to use on the final video. Then burn BOTH clips to the same disc, and view on a TV and make your own comparison.

Everyone is going to insist you use 2-pass VBR, but in fact I often use CBR and to me it looks fine. The average person won’t know the difference. We as videographers are too picky I think. If you can afford the time to render for 11 hours, then use 2-pass, why not?

If up against a deadline and can’t wait that long, then use CBR and get it done. Ultimately, YOU need to be the judge of what looks acceptable for delivery. You’ll get all kinds of opinions online.

Thanks

Jeff Pulera
Safe Harbor Computers

[Walter Soyka]

2-pass VBR is all about improving compression efficiency, and this often (but not always) leads to better results.

As Walter describes, the first pass is meant for analysis. This allows the compressor to know which parts of the source are easy to compress and which parts are hard. From there, it can build a “bit budget,” figuring out where it can safely allocate the lower end of the variable bit rate range, where it could best allocate the higher end, and potentially how to smooth out spikes in bit rate.

Because the compressor knows ahead of time where the easy parts are, it can lower the bit rate for those passages, decreasing the file size without compromising much visual quality, and saving bits for the hard parts.

Does this make it higher quality? Not really! The maximum bit rate will ultimately determine the quality of the output.

As an example, 2-pass VBR with a average rate of 10 Mbps and maximum bit rate of 15 Mbps will not beat CBR with a constant bit rate of 15 Mbps in visual quality — the most bits the VBR would possible spend on any given passage are the same number of bits CBR spends on every passage — but it will beat it in file size.

For shorter pieces, or for pieces where the file size doesn’t matter, CBR is perfectly fine.

For longer pieces, or for pieces where the file size does matter (like a Blu-ray), 2-pass VBR will almost always give you better perceived quality because it saves bits on the easy spots so it can spend them on the hard spots.

1-pass VBR combines the analysis and compression passes. It’s faster, but the compressor doesn’t have the luxury of understanding its entire bit budget, or how simple or complex upcoming material may be. You can get some space savings versus CBR, and some encode time savings versus multipass VBR, but you are trading off quality.

So in summary, it’s not that 2-pass VBR gives the absolute best results — it’s that 2-pass VBR gives the best results for a given file size.

Compression is all about trade-offs. You balance quality against file size; you balance quality and file-size against encoding time and decode complexity. You have to make compromises with compression and figure out what’s best for your specific workflow and delivery needs.

I agree with Jeff’s advice to test. Find the worst spot in your footage for a compressor to handle — the biggest lighting changes (flashes?), the fastest motion, the fastest cuts — combine that with some average footage, and see what outputs the different compression options yield within a given file size (at the same rate you’d need for the whole piece to fit on Blu-ray).

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog – What I’m thinking when my workstation’s thinking
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[Walter Biscardi]

[Walter Soyka] “So in summary, it’s not that 2-pass VBR gives the absolute best results — it’s that 2-pass VBR gives the best results for a given file size.”

Very interesting post Walter. Great analysis and good food for thought moving forward. I’ve just had so much better luck with 2 pass that we never even consider one pass or CBR. Thanks.

Walter Biscardi, Jr.
Editor, Colorist, Director, Writer, Consultant, Author, Chef.
HD Post and Production
Biscardi Creative Media

Foul Water Fiery Serpent, an original documentary featuring Sigourney Weaver. US & European distribution by American Public Television
MTWD Entertainment – Developing original content for all media.
“This American Land” – our new PBS Series.
“Science Nation” – Three years and counting of Science for the People.

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[Tim Kolb]

I tend to use Constant BitRate for quick client proofs or something like that. It’s usually faster to output and also can be easier for less powerful computer systems to play back within a given data rate range…

I agree with Walter Soyka’s explanation, and will only do final deliverables as Variable BitRate, 2 pass where available. The efficiency is worth the time investment.

TimK,
Director, Consultant
Kolb Productions,

Adobe Certified Instructor

[Ed Grogan]

I’ve loaded a two hour ProRes file onto the timeline for export media to control the compression. After compression I load the audio and video file onto the Encore Timeline and note that there is a discrepancy in the length of the audio (shorter than the PrPro timeline and the Video (longer than the Pr Pro timeline). This causes the audio and video to be out of sync.

In discussions with Adobe engineering they have confirmed this problem with CS5.5 and CS6 with Prores files.

I’ve done major testing with both the Mac and Windows version of the compressor that is part of the Adobe Master Suite.

I can repeatably demonstrate that a file (I have determined that it is not Prores dependent) compressed with the setting “H.264-Blu-Ray” produces a compressed file that contains both repeated frames and dropped frames. Repeats start at just over one minute into the video. On five different files, made with VC-1, ProRes, Uncompressed, AVI and WMF all of length 00:02:07:27 produced compressed H.264 that was five frames too long. (Source was 720p59.94). Errors started about one minute in. all showed a random pattern of compressing a frame and then repeating the last two frames. The uncompressed showed more duplication and some frames dropped.

THe compressor setting H.264 (without the Blu-ray tag) worked perfectly. The only problem is this compressor does not bring chapter marks into Encore. Also, not verified, that Encore does not like mux’d files as sources. Still looking into issues as to why Encore wants to transcode anything intended for Blu-ray.

The H.264 Blu-ray setting also damages the audio file but I haven’t started to investigate that.

Update: Only did two tests: One with Windows version & 5 on the MAC

If a timeline in PrPro is dynamically linked to Encore and the “Set Quality Settings” is evoked, you get the same setup for compression as you do in Media Encoder and PrPro. (Except Encore calls it transcoding settings). Entering the exact same setup for compression settings as those that failed in PrPro and ME produced a BluRay mts file for the BluRay disk that had no errors. This seems to indicate that there is an error somewhere in the underlying configuration being handed off by the programs that does the actual encoding (compression).

Ed Grogan

[Jonny Webb]

This was so useful I just had to login and say Thankyou.

And Jeff – always a good idea to test it ourselves.

🙂

++ As we’re all here, i guess we’re not all there ++

[Walter Soyka]

[Oscar Gotti] “could you elaborate on this? Why is it that 2-Pass VBR works better for longer pieces? “

VBR works better on longer pieces, given a specific maximum file size. If you must fit your movie within a specific data size, like a 25 GB Blu-Ray disc, 2-pass VBR will give you higher quality than CBR.

Why?

Some scenes will be easy to compress: lower action, less detail, etc. Some scenes will be hard to compress: high action, high detail.

The maximum data size is your “bit budget” — the total amount of bits you can use to store your movie.

CBR tells the compressor it must use the same amounts of bits on every second of the film. That means an easy-to-compress second takes up as much space as a hard-to-compress second. An easy-to-compress second wastes bits; it doesn’t need all the bits it’s using to achieve that quality. A hard-to-compress second could look better if it were encoded at a slightly higher rate, but there are extra bits in the budget available because they are spent equally.

VBR tells the compressor it’s ok to spend less bits on simpler scenes are more bits on complex scenes. An easy-to-compress second of video can use fewer bits out of your total bit budget, meaning you have more bits to spend elsewhere on a hard-to-compress second.

2-pass VBR specifically lets the compressor analyze the video beforehand, figuring out where the easy and hard parts are, so it can allocate its bit budget intelligently.

Does that help? Is there more I can clarify?

Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
@keenlive   |   RenderBreak [blog]   |   Profile [LinkedIn]

[Walter Soyka]

[Oscar Gotti] “I guess I’m not sure what the length or size of the video has to do with it.”

There are a confluence of factors here: the maximum data size, the maximum bit rate, and the length of the program.

Let’s take an example with Blu-Ray disc. The maximum data size is 25 GB for a single-layer disc. The maximum recommended video data rate is 40 Mbits/s. Here’s a little math:

25 GB = 204,800 Mb (25*1024*8)
204,800 Mb @ 40 Mb/s = 5120 seconds
5120 seconds = 85.33 minutes

At the maximum bitrate of 40 Mb/s, we can only fit 85 minutes in our data size. If we have a 2-hour (120 minute) program, we need to lower the video bit rate down to about 28.4 Mb/s to get everything to fit.

With CBR, we’d be lowering the bit rate across the board. With VBR, we could lower the bit rate MORE in sections that can’t use the extra bits, allowing us to use a higher temporary bit rate in sections that could benefit from it.

VBR gets you higher quality at a given file size, because you can use the same number of bits more efficiently.

[Oscar Gotti] “I guess I’m not sure what the length or size of the video has to do with it. Surely with your explanation VBR Pass 2 will also save bits to work on harder sections?”

There is no need to save bits if you don’t care about the file size. You can set everything to the maximum bit rate to optimize for the hard scenes and just waste the extra bits in the easy ones with no penalty.

You only need to be careful about how you spend bits when the bit budget is limited — that is, when you have a total file size that you cannot exceed.

Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
@keenlive   |   RenderBreak [blog]   |   Profile [LinkedIn]