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VBR Stays Low, Regardless of What I Set It To!
As the title says…. This happened around 3 months ago. No matter what I set it to before rendering, it always comes out as 21, or 22k…. Here’s a video showing the problem.
https://www.youtube.com/watch?v=CwuS7AI8-Po
Do I need to re-install, or is it something that’s so minor, I will headdesk when told the solution? lol.
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4 Replies
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Hi Jordan,
Are you sure the info displayed on Windows is accurate? Are you seeing a difference in quality with different bit rates?
Try opening the video in Quicktime and use show movie info just to be sure.
Vince Becquiot
San Francisco Bay Area
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Oh, it’s definitely accurate. I just updated to 6.0.5, and the cap has been raised from 21 or 22k, to 55,572kbps in 2 pass, and 54,621kbps in 1 Pass at max “level” 4.2 (62.5). If I go to “level” 5.0, and even raise it a hair higher, the ending result will be 737kbps…. lol.
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Usually there are 3 parameters for VBR encoding : target, maximum, and minimum bitrate.
In your video, I didn’t see the setting for the minimum bitrate. Are you sure this number is not too low? -
Jordan, it looks like there are a few factors at play, and most are normal behavior. One is a bug, but it only relates to the case with the very low bitrate (737 kbps).
Unconstrained Maximum: Bitrate can only be so high based on the footage and encoder settings. The bitrate settings go up to 300,000 kbps using HiP@L5.1, but that doesn’t mean the encoder needs that much data.
For reference, take a look at the maximum allowed settings for each H.264 Level. HiP@L5.0 has a maximum allowed bitrate of 168,750 kbps, which correlates to 1920×1080 at 72.3fps. Doing the math, this is equivalent to 1.15 bits/pixel. Therefore at 24fps a 1.15 bpp video stream would be about 56 Mbps. Actual results will vary depending on encoder settings and the nature of the footage. The 21 Mbps rate in your example is equivalent to 0.45 bpp. This might seem low, but we need to consider other factors (see below).
GOP Length: If you want to force bitrate to be higher, set Key Frame Distance to a very low number. This will increase file size, but will not necessarily improve quality beyond a certain point. I typically set Key Frame Distance to 10-15 frames for high motion footage.
The chart shown below is from a characterization study. Unconstrained average bit rate is relatively high when Key Frame Distance is set to one frame. In the study average bitrate stabilized around 0.5 bpp as Key Frame Distance increased. Therefore the 0.45 bpp rate of your 21 Mbps sample file is within reason, especially if the video is relatively static.
Bitrate Tuning: Tweaking the maximum bitrate setting will sometimes produce incremental changes in the actual encoded average bitrate. This relates to the size of the data blocks the encoder uses when breaking down larger frames.
CBR vs VBR: At very high bitrate settings CBR will be relatively high while VBR does not exceed its “natural” maximum. Beyond a certain point the CBR encoding just becomes inefficient without noticeable improvements in image quality. (CBR offers other advantages especially for broadcast video.)
Multiplexer: Under the Multiplexer tab be sure Stream Compatibility is set to Standard. It defaults to iPad which forces the H.264 Profile to Baseline.
Encoding Bug: One of the Premiere Pro updates last year introduced a glitch when trying to encode high bitrate H.264 files. Specifically, if the Target Bitrate is set too high the encoder spaces out (technical term) and produces files at only a few hundred kbps. This behavior was observed with direct exports; sending jobs to the Media Encoder queue seems to be OK.
I submitted multiple bug reports to Adobe; feel free to submit another. Perhaps the company will be more responsive if more people raise the issue. Note that this only relates to your 737 kbps file.
Other:
– The data rate reported in Windows Properties is not always accurate. As Vince noted above QuickTime is more reliable. Also consider downloading the MediaInfo application.
– Please upload a screen capture or short video of the exported file. How does image quality compare between the source file and the encoded H.264 file?
– How will the video be used? There are not many reasons to encode 1920×1080-24p H.264 video at 88 Mbps.
– Compression is relatively complex. Be prepared to try different things and read reference material to become more familiar with the settings and practices.
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