I have a question about bitrate that I'd love some help with...
My source video is 1080p using a Canon 6D. In Premier Pro CS5 my project sequences are set to HDV 720p30, as I intend to export at 720p.
My export settings are:
NTST, 1280x720, 29.97fps, Progressive AAC 320kbps, 44.1 hKz, Stero
VBR, 2Pass, Target 6Mbps, Max 8 Mbps.
Square Pixels, Baseline Profile, Level 3.2
Everytime I render my movie, the resuting mp4 shows a bit rate of about 3200 kbps. Even if I select CBR / 6 Mbps I still get roughly 3000 kbps out.
Why so low? Wouldn't you expect something between the Target and the Max bitrate? My "movie" content is a video of someone drawing a picture, so a lot of frames are blank white-paper until the drawing progresses - so I guess there is a lot of potential for compression, but still seems really low.
Should I be using 1080 for my sequences and exporting at 720 instead? The source files at 1080p are showing a bitrate of approximatley 31,470 kbps.
Thanks for any pointers!
Just for grins, I set up 1080i HDV in a 720p sequence. I trimmed the clip to 10 seconds and exported six times.
Each time I used VBR 2 pass, 6 to 8Mbps. The first three I used level 3.2 and the second three I used 4.2
For each level I used baseline, main and high. I chose a clip without much movement. I was just getting the camera set up, but it seemed like using a low motion video was appropriate for this example.
The results surprised me a bit. The file sizes ranged from 7587K (four of the files) and there was one at 7569K and one at 7571K.
According to MediaInfo, the bitrate remained pretty constant at 6007Kbps (four of them) and 5993K, 5992K. The low end of the scale, but not surprising for a low motion video on a tripod.
Unfortunately, my eyes are not good enough to tell the difference in the quality of the clips either. I would have thought it would be more obvious. I guess I need to research profiles and levels. I can tell you that when I tried to move up to 4.2 the software decided I needed to be using 1080p so I had to manually change the frame size again.
Just to make sure that the blank white frames were not the issue, I added 10 seconds of a white color matte. This time the High, 4.2 came out to almost exactly twice the file size and MediaInfo says it is 6022Kbps. Why it is a higher bit rate is a mystery to me, but one I will have to figure out at a later date unless we can get someone like Kevin to pitch in and straighten me out.
So, Brett, there is no reason that I can see that your bitrate should be that low. Which leads me to ask how you are measuring it? Can you post a screen grab of the export settings?
The bitrate is stamped in the EXIF info of the mpg file, I assume. I'm running on Windows 7, and just look at the properties of the file in the details tab. I was first alerted to it, as when I upload to Vimeo I get a warning from Vimeo that my bitrate is < 5 Mbps and may be poor quality for HD. Vimeo shows the same bit rate as is shown by windows explorer file-properties.
OK, stepping back a bit, I would ignore the intended Export, and use that footage in a Sequence, that matches the footage 100%. That is the best start. Then, when done, choose the intended Export attributes.
That does not directly address the question (Steven is doing a good job of that), but will yield a better workflow for you.
The bitrate is stamped in the EXIF info of the mpg file, I assume.
That is not the most accurate way to see a file's bitrate.
Open it up in VLC. In the menu, go to Tools>Codec Information. Switch to the Statistics tab and you'll see Content bitrate. This will show you realtime what the bitrate actually is at any moment of the video.
Thanks Jim - great tip.
Interesting to see it fluctuate so much. It's weird, as according to VLC the content bitrate jumps pretty high in some scenes - over 12 Mbps (despite my 8Mbps encoding-max), but for a lot of scenes - especially scenes where there is almost the same still for many seconds with nothing changing but audio - it gets REALLY low - 900 kbps. In the scenes where I have a more normal video of a the model talking - it shows around 8500kbps, which is closer to what I'd hope.
So, if I try and conclude from this - it sounds like my content is very compressable and despite the target 6Mbps - the very static scenes bring the average bitrate over the whole movie down dramatically.
I'm guessing the EXIF stamp is an average over the whole movie, and Vimeo is looking at this on import, which is why it's not that accurate to look at in isolation.
So ignore the Vimeo warning and carry on?
I'll also move to Bill's suggestion of keeping my sequences in 1080 from now on, seems smarter.
Thanks for the input everyone!
If one is using multi-pass Encoding, then fluctuations in Bit-Rate should occur. Sections with more motion, and especially off-axis motion, should have a higher Bit-Rate than more static sections. That is what multi-pass is designed to do, and one reason that Hollywood uses Encoders, that might sample 20 passes. They want both the highest quality, but with the lowest disc overhead. To get that, Hollywood spends $ 100,000's, and hire people, who are experts, plus do nothing all day, BUT Encode to disc.
One-pass does just that - uses what you set, and sticks to it. Multi-pass samples the full Video, and does its best to get high motion material at a higher Bit-Rate, and static at lower Bit-Rate.
OK, I understand what everyone is saying, however, I still have a question.
Why does his say 3822 and mine says 6203? I get a little difference, but not that much.
Also, while I agree that editing in 1080 makes more sense most of the time, I often edit in 720 if the scene was far away and I need some digital zoom, so to speak, without losing quality. Heck, I have edited in 640X480 to zoom way in on something very far away, rather than actually use Motion to make it bigger.
Anyway, I suggest that posting a screen shot of the export settings would be the next best step.
That is what multi-pass is designed to do
One-pass does just that - uses what you set, and sticks to it.
That's inaccurate. One or two passes doesn't define whether or not the bitrates fluctuate. With anything but CBR, the bitrates fluctuate based on the content. The point and purpose of two passes is to simply to do the job better. One pass to analyze and one to encode, instead of all at once.
It's like shooting a play you've never seen (one pass) or seeing it first and then shooting it (two pass). With the latter scenario, you have a much better idea of what's coming up and where you should be pointing the camera. With two pass VBR, the encoder has a better idea of what's coming up and where best to use the available bits.
Having said that, in my experience there is little or no visible difference between one and two passes. More often than not, all you get with two pass is a longer coffee break.
Here's a screencap of my settings. It's pretty close to the Vimeo HD preset. However, with the Vimeo HD preset, I find I get Lip-sync issues. A help article on Vimeo suggested using 44.1 kHz for audio vs. the default 48 kHz in the preset. The only other difference, is for some reason my 'Set keyframe distance isn't checked and the Vimeo HD preset setting is'.
I'm pretty convinced now that it is the unique static nature of my content over time that's driving the av. bitrate down. Looking at some of my other videos that have more normal content I'm getting av-bitrate numbers that feel much more reasonable.
There are some things in your export settings that are not in mine.
First of all, I don't use the MainConceot H.264 codec, so maybe that is why your settings indicate NTSC where mine do not. In fact, using a codec that did not come with Premiere Pro eliminates a lot of comparisons. Perhaps someone with MainConcept can assist you further.
By the way, if you change to the YouTube settings, the bit rate goes up to 8Mb/s, but you can lower or raise it if you wish.
Jim Simon wrote:
That's not normal. Can you post a link to an example, or maybe upload a clip?
It's not a Premier issue. The video comes out and plays perfectly locally in VLC/Media Player etc. But when transcoded by Vimeo the audio gets several frames out of sync - a help artcile on Vimeo's site suggested switching to 44.1, which seemed to fix the issue. Topic for another thread