A Little Confusion On Bitrate Render Settings

BobWard wrote on 6/26/2012, 8:01 PM

I have done searches on the web on this topic and read several articles but I am still a little uncertain/confused about the impact that bitrate settings have in the render process.

I just rendered a 95-minute movie to MPEG-2 and AC3. I used the Variable Bitrate, 2-Pass option in VMS10 HD Platinum with the following settings:

Max: 7,656,000
Avg: 6,120,000
Min: 3,672,000

I got these settings from one of the online bitrate calculators. I really do not have a good grasp of what these settings mean and how they impact the quality of the rendered movie. The rendered movie was successfully burned to a DVD using DVDAS 5.0.

I have read where some people say to never set the Max value higher than 8,000,000 and the Min value lower than 1,500,000.

Would someone have a relativley short explanation of what these bitrate settings do and how they impact the movie qualiity? Do the settings I used for the referenced movie look reasonable?

Thanks for your time.

Comments

Chienworks wrote on 6/26/2012, 9:15 PM

Depending on the material those settings are quite reasonable. If you have a lot of action or contrast then you might want higher max and lower min.

The average specifies the finished size of the whole rendered file. It will use 6,120,000 bits per second, or about 43.77MBytes per minute. This rate has to be low enough to allow the entire video to fit on the disc. However, sometimes this rate is too low for the high action/high contrast scenes. These need a higher bitrate in order to get a quality encode. But, you can't raise the bitrate of the entire disc or the video won't fit anymore. Thus the concept of a variable bit rate was created. The idea is that in addition to the scenes that need more bits, there are also scenes that can be encoded acceptably with much fewer bits. Scenes with very little motion don't need as much data to reproduce them accurately enough.

So, the Max and Min specify the allowable variation. When the encoder can during slow scenes it will use fewer bits and "save up" the surplus to spend them on higher action scenes. That way the bits are used where they're most needed without making the overall file bigger.

Thing is, with the settings you have there, there isn't much difference between the average and the max, so the max scenes won't be improved as much as they might be. This is the reason to use a higher max. In fact, i don't see any reason to ever use a max lower than the maximum possible that allows good playback. If you go too high then some DVD players can't read the data fast enough for smooth, full frame rate playback. I find 8,500,000 to always work well so i have this fixed as the max i use for every encode. By the same token a minimal minimum allows the encoder to use those extra bits for the max. I find 1,500,000 to be an acceptable minimum so i use that on every encode. Note that the encoder will rarely ever go all the way to the max or min, i just like to have the range available so it can approach those values when it needs to.

The only thing i calculate is the average so that the file fits on the disc.

BobWard wrote on 6/26/2012, 10:03 PM

Thanks for a very good explanation, Chienworks.

So I am assuming that the "bits" are the chunks of rendered computer code that are being used to construct the mpeg-2 video image and the AC3 sound track?

Action scenes incorporate quicker changes to the image, so the bitrate (data transfer) has to speed up to keep up with the rapidly changing image, otherwise the image quality suffers?

Chienworks wrote on 6/27/2012, 6:45 AM

Precisely exactly.

musicvid10 wrote on 6/27/2012, 10:09 AM

You've confined your headroom and footroom a bit with those settings, reducing the efficiency of the VBR method.

The 8,000,000 headroom recommendation was for very old players, which would stutter. The 1,500,000 minimum is the lowest I would ever consider.

9,500,000 peak headroom (rarely achieved)
6,000,000 good for about 1 hr. 40 min.
2,000,000 eliminates almost all blocking in fades

Jack S wrote on 6/27/2012, 2:14 PM

Hi Guys. On the same subject, I wonder if I could pass this by you.
I render at a maximum of 9,100,000, purely because my Sony camcorder produces video at this maximum bit rate (as reported by MediaInfo). Therefore I get no recompression. The downside of this is that some of my projects are too big to fit on a standard DVD. I then have to use DVDShrink to bring the size down. Although DVDShrink does a wonderful job with no noticable degradation, would I get the same level of quality lowering the bitrate in VMS?
I would value your expert opinions.

musicvid10 wrote on 6/27/2012, 3:11 PM

There have been anecdotal reports on the forums that DVDShrink does a bit better than Vegas on the downsample.

I have not seen any actual examples to back this up, so it may be better, or it may be a pygmalion effect.

Chienworks wrote on 6/27/2012, 3:21 PM

Are you sure Vegas is not recompressing? Does the preview screen go black during the render with "no recompress" displayed? If you see the frames go by while it's rendering then it is recompressing. In that case, there's nothing "magical" about using the same output bitrate as the input, and you might as well use a lower bitrate when rendering to avoid recompressing twice.

Jack S wrote on 6/27/2012, 4:37 PM

Yes, the preview shows 'No Compression' except for the transitions and titles. The beauty of using DVDShrink is that I don't have to worry about working out a suitable bit rate for a particular project. If DVDAS tells me the project is too big to fit on a single layer DVD, I just ignore it and proceed anyway. I then put it through DVDShrink which shrinks it down exactly to fit. Thank you for your comments. I will probably carry on with this workflow.