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Bitrate and Resolution Guide

2000-6-5


First: THERE'S NO IDEAL BITRATE AND OR RESOLUTION!!!!! These settings depend on what you're driving for - high resolution - good image quality, size, etc. 

You can use the bitrate calculator that's on my page in the software section to give you a rough idea about how to set the bitrate. Keep in mind that DivX is kind of VBR (variable bitrate) so if you set the exact value you calculated don't be surprised when the actual size of the movie is quite different. Especially FlaskMpeg has the nasty habit of creating larger files then expected. Therefore ALWAYS set a lower bitrate than calculated. Between 50 and 100kbit/s should do the job. Mpeg2avi is much better at filesizes, the final files are usually as large as they're supposed to be. 

You should also consider that the final size depends on the output resolution you chose, despite all logics. Somehow DivX makes better use of the bitrate at a lower resolution. Below you can see the bitrate calculator

BitRateCalculatorSince I'm doing almost only full-res movies I chose a really high bitrate, around 2000kbit/s usually. I noticed that a bitrate below 1700kbit will get you a lot of blockyness in fast moving scenes. The situation is quite different at lower resolutions. Since you might want to put a movie on 1 or 2 CDs you should consider lowering the resolution.

Another issue to keep in mind are the black bars above and below the movie - this applies only to widescreen movies, of course. In most cases it's smart to get rid of them since they only take up a lot of space.



You can try to select one of the profiles included in the mpeg2avi GUI and see what you get. I also suggest you use the preview my Avi button in the mepg2avi GUI, you'll be able to see what the current settings will do to your video.

Here's some settings you might use in mpeg2avi - just enter them in the GUI at the corresponding places - Output cropping X*Y and Downsizer Y:

1:2.35 movies: 720x304, 384 / 640x272, 352 / 560x240, 304 / 480x208, 256 / 400x176, 224 / 320x144, 176

1:1.85 movies: 720x384, 384 / 640x352, 352 / 560x304, 304 / 480x256, 256 / 400x224, 224 / 320x176, 176

Those of you who are a adept in math will see that I simply used a calculator, tried to have the same aspect ratios while respecting the rule that width and height have to be a multiple of 16, and calculated the values. You can take any values lower than these above or in between if you respect the multiple of 16 rule. 

If you want to make a 1CD movie, you should select a resolution as low as 320x1XY, and set the audio bitrate to 96kbit/s if using mp3 or 64kbit/s when using DivX - enter those values along with the length of the movie in the calculator and you'll get the bitrate. Keep in mind that this resolution is rather small - you can get way better quality if you reduce the resolution just slightly - say 640xYYY - and go for 2CDs. Dig this example: 8mm NTSC at 640x288, 23.976fps in mpeg2avi resulted in a 960mb file. I can add the full ac3 soundtrack to this and put it on 2 files and when I reduce my resolution to 640x480 when playing back it's fullscreen without stretching and the sound is awesome.. :)

Resolution in FlaskMpeg

For FlaskMpeg there's an easier way since it has the Output pad - select Options, then Global Project Options, then click on the Output pad button in the lower left corner. Click Reset settings. Here's a sample of what you'll get:

Now it's time to decide how we're going to reduce the image size. There's 2 ways: Just reduce the overall size, or reduce the size and crop the black borders away. Generally cropping is a good idea since we're going to watch the movie on a computer so we don't have to worry about having the right image size for a TV - most TV output cards can add the black bars all DivX players will add if you play them at full screen. 

If you want to keep the black bars for some reason - keep in mind that it's a waste of space, they take quite a lot of the bitrate for nothing - then all you have to do is to reduce the output size using the Width and Height buttons in the Output size section in the Output Pad. FlaskMpeg will automatically respect the resolutions needed for the DivX codec - width and height must be a multiple of 16. To keep the aspect ratio simply reduce the horizontal resolution by the same amount as the vertical resolution. For instance press both the Width and Height button 5 times to reduce the resolution from 720x480 to 640x400. 

Another issue to consider is this: Both DVD players and DVD Software players display NTSC and PAL movies at the same vertical size - although the vertical resolution of NTSC is 480 while for PAL it's 576. If you want to keep this behavior, increase the Height to 576 using the buttons in the Output size section before you reduce the size. 

Cropping

You don't need to change the starting resolution to 576 if you're cropping... Reduce the Output size to the desired format using the Width and Height buttons in the Output size section. Keep in mind that we're not going to change the "real" aspect ratio - the part of the picture containing the actual movie - right now. You're screen will look like this after this operation:

Note that the picture looks kind of messed up. Clearly we have to stretch the picture again to look normal. So next we're going to click on Crop. From now on, everything that we'll do is going on within the selected boundaries for the screen size. Therefore DON'T touch the Height and Width buttons in the crop section. If you press the Width and Height buttons in the Output size section, you're going to stretch the movie - don't worry that's how it has to be done. Increase the Top offset and the Height in the Output size section till the movie fits the selected screen-size. In the end you'll get something like this:

So.. you don't believe me that you have to stretch the picture? Then check out this page - beware, the pictures are huge -  it contains a screenshot taken from the FlaskMpeg player, after having reset all setting in the output pad, and the same scene taken from PowerDVD. You can clearly see that the picture in Flask is squeezed when you don't change any of the settings.

If you're not cropping, you can just change the output size in the FlaskMpeg output options. Keep in mind that in case of an NTSC movie you still have to stretch the movie.

 

 

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