digitaldecay
01-27-2007, 03:35 AM
I am trying to figure out what exactly the keyframe interval does, and if there is a way to convert a file while keeping its original bitrate.
XerXes
01-27-2007, 07:49 AM
The keyframe interval is the number of frames that pass before a new keyframe is created. It is calculated by multiplying the frame rate by the time interval (in seconds). Keyframes are directly related to the frame rate. The higher the frame rate, the more uncompressed keyframes are required. Keyframes are calculated on a per-second basis. Adjusting the keyframe interval does not affect the data rate value directly but does affect the physical size (on your hard drive) of the compressed video file. Alternatively, the data rate value does affect the image quality of the keyframes: The more keyframes you have, the more bits per second are consumed.
Formula:
frame rate (fps) x interval (sec.) = keyframe interval
Example:
15 fps x 4 sec. = 60
This places a keyframe every 60 frames starting at frame 1.
The greater the interval between keyframes, the harder it is for the codec to rebuild the missing frames. For example, a high-motion video clip with a large keyframe interval results in the perception of poorer quality. If your keyframe interval is too great, you run the risk of not having enough data rate to compress and transmit the data bits.
In other words, 250 is a good enoph KFI to not effect quality.
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In answer to your second question you have to manualy type the bit rate you want into the dropdown menu. As far as I know there is no way to automaticaly keep the original bitrate.