Marsman2k
8th July 2004, 16:31
I see a lot of people struggling with getting the desired output rate. One thing that I don't think many people have realized yet is that the idea of adding the video rate plus the audio rate equals the output rate is only valid at 30 frames per second!
Example: for a 128k output: Video: 104 / Audo: 24 = 128k <- but ONLY at 30 fps!
For other fps rates, if the desired output rate is to be maintained, the video data rate has to be increased. Lets use an example of 15 fps. Here we have exacly half the number of frames per second as with 30 fps. Since only half the frames are rendered, the output total for video is only 52k - even though we specifed 104k. This is because the video rate is calibrated for 30 fps. The picture quality will remain the same as the process always gives each frame 1/30th of the video rate, but since there are only 15 frames, the total output is one half.
In order for those 15 frames to total 104k, they need to be encoded twice as big as with 30 fps. By doubling the video rate to 208k - with each one of the 15 frame's receiving 1/30th of the data rate, the result is 104k - exactly what we wanted. Note that the picture quality is increased as each frame is given the same encoding as it would for a 30 fps @ 208k video. So each frame looks like a 208k encode, but since there are only 15 frames, the output rate is just 104k instead of 208k if there were 30 frames.
Therefore, for any encoding OTHER than 30 fps, the video + audio = output is not valid and must be adjusted. Here is a simple formula you can use to achieve the desired output:
(30/fps) x (video rate) + (audio rate) = actual output rate
Lets use our 15 fps in the formula:
(30/15) x (104) + (24)
(2) x (104) + (24)
(208) + (24)
Result: 128k @ 15 fps
So base your initial video rate on 30 fps - then use the formula to adjust the video rate to compensate for any drop in fps. Remember also that lower fps's using this method yield better video qualities as more video processing is given to each frame. I hope this help sort out some of the confusion regarding output rates at fps's other than 30.
Example: for a 128k output: Video: 104 / Audo: 24 = 128k <- but ONLY at 30 fps!
For other fps rates, if the desired output rate is to be maintained, the video data rate has to be increased. Lets use an example of 15 fps. Here we have exacly half the number of frames per second as with 30 fps. Since only half the frames are rendered, the output total for video is only 52k - even though we specifed 104k. This is because the video rate is calibrated for 30 fps. The picture quality will remain the same as the process always gives each frame 1/30th of the video rate, but since there are only 15 frames, the total output is one half.
In order for those 15 frames to total 104k, they need to be encoded twice as big as with 30 fps. By doubling the video rate to 208k - with each one of the 15 frame's receiving 1/30th of the data rate, the result is 104k - exactly what we wanted. Note that the picture quality is increased as each frame is given the same encoding as it would for a 30 fps @ 208k video. So each frame looks like a 208k encode, but since there are only 15 frames, the output rate is just 104k instead of 208k if there were 30 frames.
Therefore, for any encoding OTHER than 30 fps, the video + audio = output is not valid and must be adjusted. Here is a simple formula you can use to achieve the desired output:
(30/fps) x (video rate) + (audio rate) = actual output rate
Lets use our 15 fps in the formula:
(30/15) x (104) + (24)
(2) x (104) + (24)
(208) + (24)
Result: 128k @ 15 fps
So base your initial video rate on 30 fps - then use the formula to adjust the video rate to compensate for any drop in fps. Remember also that lower fps's using this method yield better video qualities as more video processing is given to each frame. I hope this help sort out some of the confusion regarding output rates at fps's other than 30.