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Old March 28th, 2008, 11:49 AM   #1
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What kbps and mbps?!

I've been exporting mp4's of an HDV video in quicktime. I have to type in kbps and I don't know what this is. In the final video I can get info in the form of mbps. Is this the same thing? If so how many kbps are in a mpbs? I tried 20000 and 30000, but then the final mpbs are inconsistent and seem to depend on the clips length.
When I select multipass encoding I have a choice of main and baseline. I don't know what that is either, but is it a factor in these seemingly inconsistent mbps?
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Old March 28th, 2008, 02:04 PM   #2
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There should just be a constant factor between "kbps" (kilo-bits-per-second) and "mbps" (mega-bits-per-second): either 1,000 or 1,024 - which one is a bit of a religious question; computer engineers like the power of two (1,024), whereas others seem to be more comfortable with one thousand even.

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Old March 28th, 2008, 03:59 PM   #3
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kbps and mbps are terms for the amount of data per second in the final video: bits per second, also commonly known as bit rate. m is mega, and k is kilo, so mbps is roughly 1000 kbps - depending on religion.

The higher the number, the lower the video compression, the better the quality and the bigger the file size. If your original was DV then the video is 25mbps uncompressed. If your original was HDV then it is 25mbps in mpeg-2. For the sound you can also toggle bit rate, same thing, the more bps the better the quality. There comes a point where higher bit rate will not provide better quality because the source does not contain more information.

When you export video you can often choose between fixed bit rate (that is fixed number of bits per second) or average bit rate. The average bit rate allows you to encode simple pieces using less data and use more for complex pieces, providing an overall better quality with the same amount of data. You can also export to make sure the final file does not occupy more than a certain amount. This is great if you need to burn a dvd.

The 2-pass compression may provide a better result. In the first pass the video is analyzed to determine the optimal compression. The reason for two pass AFAIK is that mpeg compression use interframe compression, that is a sequence of frames is compressed together, so the compression needs to analyze across multiple frames.

Of course, the final file size depends on the bps you set and the length of the clip: size (bytes) = (video bps + audio bps)*length in sec/8.

Cheers, Erik
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Old March 28th, 2008, 04:39 PM   #4
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Erik,

Good summary, except:
Quote:
Originally Posted by Erik Norgaard View Post
The 2-pass compression may provide a better result. In the first pass the video is analyzed to determine the optimal compression. The reason for two pass AFAIK is that mpeg compression use interframe compression, that is a sequence of frames is compressed together, so the compression needs to analyze across multiple frames.
Not quite. Two pass compression has nothing to do with interframe compression; rather, it is what enables what you described in your previous paragraph: "to encode simple pieces using less data and use more for complex pieces, providing an overall better quality with the same amount of data". In the first pass, the encoder analyzes the footage and keeps track of where the simple pieces and the complex pieces are; in the second pass, it does the actual encoding of all pieces using the respective bitrates computed in the first pass.

If you do a single-pass encode, the bitrate will be constant - average bitrate will be used throughout the whole video.

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