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The QTKit/QuickTime Player mystery

Subject: The QTKit/QuickTime Player mystery
From: Jean-Olivier Lanctôt <email@hidden>
Date: Thu, 30 Jun 2005 15:59:16 -0400
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Yo, list,

I'm working on getting the bitrate of a simple CBR (constant bit rate) MP3.

I figured I needed the size in bytes (to be xformed in kilobits), and the duration in seconds

I'm always using QuickTime Player.app's Movie Info window as a reference:

It says: ------ Filename.mp3 DataSize: 3.48 MB = 28508 kbits (http://www.google.com/search? hl=en&lr=&c2coff=1&biw=1024&q=3.48+megabytes+in+kilobits&btnG=Search) BitRate: 160.2 Kbits/sec Duration: 00:03:02.23 ------

Now, logically

Bitrate should = total number of kilobits / duration in second
= 28508 bits / ((3 * 60) + 2.23)
= 156.638242 kbps.

Bah, it's off less than 4 kbits. not so bad. But still, there's a first mystery going on here. I suspect 3.48 isn't the right size? Whatever.

So I try to do it in QTKit.

QTStringFromTime([myMovie duration]) gives me "0:00:03:17.322/600", so now there's a time diff of approx. 15 seconds between QTKit and Qt Player.app the size property gives me exactly 3.48 MB, though.

Without the proper time, i cannot calc. the bit rate...

Please, someone explain what's going on here..

Thanks,

-- Olivier

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