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Question about MP3s, compression rations, kbs, etc.

Old 05-08-02, 02:58 PM
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Question about MP3s, compression rations, kbs, etc.

How do I tell the amount of compression on an MP3 on a website?

For example, one song I was looking at and downloaded is 14:13 in length (853 seconds), and is 8.3MB in size which works out to 9.7 kb/sec (compressed). When played, realplayer tells me that it is playing at 128 kb/sec and the sound is OK. Not great, but not bad, though part of it may be due to the fact that it was a live recording. Is there any correlation between the 9.7 kb/sec and the 128 kb/sec?

A typical blank CD-R has 700MB storage capacity, which is 80 minutes worth of standard, uncompressed music. 700 mb = 700,000 kb, 80 minutes = 4800 seconds. Thus, is if fair to say that a standard CD-R plays at 145.8 kb / sec?

Does realplayer play everything at 128 kbs?

Thanks!
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Old 05-08-02, 04:33 PM
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Well, the encoded rate is directly proportional to the size of a file. So, if you have a 1 second long song at 128 kbps, then the file size should be around 128. (You also have to accomidate for file name and ID3 tags, which can add a couple KB to the file)

BUT, the encoded rate of an MP3 (kbps) is not the same as kb/s. KB/s implies kilobytes per second. kbps implies kiloBITS per second. Same applies to mbps and MB/s. SO, to get KiloBYTES per second, simply divide 128/8. OR, to get kiloBITS per second, multiply 9.7 by 8. (This is why modem speeds are misleading. 56 kbps is not 56 kilobytes/second. It is 56 kiloBITS/second. You must divide by 8 to get KB/s. A 56k modem should download around 6-6.5 KB/s, as they are only allowed to operate at 53 kb/s according to FCC regs. And you also have to allow for packet overhead.)

Looks to me like your song is encoded at around 80 kbps...

Also, MP3s can be encoded at continuious bit rate (CBR) or variable bit rate (VBR)

CBR uses a constant bit rate throughout the song, while VBR varies the bit rate. With VBR, the encoding software decides what the optimum bit rate is for any given instant and encodes it at that. This could lead to inconstincies with file size and bit rate. Some programs only display one bit rate for VBR files. Other display the bit rate in real time. WinAMP is one program that displays the bit rate in real time.

I hope that answered your question.
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Old 05-08-02, 06:21 PM
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To add to what shaun3000 posted ...

80 min, 700 MB CD
700 MB is approximately 700,000 KB
80 minutes equals 4800 seconds
Result is around 145.8 KB / s which is approximately 1200 kbps.
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