CD bit rate question? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 5 Old 09-12-2000, 09:44 PM - Thread Starter
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While sitting in Archaeology class today my mind began to wander...

I started to calculate various data rates of media (exciting, I know, *grin*) and entered the following quandary...

(Please check my math)

CD audio
16-bit, 44.1Khz

16 X 44.1 X 2 (two channels) = 1411.2 kbps

Traditional CD-Rs advertise 650MB and 74 mins of recording capability, however….

1411.2 X 74 (mins) X 60 (seconds) = 6,265,728 kb / 8 (bits in a byte) / 1024 (KB in a megabyte) = 764.86 MB needed for 74 mins of audio

Conversely, 650 MB equates to 62.88 mins of CD-quality audio

(650 X 1024 X 8) / 1411.2 = 3773.2426 / 60 = 62.88

What is the deal?
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post #2 of 5 Old 09-13-2000, 07:10 AM
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I do not have a complete answer for you and will probably make your calculations worse In addition to your calculations you have to allow for the 2 parity bits introducted by the EMF (eight-fourteen modulation)which breaks the bitstream up into an 8 bit segment + a 14 bit segment + 2 parity bits. Further algorithms are applied to this to scramble the stream to help with error correction.
I suspect that the 650 meg number is the effective amount of storage you can anticipate after the addition of the error correcting steps taking in encoding the data.

Also, you showed the results of your multipications as 6,265,728kb, I think you should have determined kb by dividing 6,265,728,000 by 1024.


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post #3 of 5 Old 09-14-2000, 09:34 PM
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I think your math is basically right (including your bits --> MB conversion), the confustion is that 650MB is a limit in the file system for CD-ROM. Once you've added the overhead of a PC filing system for storing data files instead of a music bitstream with track markings, you lose a lot of space. Take for instance a hard drive, there are actually 3 (or more) different numbers a marketer could tell you about the size:
1. the unformatted capacity in millonbytes (bytes x 1,000,000) eg 700MB
2. the unformatted capacity in Megabytes (bytes x 1,048,576) eg 667MB
2. the formatted capacity in Megabytes (bytes x 1,048,576 - FAT overhead) eg 610MB
That's a big difference in perceived size caused by just naming differences (and which scheme do you think the advertisers use?).

I don't know this is the full cause of the discrepancy between 650MB and your 764MB figure, but I do know that the formatting causes SOME overhead differences between music and data.


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post #4 of 5 Old 09-16-2000, 08:54 PM
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Data and audio are stored differently on a CD. Have you ever tried ripping a CD to your hard drive,if you do a 74 minute Cd will take about 780 MB+/-.
Audio uses 2350 bytes/sector instead of 2048 because it uses the space that the data uses for error correction and some other things I can't remember right now. Plus most CDs give a conservative number on the packaging and can be "overburned",some by up to 8 minutes.
332800 sectors*2048bytes/sector=650MB for data
332800 sectors*2350bytes/sector=745.85MB for audio


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post #5 of 5 Old 09-18-2000, 06:46 AM
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The actual 'bit rate' required on any digitally recorde media, to equal that of the human ear's funtion is a MIMIMUM of 225k samples a second, at a bit depth of over 20. NO EXCEPTIONS. I proffered this information about 6 years ago on the old (almost pre-net) Highend audio forum in the news groups, and nearly had my head bit off for stating such foul blasphemy.

Now it is the accepted standard.

Go figure.

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