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June 2017 Giveaway

Why do CDR discs hold 74 minutes of audio, but only 650 MB of data?

It must be that New Math.

“I’ve noticed what I think is a discrepancy regarding the listed capacities of CDR discs. Many of them say something like 650 MB, while they list the recording time at 74 minutes. Regardless of the specific numbers none of them add up. I thought a stereo 16-bit recording at a 44.1kHz sampling rate (CD Audio) required approximately 10 MB of space per minute. If that were true then a 74 minute CD would have about 740 MB of data. Why then, can they only hold 650 MB of data when using software like Toast to make a CD ROM?”

First of all, (and for the clarity of our readers) the numbers you lay out above are correct. CD audio does take about 10 MB per minute (5 MB per track). So it does appear that the numbers don’t add up. The explanation lies in the amount of and type of error correction data.

Keep in mind that when we talk about disc errors we are talking about the interaction of the disc with the mechanism that it is being played on. The performance or error rate of a CD can only be measured by reading the disc; therefore, the performance of a particular disc is a combination of the disc and the mechanism used to read it. Disc errors are not absolute; they are a reflection of how well the disc works with the drive it is being accessed on.

An audio CD does not need as much error correction as a data CD-ROM because an incorrect bit in an audio stream will not be heard, while an incorrect bit in a computer program could crash the program (or the computer). When an audio CD player encounters an error that it cannot correct using the CIRC, it just skips 1/75th of a second, and you are unlikely to hear any difference. Many errors, however, will make the audio sound bad. These errors can be caused by scratches or dirt on the disc, or can be a result of a poorly manufactured or recorded disc.

A CD-ROM disc uses additional error detection and correction code on top of CIRC. This code is called Error Detection Code/Error Correction Code (EDC/ECC). When a CD-ROM drive has trouble reading a sector correctly (and it knows this because the EDC tells it so), the ECC can usually correct that error by calculating the correct value of what is being read. It does this by using other data on the disc that exists for the specific purpose of correcting errors. This data takes up room on the disc, and CDR media space is rated with this in mind. That is why it is common to see them rated in minutes and Megabytes. Depending upon which type of data you are burning to the disc the functional capacity of the disc is different.

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