Linear Pcm Recording Time

[bptr]

Why is it that a 700 meg CD can record 80 minutes but a 1 gig minidisc can only hold 94 minutes using linear PCM recording? If it is proportional, it should be 114 minutes (8.75 meg per minute).

[dex Otaku]

It's a rounding error.

There are two standards for showing kB/MB/GB and the like.

The 'correct' one of computer capacity terms uses powers of 2, so 1B = 1024 bytes. The other, less honest standard uses 1000 bytes as 1kB.

As a result 1MB does not = 1MB.

[This is much the same as why hard disc manufacturer's reported hd sizes are usually totally incorrect - the actual capacity is always lower than what the mfg specifies simply because they're rounding from powers of 10 rather than 2.]

The short version of all this is that a blank 1GB HiMD has less than an actual 1GB free on it; it's pretty close to 1,000,000,000 bytes, whereas 1GB = 1,073,741,824 bytes.

There are other thins to take into account as well, such as the DRM table, track info database, and the like.

[Peel]

Hm, guess my original reply got eaten by the forum crash.

Anyway, the rounding/ambiguity issue accounts for a portion of the difference -- about 50MB. But the bulk of the difference is because on CD, audio is stored in a different format than data -- that CD can hold 80 minutes of CD-audio *OR* 700MB of data. If you rip an 80-minute CD into .wav files, you'll see that it takes up more than 800MB of space on your HD (or MD). The reason is that data files need additional error-checking information which takes up some space. Given that 1GB = 1 trillion bytes (not technically correct but standard practice), 94 minutes worth of 44/16 stereo PCM in 1GB is actually quite efficient (95 minutes wouldn't fit even if there were no file system overhead).