Sunday 8 April 2007

What is the x in 8x, 24x, 56x in CD and DVD drives

CD-R, DVD-R, CD-RW, DVD-RW and such discs and drives that read and write them, mention speeds as 2x, 8x, 24x, etc. What is this x in normal data transfer terms of MB/sec? I did some research and this is what it means.

In CD drives and discs, 1x is 150 KB/sec (KiloBytes per sec). This the speed that floppy drives wrote and read at. So CD drive makers expressed their speeds as multiples of this to help people understand how fast it worked with respect to floppies.

So for CD-ROM, CD-R, CD-RW drives and discs...
1x = 150 KB/sec
4x = 600 KB/sec
16x = 2.4 MB/sec
24x = 3.6 MB/sec
32x = 4.8 MB/sec
48x = 7.2 MB/sec

Its different for DVD-ROM, DVD-R, DVD+R, DVD-RW and DVD+R DL
1x = 1.375 MB/sec
2x = 2.75 MB/sec
4x = 5.5 MB/sec
8x = 11 MB/sec
16x = 22 MB/sec

And the new Blu-ray disks have a different measure too.
1x = 4.5 MB/sec
2x = 9 MB/sec
4x = 18 MB/sec

As a comparison, 2x of Blu-ray is equal to 6.5x of DVD is equal to 60x of CD.

For digital video working, the speed and capacities of these drives can be expressed like this.

CD-R and RW discs can record about 4 mins. of DV quality video in about 2 mins at 48x
DVD-R and RW discs can record about 21 mins of DV quality video in about 4 mins at 16x
or about 4 mins of 8-bit uncompressed video in about 4 mins
DVD-R DL discs can record about 40 mins of DV quality video in about 50 mins at 2x
or about 8 mins of 8-bit uncompressed video in about 50 mins
BD-R and RE discs can record about 2 hrs of DV quality video in 46 mins at 2x
or 25 mins of 8-bit uncompressed video in 46 mins
Dual layer BD-R and RE discs can record about 4 hrs of DV quality video in about 92 mins at 2x
or 50 mins of 8-bit uncompressed video in about 92 mins

So Blu-ray discs have the potential of replacing DV tapes completely. They are already available as video recording media in XDCAM systems but that's not DV, its MPEG compressed.
And DVD-R DL discs are useless for any kind of video media backup because they are so slow. You can write 8 DVD-R discs in the same time that you can write just one DVD+R DL.
And when Blu-ray gets to 8x it will be fast enough to write uncompressed 10-bit SD video in faster than real time, as well as write compressed HD at many times faster than real time.

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