On 22/07/2010, at 1:22 PM, James Shatto wrote:
> CD quality is basically 1GB per hour with a surplus. CDRs being 700MB an=
d 80 minutes (or was it 74?). And is that actual computer MB or marketing =
MB? In rough math, and extra hour (disc) for every three hours. So 21 hou=
rs minimum for a 16GB card (16 + 5). Assuming that the card was blank when=
you started. And that the filesystem uses space semi-efficiently. Not th=
at you normally want to record files of that girth, but if you're curious i=
f you snore or not? In less rough math, (((16000 / 700) * 80 ) / 60 ) =3D=
=3D's 30.4762 -ish hours. Or 28.2-ish hours at 74 minutes. Since meta dat=
a and filesystem occupy some of that space, a safer bet IMO.
Card manufacturers typically use 1MB =3D 1 million bits, whereas file size =
calculations are based on the correct 1MB =3D 1.024 million bits. The adver=
tised card size is also based on unformatted capacity, and formatting the c=
ard with a file system will reduce available capacity. An example is a 16G=
b card might have a "real world" capacity of 14.9Gb when formatted and capa=
city read using the correct 1MB=3D1.024million bits.
For a "16Gb" card (in this case a SanDisk Extreme) - with an actual capaci=
ty 14.9Gb - the maximum recording time ignoring file headers and metadata =
is:
time in minutes =3D data size in Gb * 1024 * 1024 * 1024 / (sample rate in=
hz * number of channels * (bits per sample/8) * 60)
time in minutes =3D 14.9 * 1024 * 1024 * 1024/ (44100 * 2 * 2 * 60)
NB: "1024 * 1024 * 1024" converts from Gb -> Mb -> Kb -> bytes, if you hav=
e capacity in Mb only use 1024 * 1024.
time in minutes =3D 15998753177.6 / 10584000
time in minutes =3D 1511 or 25 hours 11 minutes
After you make allowance for overhead in file headers, system files, saved =
setups, etc, etc, you'd get around 24 1/2 to 25 hours recording time.
You cannot trust the marked capacity to reflect the usable storage space on=
ce the card is formatted when calculating recording time - you need to chec=
k the formatted disk's capacity in a reader to verify, and calculate on tha=
t basis.
cheers
Paul
|