naturerecordists
[Top] [All Lists]

Re: SD cards how much will they hold

Subject: Re: SD cards how much will they hold
From: "Paul Jacobson" thebrunswicktwitcher
Date: Wed Jul 21, 2010 10:45 pm ((PDT))
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







<Prev in Thread] Current Thread [Next in Thread>
Admin

The University of NSW School of Computer and Engineering takes no responsibility for the contents of this archive. It is purely a compilation of material sent by many people to the naturerecordists mailing list. It has not been checked for accuracy nor its content verified in any way. If you wish to get material removed from the archive or have other queries about the archive e-mail Andrew Taylor at this address: andrewt@cse.unsw.EDU.AU