3.2
Calculation of deployment endurance
An excel spreadsheet is provided on Ocean Instruments website to help with the calculation of
deployment times for differing deployment parameters. The spreadsheet provides deployment
estimates, as well as identifying one of three possible deployment constraints: battery capacity,
memory capacity, or file count. While the first two are obvious, the file count constraint requires
explanation.
The file system used by SoundTrap currently has a minimum file size of 4 MB. When using periodic
recording, if the programmed ‘record for a period of’ period is very short, the resultant file may be
smaller than the minimum file size. This will result in inefficient usage of memory, and the
deployment may be artificially memory limited. For example, a SoundTrap ST400 has 256 GB of
memory; dividing this by the minimum file size of 4 MB gives a maximum of 64000 files.
This limitation becomes an issue when using low sample rates in combination with X3 compression.
By experimenting with the periodic recording parameters in the deployment spreadsheet it will
become clear how to best avoid this limitation.
3.3
Clock details
SoundTrap’s clock is synchronised to your PC whenever you click on the ‘deploy’ butto
n, with an
accuracy of +/- 0.5s.
Internally the clock is maintained in UTC time. When files are offloaded to the PC file times are
converted to local time (using the PCs location settings) for population of the filenames. This means
that the dates and times in the filenames are always in the time zone of the PC that was used to
offload the files. This can be important where files are offloaded in a region other than that in which
they were recorded.
Alternatively, filenames can be assigned UTC times by selecting the ‘Use UTC
time’ option under the tools menu.
Like most electronic devices, SoundTrap relies on a crystal oscillator for time keeping. The oscillator
used in the ST400 has an accuracy of +/- 5PPM, or +/- 0.44 seconds per day. The audio sample rate is
derived from the same oscillator, meaning the sample rate is subject to the same accuracy. The
oscillator will drift with temperature with this range. For more information on the clock accuracy
please see documentation for the oscillator - SiTime part SIT1552AC-JE-DCC-32.768D.
Sampling rate accuracy is also subject to occasional sampling gaps. The nature of SD flash memory
cards is such that occasional the memory card will fail to keep up with sampling, thereby resulting in
