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Science Manual – Using Video Images for Fisheries Monitoring 19

At the other end of the scale are the ‘MicroDVR’ class of recorders. As the name

suggests, these are small and very portable 12-volt recording options. They are also

cheap, with prices starting at around £130 (Table 3.4). These units can often be

in MPEG4 format onto removable media such as ‘Secure Digital’ (SD) cards; these

can be removed and files copied to a computer using an SD card reader. The

MPEG4 format allows the data to be analysed using image processing software if

necessary, although the software cannot be installed and used on site (Table 3.2).

The relatively small capacity of SD cards at the moment limits the amount of data

that can be recorded; for example, a two-gigabyte (GB) SD card can store 2.5 days

of data. These types of DVRs are therefore more suited to exploratory, short-term

deployment. Despite being small, some of the useful features available with the

other two systems are retained, such as adjustable recording frame rate and

schedule recording.

Low-power computers

There are a variety of small form factor PCs on the market which can operate on a

12-volt power supply, where the boards are assembled and powered differently to

standard computers. The boards are stacked on top of each other and the whole

system can be enclosed in a case 210 x 190 x 85 mm (Table 3.2). Before

purchasing a low-power computer, check that it can run the Windows operating

system and that it is compatible with DVR cards, such as those manufactured by

Voltek or Swann (Table 3.4). If it is compatible with these cards, it will be possible to

produce MPEG4 codec files (Table 3.2). Low-power external USB hard drives

(Section 3.6) are available, which obtain power from the computer and do not need

an additional power supply. Small 12-volt computer monitors can be bought to set

the system up on site (Table 3.4).

As part of the project, discussions have been taking place with the suppliers of one

type of low-power PC, the Mini-ITX system. The machines can be built to

specification to ensure they are suitable for purpose and standard specifications for

fish counting systems have been agreed. A specification for a machine suitable for

use as a basic digital video recorder can be found in Appendix 4.

A system like this can provide all of the functionality of a standard computer-based

recording system which means that, unlike the DVRs, image processing software can

potentially be installed on the machine and operated on site. Testing of the suitability

of the ITX machine to run Fishtick on site is underway and the results will be

circulated when this is complete.

Table 3.2: A comparison of the low-power recording systems tested for this

project

Timespace Mini-ITX MicroDVR

Cost (£) 2,400 550 129

Power (watts) 7.2 2.5

Size (mm) 220 x 116 x 52 210 x 190 x 85 90 x 65 x 30

Storage capacity

(GB)*

250+ 160 2

File format Proprietary MPEG4 MPEG4

Suitable for…? Long-term use Long-term use Short-term use

Software Not compatible Can be installed Used to playback files

*dependent on the maximum size of compatible storage media.

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