CAN WE HELP?

Q There are increasing numbers of digital products appearing in the market to fill roles traditionally taken by the multiplexer/time-lapse VCR combination. How are storage capacities and times calculated for these products?

A When dealing with digital systems, users want to know how long it will take for the disk to overwrite or for the archive media to fill. This is important because you need to able to ensure that an archive tape will last over a weekend, or you may wish to adjust the record rate to ensure the same. Not knowing the capacities in terms of time may lead to lost images.

Working out capacities and times is not difficult and a standard calculation can be used. There are a number of factors which must be considered before an approximate value can be arrived at:

a)Digital recording systems generally use data compression. Normally an estimated file size is set for the desired picture quality. In reality recorded images will be close to and around this file size but not the exact file size. JPEG based systems give a steady average picture size due to a new picture being collected every time.

b) Non JPEG compression methods can be sporadic in nature and generate very small amounts of data and equally very large amounts of data making it necessary to average out the amount of data generated per minute or second. Even when this is done it can only be used as a rough guideline and actual site performance can differ from a laboratory based figure.

c) When the disk or media size is quoted, it often is not all available for image recording. Certain areas of the disk or media are frequently reserved for operating systems and other required mechanisms such as logs and indexing. The size of the area reserved by these items must be subtracted form the total media capacity.

d) Media size is often quoted as unformatted. When a format is applied to the media then some capacity is lost, although this is usually negligible.

The actual calculation for storage is fairly easy to implement, it is basically the number of images which can be recorded to the media / record rate. Two examples follow to show how the calculation works.

1) Average picture size of 20 kilobytes