The calculations

Using these figures, we can calculate that the number of hours of information per cubic metre of housing is 12500/25 = 500 hours.

Considering that one hour of information is, in fact, the “final product” of the archive, all the housing costs of the complete archive should be charged to the product. Therefore, a cubic metre of storage space costs 70000/25 = f.2,800 a year and, in this example, the storage of one hour of information costs 2800/500 = f.5.60 a year.

It seems clear that, with the space available in the existing store in mind, when using a information carrier with a higher storage capacity, the total number of hours of information stored can be increased, with the result that the annual storage costs per hour of information decrease.

An information carrier with a lot more storage capacity than the analogue recording tape used in this example is the R-DAT (Rotary-head Digital Audio Tape). Accepting that the size of an R-DAT cassette is much smaller and the recording capacity is three times as high as a professional audiotape, the use of R-DAT could create a great increase of storage capacity in an archive.

Using the same figures as before, we can make a comparison of analogue tape and R-DAT:

$ The measurements of an analogue recording tape are 0.27 x 0.27 x 0.015 = 1.09 x 10-3 cubic metre.
$ The measurements of a R-DAT cassette are 0.08 x 0.06 x 0.015 = 0.072 x 10-3 cubic metre.

The volume ratio between the formats is, therefore, 1.09/0.072 = 15.19:1.

The capacity of a R-DAT cassette is 3 hours. The information storage ratio between a R-DAT cassette and an analogue tape is, therefore, 3 x 15.19 = 45.57:1.

However, storage capacity is reduced when storing smaller objects: wasted space is increased. From past experience, it is necessary to introduce a correction factor of 0.8. This gives an increase of 45.57 x 0.8 = 36 times.

The conclusion is that the capacity of a storage space with R-DAT cassettes is 36 times higher than analogue tapes.

The cost of tapes and machines are about the same for R-DAT and analogue tape. At present, the price of a R-DAT-cassette is roughly the same as the price of an analogue tape. The price of a professional R-DAT-recorder (Pro-DAT) is about the same as the professional analogue audio-recorders normally used in archives.

We have ignored the costs of new shelving that is needed for housing the R-DAT cassettes. The most important fact is the possibility of increasing the storage capacity by 36 times and that results in the number of hours per cubic metre housing in this example to be increased to 36 x 500 = 18.000 hours.

Taking into account the figure of housing per cubic metre being f.2800, we can calculate that the storage costs of one hour information on a R-DAT cassette are 2800/18000 = f.0.16 a year: a dramatic and attractive reduction by a factor of 35 from the f.5.60 a year for analogue tape.

The example shows us the following:

$ The storage costs using analogue recording tape are 35 times more expensive as when using the R-DAT-cassette when using the same capacity storing space.
$ The existing archive space can be used for an extra 36 years at present production level.

The result that, when compared with using analogue tape, no building activities have to be undertaken to expand the storage space is, of course, very attractive. If one has to do so, the housing costs per year are increasing only slightly.

When re-calculating, we can compare the results with each other in order to make a decision. When we look at the storage costs only, and we do not take the other aspects of information carriers into account, the result of this comparison is very clear.