Digital vs. analogue

One fundamental feature of computers is that they represent data in digital form, i.e. using discrete numbers instead of continuous values. For some data types this makes sense. A character is either an 'A' or a 'B', it is not somewhere in between - characters are discrete. However, not all data we surround ourselves with have this property. E.g., the colour of the sky is not blue or red, it is more likely to have elements of both and change continuously.

Audio is another data type that from nature comes as continuous signal, i.e. variations in the air pressure. An audio signal can be represented as a function of time (figure 5a).

If the audio signal is to be transferred through a digital telephone system or stored in a computer, it needs to be digitised. In the digitisation process, the continuous changes in the air pressure are sampled at a certain frequency, and the sampled values are rounded off to the closest discrete value (figure 5b).

Images are also represented digitally through a sampling process. A scanner looks upon the image as a two-dimensional grid, and measures the chrominance and luminance of each pixel.

The move from analogue to digital systems is a fundamental change that is taking place in all media. The digital CD replaced the analogue LP in just 10 years, and the next generation radio and TV broadcasting systems will be digital. Telephone systems are digital, except for the last kilometer into our homes - and that is about to change as well.