ng quantisation levels, which increases the degree of accuracy and reduces quantisation noise. This increase is however at the expense of a larger data size which may take a longer time to transmit. Therefore, two factors determine the number of quantisation levels. One is the necessary accuracy for a particular application. The other is telecommunications systems capacity and its associated device structure and the protocols. In practice the size is also determined by equipment standards. Computer systems usually use memory and processing structures based on units of one or more bytes, where each byte is 8 bits. Quantisation levels are therefore designed to produce binary digits of multiples of 8 bits. Voice for example uses 256 quantisa- tion levels, and produces 8 bits per sample (2 superscript(8) = 256). Music (as in compact discs, CDs) uses 65 536 quantisation levels, and produces 16 bits per sample (216 = 65 536). Furthermore, quantisation level boundaries may be set to produce equal probability of signal occurrence within each band. This usually produces finer quantisation levels at lower amplitudes and coarser levels at higher magnitudes. An example of this is given in relation to digitisation of voice signals below. From Analogue to Digital Why Digital? While capable of perfectly reproducing an analogue signal, a digital representation is still less than perfect in practice for two reasons. One is because of the filtering required to limit the signal bandwidth, which discards some information. The other is imperfect rep- resentation of the signal sample due to limited quantisation levels, which may lead to reproduction quality impairment. This partly explains why aficionados of vinyl records (analogue) have long bemoaned the lower quality of music CDs (digital). So why is digital SO popular? There are a number of technical and business reasons, but the most important technical reason is the ability of digital communications to remove the impact of thermal noise. Noise impact is cumulative: it is added to