input signal increases in level, the output remains fixed at a quantising level before suddenly jumping to the next level. Consequently, the transfer curve looks like a staircase instead of a desirably straight line. Analogue tape recording suffers a similar non-linearity problem because the properties of magnetic tape are far from perfect. However, in this case, the problem is overcome by employing an inaudible, very high-frequency signal (called bias) which makes the magnetic properties behave more linearly. Digital audio systems have to resort to a similar tactic to create a linear transfer characteristic using an audible signal called dither . Dither is essentially a very small amount of white noise (equivalent in amplitude to one quantising level -- about 90dB below peak level in a 16-bit system) which is deliberately added to the analogue audio signal as it enters the A-D converter. Thinking back to the transfer plot, this dither noise effectively 'fills in' the steps in the transfer curve so that a straight line can be drawn through the noisy staircase. When correctly dithered in this way, the digital system behaves perfectly linearly, but there is a small amount of noise always present -- the amplitude of which rises and falls as the input signal increases in level through each quantising band (see Figure 4). DIGITAL HEADROOM & METERING In the analogue recording world, we are used to the idea that there is a nominal working level -- say 0VU or +4dBu -- but that it is permissible for