The bar chart representation of Distortion Residue is particularly interesting as it contains a lot of information designed to allow at-a-glance assessment of performance. The first bar represents the Noise level, followed by bars for relative Distortion Residue at up to six levels. The first of these (at –20dB) is often the most revealing. In the absence of significant distortion it will indicate a figure 20dB worse than the noise bar (because it is showing noise relative to the –20dB level and not alignment level) but crossover distortion (in amplifiers), quantising errors (in digital convertors), and modulation noise (on tape) will all show as further worsening. Manufacturers are keen to produce very low figures for distortion measured at MOL (maximum output level) or FS (full scale), but in practice these are of little relevance compared to performance at –20dB AL (typically -46dB on CD) where low relative levels are harder to achieve and defects are more likely to be heard.
|
