Sonnox Oxford Limiter User Manual
Page 22
9.2 Noise Shaping Dither
9 DITHER AND NOISE SHAPING
requirement.
This means the noise level necessarily increases in some ranges of the spectrum. A level
increase anywhere in the spectrum must be accommodated by an increase in total peak
noise level. The following graphs illustrate this in action, under the previous test
conditions.
The first graph above shows a sample value plot of the conventional
TPDF dithered signal.
The second graph shows the increase in values caused by the application of the
TYPE 1
noise shaping at 100%. Focussing the dither energy into a more restricted range than
would naturally occur causes the level to increase.
From the effective level values highlighted in the graphs it can be seen that the application
of noise shaping has increased the effective noise level from around —93dBr to —80dBr,
an increase of roughly 13dB.
From this it can be understood that the design of a suitable noise shaping frequency
curve is a trade off between the perceived loudness of the noise under certain conditions
and the increase in overall level of the dither signal; much of this trade off relies on what
we can hear (psycho-acoustics). Significant research has been carried out over the years
into various approaches to this issue, and several accepted curves are in use around the
industry.