Sonnox Oxford Limiter - Manual (Page 21)

9.2 Noise
Shaping
Dither 9 DITHER AND NOISE SHAPING
9.2 Noise
Shaping
Dither
If for some reason SNR figures of 93dB at 16bits (or 143dB at 24bits) prove insufficient,
noise shaping can create an apparent increase in SNR, but there are some potentially
hidden costs. Noise shaped dithering is a mechanism that aims to reduce the perceived
loudness of the noise of a dithered signal by either forcing the spectrum of the noise out
of the audible range or placing it into frequency ranges to which we are less sensitive. In
this way the noise at very low levels may be reduced and even lost entirely, if they are at
the limit of our hearing within ambient noise conditions. The following graph illustrates
this process.
The red line shows the original 16-bit dithered output with the —120dBr signal passing.
The blue line shows the effect of noise shaping (
TYPE 1 at 100%) on the same signal
transfer.
In use, the DITHER TYPE button (see right)
toggles TPDF, TYPE 1, TYPE 2, TYPE 3, and TYPE 4.
It can be seen that the noise
has been substantially reduced in the regions up to around
8KHz where we are most sensitive, at the expense of extra
noise energy in the higher ranges above 10KHz at which
we are less sensitive. Such processing can have a dramatic
effect on the perceived intrusion of low-level noise.
However, as is always the case, one cannot get something
for nothing and it can be seen from the above graph that the
total noise power across the whole range must remain constant to satisfy the dithering
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