Filter
Top Products
dCS 974 User Manual Manual for Software Version 1.0x
dCS Ltd May 2001
Manual part no: DOC1241121A1
Page 105
Document No: OS-MA-A0124-112.1A1
Contact
dCS
on + 44 1799 531 999 email to: more@dcsltd.co.uk
(inside the UK replace + 44 with 0) web site: www.dcsltd.co.uk
The 0 dB reference level is taken as the minimum noise we could possibly get
away with – the amount that simple 16 bit truncation (16 bit Q noise) would give,
if it were well behaved, which it is not.
Straight forward dither always adds noise – it can only produce signals with a
noise floor higher than Q noise on its own. However, the noise power added is a
few dBs for simple types. Noise shaping adds rather more noise, but it can be
made to add it in parts of the spectrum that the ear is less sensitive to, so the
perceived noise (F weighted noise) is lower – up to three bits lower. It results in
a signal that the ear hears as having a far lower noise floor than a 16 bit
truncated signal, rather than the “not much worse” of dither alone, even though
there is really more noise present
29
.
Truncation Type, with
44.1 kS/s data rate
Noise,
unweighted,
rel 16 bit Q
noise
30
Noise,
F weighted,
rel 16 bit Q
noise
Comments
16 bit truncation 0 dB 0 dB Unpleasant low
level effects
16 bit truncation with
Top Hat dither
3 dB 3 dB
Okay – can show
noise modulation at
low signal levels
16 bit truncation with
Triangular dither
4.8 dB 4.8 dB All noise
modulation and
unpleasant effects
removed, but noise
floor is high
16 bit truncation with
Noise Shaped
Triangular dither
4.8 dB 1.2 dB
All noise
modulation and
unpleasant effects
removed. Not much
perceived noise
penalty
16 bit truncation with 2
nd
order noise shaping and
no dither
6.2 dB -10.4 dB Okay with input
noise floors down
to –102dB0
16 bit truncation with 2
nd
order noise shaping and
Noise Shaped
Triangular dither
11.0 dB -9.2 dB Unconditionally free
from truncation
effects with all
inputs
16 bit truncation with 9
th
order noise shaping and
no dither
23.4 dB -17.9 dB Okay with input
noise floors down
to –120dB0
16 bit truncation with 9
th
order noise shaping and
Noise Shaped
Triangular dither
28.2 dB -16.7 dB
Unconditionally free
from truncation
effects with all
inputs
Table 16 – Dither and Noise Shaping Noise Powers
29
DSD carries this further. The principle is the same, but with DSD, there is more noise than there is signal,
even at full scale. It is just that it is in a part of the spectrum the ear cannot hear.
30
16 bit Q noise is -98.1 dB relative to a full scale sine wave.