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dCS 900E / 902E
User Manual
Manual for Software Versions 1.3x to 1.5x
dCS
Ltd
12
th
June 2000
Manual part no: DOC0029021E1
Page 52
Document No: OS-MA-D0002-902.1E1
Contact
dCS
on + 44 1799 531 999
email to: [email protected]
(inside the UK r 44 with 0)
web site: www.dcsltd.co.uk
There is another option not supported by the
dCS 900
&
902
– generate the dither
independently of the signal and frequency shape it prior to addition, but do not
add it in an error shaping loop. This seems to
dCS
to combine the worst of all
worlds – the high noise floor in the 0-6 kHz area of straight dither, and the high
total noise of noise shaping. However, some people use it.
What does it look like?
-
Figure 28 gives the spectra of 16 bit truncated 44.1 kS/s signals with a –90dB
sine present, for two dither only signals (Top Hat, Noise Shaped Triangular), and
with a 10
th
order noise shaped
13
signal, generated and processed by a
dCS 972
.
The equivalent simply truncated spectrum is shown in Figure 29, separately
because it is so revolting. In it, we can see that at the signal level shown
(-90 dB) error power from the quantising/truncation is beginning to pile into the
fundamental, which is showing an amplitude error of +1.3 dB, as well as all the
unwanted harmonics. This would show up on a conventional linearity plot,
although the sign of the error could be either way.
We see that the noise shaping approach maintains low noise in the critical audio
mid band.
Noise Shaper and Dither Only Comparison
dCS 972 SW v1.54
-160
-140
-120
-100
-80
-60
0
2
4
6
8
10
12
14
16
18
20
22
Frequency (kHz)
Amplitude, Real Data (dB)
16 bit truncation, Noise Shaped Triangular dither
16 bit truncation, Top Hat dither
16 bit truncation, high order noise shaping only
44.1 kS/s sampling, 1024 point FFTs, H6 window, -90 dB 1.44 kHz input
Figure 28 – Noise Shaping and Dither Spectra
13
for comparison with the table, 10
th
and 9
th
order noise shaping are very similar.