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dCS 904 User Manual Manual for Software Version 1.5x and 1.36
dCS Ltd June 2000
Manual part no: DOC135904 iss 2B2
Page 66
135904ma2b2.pdf file available from website
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dCS
on + 44 1799 531 999 email to: more@dcsltd.co.uk
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Noise shaping on its own is not perfect. It relies on a small amount of noise in
the input signal to generate the frequency shaped correction signal, and if there
is very low noise in the input signal, this mechanism can break down. With
ADCs, however, this situation does not arise, because of the analogue noise in
the ADC and the input signal.
There is another option not supported by the dCS 904 – 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 35 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 36, 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 10121416182022
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 35 – Noise Shaping and Dither Spectra
13
for comparison with the table, 10
th
and 9
th
order noise shaping are very similar.