dCS 904 User Manual

Manual for Software Version 1.5x and 1.36

dCS Ltd

June 2000

 

 

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 10th order noise shaped13 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

Amplitude, Real Data (dB)

-60

16 bit truncation, Noise Shaped Triangular dither

-80 16 bit truncation, Top Hat dither

16 bit truncation, high order noise shaping only

-100

-120

-140

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

44.1 kS/s sampling, 1024 point FFTs, H6 window, -90 dB 1.44 kHz input

 

 

-160

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

2

4

6

8

10

12

14

16

18

20

22

 

 

 

 

 

 

Frequency (kHz)

 

 

 

 

 

 

Figure 35 – Noise Shaping and Dither Spectra

13for comparison with the table, 10th and 9th order noise shaping are very similar.

Manual part no: DOC135904 iss 2B2

Page 66

135904ma2b2.pdf file available from website

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