dCS 974 User Manual

Manual for Software Version 1.0x

dCS Ltd

May 2001

When is Dither Needed?

Dither (on its own or with noise shaping) is not always needed. The condition for not being needed depends on the level of noise in the incoming signal, the amount of noise shaping being used, and the output word length. The user may want to avoid having to think about this at all – in which case, use Noise Shaped Triangular dither – or may wish to minimise noise and added noise.

As a rule of thumb, if the following condition is met, dither is not necessary:

Noise in input

+

Noise gain of

>

Q noise of wanted

signal

noise shaper

output word length

 

 

The noise gain of the noise shaper is the same as the unweighted noise power given in Table 16, and is shown in Figure 50 below.

Noise Power (dB rel no shaping)

30

24

18

12

6

0

-6

-12

-18

-24

-30

-36

-42

-48

Noise Gain and Weighted Noise Gain vs Shaper Order

dCS 972 SW v1.54

Actual Increased Noise

Power and Noise Gain

(top section)

96 kS/s

88.2 kS/s

48 kS/s

44.1 kS/s

32 kS/s

Perceived (F Weighted)

Noise Power (bottom

section)

0

1

2

3

4

5

6

7

8

9

10

 

 

 

 

 

Noise Shaper Order

 

 

 

 

 

Figure 50 – Noise Gain and Weighted Noise Gain by Shaper Order and Sample Rate

As an example of reading this, consider a 24 bit signal in with a –110 dB0 noise floor, to be output at 48 kS/s 16 bits. The Q noise in the output signal would be -98 dB0, so if a noise shaper with more 12 dB of noise gain was used, no dither would be necessary. Looking at the chart, a 5th order shaper (14.2 dB of noise gain) would avoid the need for dither, whereas a 4th order one (10.9 dB of noise gain) would need dither. The 5th order shaper would give a weighted noise improvement of 16.4 dB, whereas the 4th would only give 13.9 dB perceived improvement, less a further degradation from adding dither.

Manual part no: DOC1241121A1

Page 108

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)

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