Audio Engineering

Compact Disc 519

employ “ noise shaping, ” a procedure in which, as shown in Figure 16.16 , the noise
components are largely shifted out of the 20-Hz to 20-kHz audible region into the
inaudible upper reaches of the new 11.29-MHz bandwidth.

The proposition is, in effect, that a decoded digital signal consists of the pure signal, plus
a noise component (caused by the quantization error) related to the lack of resolution
of the decoding process. It is further argued that if this noise component is removed by
fi ltering, what remains will be the pure signal—no matter how poor the actual resolution
of the decoder. Although this seems an unlikely hypothesis, users of CD players
employing the “ bit stream ” system seem to agree that the technique does indeed work in
practice. It would therefore seem that the greater freedom from distortion, which could
be caused by errors in the quantization levels in high bit-level DACs, compensates for the
crudity of a decoding system based on so few quantization steps.

Mornington-West^1 quotes oversampling values of 758 and 1024 times, respectively, for
“ Technics ” and “ Sony ” “ low-bit ” CD players, which would be equivalent in resolution
to 10.5- and 11-bit quantization if a simple ‘ 0 ’ or ‘ 1 ’ choice of encoding levels was used.
Since the presence of dither adds an effective 1 bit to the resolution and dynamic range,
the fi nal fi gures would become 10-, 11.5-, and 12-bit resolution, respectively, for the
Philips, Technics, and Sony CD players.

However, such decoders need not use the single-bit resolution adopted by Philips,
and if a 2- or 4-bit quantization was chosen as the base to which the oversampling

0

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20 dB

10 100 1 K 10 K

‘Noise’

Signal

100 K 1 M 10 MHz

Output (dB)

Frequency

Figure 16.16 : Signal noise spectrum after “ noise shaping. ”