Audio Engineering

(Barry) #1
Compact Disc 517

smaller, as shown in Figure 16.15. This reduces the quantization noise and also increases
the effective resolution of the DAC. As a general rule, an increase in the replay sampling
rate gives an improvement in resolution equivalent to that given by a similar increase in
encoding level, such that a four times oversampled 14-bit decoder would have the same
resolution as a straight 16-bit decoder.


Yet another advantage of oversampling is that it increases the bandwidth over which the
“ quantization noise ” will be spread—from 22.05 to 88.2 kHz in the case of a four times
oversampling system. This reduces the proportion of the total noise that is now present
within the audible (20 Hz to 20 kHz) part of the frequency spectrum—especially if “ noise
shaping ” is also employed. This aspect is examined later in this chapter.


16.3.2.4 “ Dither ”


If a high-frequency noise signal is added to the waveform at the input to the ADC and
if the peak-to-peak amplitude of this noise signal is equal to the quantization step ‘ Q ’ ,
both the resolution and the dynamic range of the converter will be increased. The reason
for this can be seen if we consider what would happen if the actual analogue signal level
were to lie somewhere between two quantization levels. Suppose, for example, in the
case of an ADC, that the input signal had a level of 12.4 and that the nearest quantization
levels were 12 and 13. If dither had been added, and a suffi cient number of samples
were taken, one after another, there would be a statistical probability that 60% of these


Time

Amplitude

Figure 16.15 : Effect of four times oversampling and interpolation of intermediate values.
Free download pdf