Audio Engineering

482 Chapter 15

At this stage you will need to be sure you are confi dent that you appreciate that the
combined effect of the antialias fi ltering and the proper implementation of the sampling
process mean that sampled data contain perfectly all of the detail up to the highest
frequency component in the signal permitted by the action of the antialias fi lter.

15.10.2 Quantizing

The sampled input signal must now be measured. The dynamic range that can be
expressed by an n -bit number is approximately proportional to 2 n and this is more usually
expressed in dB terms. The converters used in test equipment such as DVMs are too slow
for audio conversion but it is worthwhile considering the outline of their workings ( Figure
15.23 ). A much faster approach uses a successive approximation register (SAR) and a
digital-to-analogue converter (DAC) ( Figure 15.24 ).

Comparator

Start

conversion

Samples

waveform

Counter

output

Ramp

Stop

Clock

reset

Figure 15.23 : The simplest ADC uses a ramp generator, which is started at the beginning
of conversion. At the same time a counter is reset and the clock pulses are counted. The
ramp generator output is compared with the signal from the sample and hold and when
the ramp voltage equals the input signal the counter is stopped. Assuming that the ramp
is perfectly linear (quite diffi cult to achieve at high repetition frequencies) the count will be
a measure of the input signal. The problem for audio bandwidth conversion is the speed at
which the counter must run in order to achieve a conversion within approximately 20 μs. This
is around 3.2768 GHz and the comparator would need to be able to change state within
150 μs with, in the worst case, less than 150 μV of differential voltage. There have been
many developments of this conversion technique for instrumentation purposes.