Audio Engineering

514 Chapter 16

However, this comb fi lter type arrangement is not very conveniently suited to a system,
such as the replay path for a CD, in which all operations are synchronized at a single
specifi c “ clock ” frequency or its submultiples, and an alternative digital fi lter layout,
shown in Figure 16.12 in simplifi ed schematic form, is normally adopted instead. This
provides a very steep-cut low-pass fi lter characteristic by operations carried out on the
signal in its binary-encoded digital form.

In this circuit, the delay blocks are “ shift registers, ” through which the signal passes in
a “ fi rst in, fi rst out ” sequence at a rate determined by the clock frequency. Filtering is
achieved in this system by reconstructing the impulse response of the desired low-pass
fi lter circuit, such as that shown in Figure 16.13. The philosophical argument is that if
a circuit can be made to have the same impulse response as the desired low-pass fi lter,
it will also have the same gain/frequency characteristics as that fi lter—a postulate that
experiment shows to be true.

This required impulse response is built up by progressive additions to the signal as it
passes along the input-to-output path, at each stage of which the successive delayed
binary coded contributions are modifi ed by a sequence of mathematical operations.
These are carded out, according to appropriate algorithms, stored in “ look-up ” tables,
by the coeffi cient multipliers A 1 , A 2 , A 3 ,... , An. (The purpose of these mathematical
manipulations is, in effect, to ensure that those components of the signal that recur more

    

Delay Delay Delay Delay Delay

A 1 A 2 A 3 A 4 A 5

Clock

Input Output

etc.

Figure 16.12 : A basic oversampling fi lter.