Direct-Form-I implementation of a 2nd-order digital filter.
The DF-I structure has the following properties:
It can be regarded as a two-zero filter section followed in series
by a two-pole filter section.
In most fixed-point arithmetic schemes (such as two's complement,
the most commonly used
there is no possibility of internal filter overflow. That is,
since there is fundamentally only one summation point in the filter,
and since fixed-point overflow naturally ``wraps around'' from the
largest positive to the largest negative number and vice versa, then
as long as the final result is ``in range'', overflow is
avoided, even when there is overflow of intermediate results in the sum
(see below for an example). This is an important, valuable, and
unusual property of the DF-I filter structure.
There are twice as many delays as are necessary. As a result,
the DF-I structure is not canonical with respect to delay. In
general, it is always possible to implement an th-order filter
using only delay elements.
As is the case with all direct-form filter structures
(those which have coefficients given by the transfer-function coefficients),
the filter poles and zeros can be very sensitive to round-off errors
in the filter coefficients. This is usually not a problem for a
simple second-order section, such as in Fig.9.1, but it can
become a problem for higher order direct-form filters. This is the
same numerical sensitivity that polynomial roots have with respect to
polynomial-coefficient round-off. As is well known, the sensitivity
tends to be larger when the roots are clustered closely together, as
opposed to being well spread out in the complex plane
[19, p. 246]. To minimize this sensitivity, it is common to
factor filter transfer functions into series and/or parallel second-order
sections, as discussed in Chapter 9.
It is a very useful property of the direct-form I implementation
that it cannot overflow internally in two's complement fixed-point
arithmetic: As long as the output signal is in range, the filter will
be free of numerical overflow. Most IIR filter implementations do not
have this property. While DF-I is immune to internal overflow, it
should not be concluded that it is always the best choice of
implementation. Other forms to consider include
parallel and series second-order sections (§9.2 below),
normalized ladder forms [32,48,86].10.2Also, we'll see that the transposed direct-form II
(Fig.9.4 below) is a strong contender as well.