On the DSP, you will implement the
elliptic low-pass filter designed using the
`ellip`

command from IIR
Filters: Filter-Design Exercise in MATLAB. You should
not try to implement the notch filter designed in IIR Filtering: Filter-Coefficient
Quantization Exercise in MATLAB, because it will not
work correctly when implemented using Direct Form II. (Why
not?)

To implement the fourth-order filter, start with a single set
of second-order coefficients and implement a single
second-order section. Make sure you write and review
pseudo-code *before* you begin programming.
Once your single second-order IIR is working properly you can
then proceed to code the entire fourth-order filter.

**Large coefficients**

You may have noticed that some of the coefficients you have
computed for the second-order sections are larger than 1.0
in magnitude. For any stable second-order IIR section, the
magnitude of the "0" and "2" coefficients
(

**Repeating code**

Rather than write separate code for each second-order
section, you are encouraged first to write one section, then
write code that cycles through the second-order section code
twice using the repeat structure below. Because the IIR
code will have to run inside the block I/O loop and this
loop uses the block repeat counter
(`BRC`

), you must use another looping structure
to avoid corrupting the `BRC`

.

#### Note:

```
stm (num_stages-1),AR1
start_stage
; IIR code goes here
banz start_stage,*AR1-
```

**Gain**

It may be necessary to add gain to the output of the system.
To do this, simply shift the output left (which can be done
using the `ld`

opcode with its optional
`shift`

parameter) before saving the output to
memory.