# One pole filter, LP and HP¶

• Author or source: uh.etle.fni@yfoocs
• Type: Simple 1 pole LP and HP filter
• Created: 2006-10-08 14:53:38
notes
```Slope: 6dB/Oct

Reference: www.dspguide.com
```
code
 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13``` ```Process loop (lowpass): out = a0*in - b1*tmp; tmp = out; Simple HP version: subtract lowpass output from the input (has strange behaviour towards nyquist): out = a0*in - b1*tmp; tmp = out; hp = in-out; Coefficient calculation: x = exp(-2.0*pi*freq/samplerate); a0 = 1.0-x; b1 = -x; ```

```Why don't you just say:

Process loop (lowpass):
out = a0*in + b1*tmp;
tmp = out;

Simple HP version: subtract lowpass output from the input (has strange behaviour towards nyquist):
out = a0*in + b1*tmp;
tmp = out;
hp = in-out;

Coefficient calculation:
x = exp(-2.0*pi*freq/samplerate);
a0 = 1.0-x;
b1 = x;
```
```There's a tradition among digital filter designers that the pole coefficients have a negative sign. Of course the other one is also valid, and sometimes these notations are mixed up.

If you're worried about the extra negation operation, then you could say

b1 = -x;
a0 = 1.0+b1;

-- peter schoffhauzer
```
```Of course, you don't need tmp.

Process loop (lowpass):
out = a0*in + b1*out;
```
```Indeed.
```
```Or...
out += a0 * (in - out);

:)
```