```Thanks, I now understand what I was doing wrong.

p.kootsookos@iolfree.ie (Peter Kootsookos) wrote in message news:<3fca8095.0411231251.6426d1c4@posting.google.com>...
> aine_canby@yahoo.com (?ine Canby) wrote
>
> > and the output filtered signal has a magnitude of 1.5*10-4, which is
> > -38.23 for an input magnitude of 1.
>
> >> 20*log10(1.5*10^-4)
>
> ans =
>
>   -76.4782
>
> Perhaps you need to do the power rather than the magnitude?
>
> Ciao,
>
> Peter K.
```
```On 23 Nov 2004 07:27:57 -0800, aine_canby@yahoo.com (?ine Canby)
wrote:

>Thanks for your answers. I tried filtering a cos signal as follows -
>
>a=fir1(1024,0.2);
>%a = a'
>%freqz(a,1,1024);
>
>k = 0:1:20000;
>fk = cos(0.3*pi*k);
>y = filter(a,1,fk);
>plot(y(1024:20000));
>
>and the output filtered signal has a magnitude of 1.5*10-4, which is
>-38.23 for an input magnitude of 1. At the same time if I plot the
>freqz() of "a" I get a graph which suggests that the attenuation at
>0.3f (f=sampling freq) is in the order of -70dBs? Why is this and how
>should I establish my stopband attenuation in dBs, because looking at
>the freqz graph it seems to be -70dBs for frequencies above 0.25f.
>
>Cheers,
>
>Aine.

Hi,
Wow, ... you used some strange values in your code.

is a sinusoid whose peak amplitude is 1.37*10^-4.

Your input sinusoid has a peak amplitude 1.  So ....

1.37*10^-4
dB = 20*log10 ---------------  = -77.23
1

Your filter input frequency is roughly 0.15Fs Hz (Fs is sample rate).
So if you use

freqz(a,1,4096,1), zoom on

and look closely (zoom in several times) at the magnitude response
exactly at the freq of 0.15Fs Hz, you'll see that the mag
response is quite close to -77.5 dB which agrees well with the
above "dB" computation.

[-Rick-]

```
```aine_canby@yahoo.com (?ine Canby) wrote

> and the output filtered signal has a magnitude of 1.5*10-4, which is
> -38.23 for an input magnitude of 1.

>> 20*log10(1.5*10^-4)

ans =

-76.4782

Perhaps you need to do the power rather than the magnitude?

Ciao,

Peter K.
```
```(...)

> and the output filtered signal has a magnitude of 1.5*10-4, which is
> -38.23 for an input magnitude of 1.

I think you mean -38.23 dB ?? Well, this isn't really correct... The
definition of the dB is :

L [dB] = 10*log10(P2/P1)

if P1 and P2 are powers (watts for example) and

L [dB] = 20 * log10(V2/V1)

if, for example, V1 and V2 are values such as voltages or currents. The
value that you read when doing your plot is not a power, so you have to
use the second equation :

20*log10((1.5*10^-4)/1) = -76.48 dB

Sam
```
```Thanks for your answers. I tried filtering a cos signal as follows -

a=fir1(1024,0.2);
%a = a'
%freqz(a,1,1024);

k = 0:1:20000;
fk = cos(0.3*pi*k);
y = filter(a,1,fk);
plot(y(1024:20000));

and the output filtered signal has a magnitude of 1.5*10-4, which is
-38.23 for an input magnitude of 1. At the same time if I plot the
freqz() of "a" I get a graph which suggests that the attenuation at
0.3f (f=sampling freq) is in the order of -70dBs? Why is this and how
should I establish my stopband attenuation in dBs, because looking at
the freqz graph it seems to be -70dBs for frequencies above 0.25f.

Cheers,

Aine.
```
```in matlab it is also possible to design FIR filters, especially with
fdatool if you have the toolbox, or using the fir1() or remez()
commands, depending on the filter you want to deign. This is helpful in
the case you don't want to copy your 1024 coeffs into a matlab function
to plot them ... :-)

sam
```
```In Matlab, use the freqz function.  Type help freqz for more info.  With FIR
filters, the attenuation may be 35dB worst case in the stop band, but still may
have "nulls" or "ripples" that are significantly deeper than this.  The freqz
plot should show this.  Or you could try a few other frequencies nearby to see
if the results differ.

"?ine Canby" <aine_canby@yahoo.com> wrote in message
> Hi there,
>
> I have a 1024 coef FIR filter bank, which I use to filter a cosine
> signal. I have calculated the stop-band supression to be -35dBs but
> this sounds really high to me. The filters turning point is at 0.1f,
> where f is the sampling frequency. For example, if I feed the FIR with
> a frequency of 0.25f, I get a signal out with amplitude 1*10^-6. Does
> this sound correct?
>
> Is there a function in matlab I could use to plot the spectrum
> resulting from my FIR?

```
```Hi there,

I have a 1024 coef FIR filter bank, which I use to filter a cosine
signal. I have calculated the stop-band supression to be -35dBs but
this sounds really high to me. The filters turning point is at 0.1f,
where f is the sampling frequency. For example, if I feed the FIR with
a frequency of 0.25f, I get a signal out with amplitude 1*10^-6. Does
this sound correct?

Is there a function in matlab I could use to plot the spectrum
resulting from my FIR?

Thanks,

Aine.

My Coefs-

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```