Databases
StackOverflow2013
Postsdbo
POChebyshev LPF introducing noise
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POChebyshev LPF introducing noise
    primarykey
    data
    text
    <p>I have created a simple Chebyshev low pass filter based on coefficients generated by this site: <a href="http://www-users.cs.york.ac.uk/~fisher/mkfilter/" rel="nofollow noreferrer">http://www-users.cs.york.ac.uk/~fisher/mkfilter/</a>, which I am using to filter out frequencies above 4kHz in an 16kHz sample rate audio signal before downsampling to 8kHz. Here's my code (which is C#, but this question is not C# specific, feel free to use other languages in different languages).</p> <pre><code>/// &lt;summary&gt; /// Chebyshev, lowpass, -0.5dB ripple, order 4, 16kHz sample rte, 4kHz cutoff /// &lt;/summary&gt; class ChebyshevLpf4Pole { const int NZEROS = 4; const int NPOLES = 4; const float GAIN = 1.403178626e+01f; private float[] xv = new float[NZEROS+1]; private float[] yv = new float[NPOLES + 1]; public float Filter(float inValue) { xv[0] = xv[1]; xv[1] = xv[2]; xv[2] = xv[3]; xv[3] = xv[4]; xv[4] = inValue / GAIN; yv[0] = yv[1]; yv[1] = yv[2]; yv[2] = yv[3]; yv[3] = yv[4]; yv[4] = (xv[0] + xv[4]) + 4 * (xv[1] + xv[3]) + 6 * xv[2] + (-0.1641503452f * yv[0]) + (0.4023376691f * yv[1]) + (-0.9100943707f * yv[2]) + (0.5316388226f * yv[3]); return yv[4]; } } </code></pre> <p>To test it I created a sine wave "chirp" from 20Hz to 8kHz using Audacity. The test signal looks like this:</p> <p><img src="https://i.stack.imgur.com/BcSDx.png" alt="test signal"></p> <p>After filtering it I get:</p> <p><img src="https://i.stack.imgur.com/reAhL.png" alt="filtered spectrum"></p> <p>The waveform shows that the filter is indeed reducing the amplitude of frequencies above 4kHz, but I have a load of noise added to my signal. This seems to be the case whichever of the filter types I try to implement (e.g. Butterworth, Raised Cosine etc).</p> <p><img src="https://i.stack.imgur.com/1ECba.png" alt="filtered waveform"></p> <p>Am I doing something wrong, or do these filters simply introduce artefacts at other frequencies? If I downsample using the naive approach of averaging every pair of samples, I don't get this noise at all (but obviously the aliasing is much worse).</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USMark Heath
    1. USMark Heath
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. This table or related slice is empty.
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload