# Rust: `no_std` digital low pass filter library
This is a `no_std` Rust library for simple digital low pass filters. You can use it for example to
get the low frequencies from a song.
**I'm not an expert on digital signal processing. Code contributions are highly welcome! :)**
## How to use
```rust
use std::path::PathBuf;
use std::fs::File;
use audio_visualizer::waveform::staticc::png_file::visualize;
use audio_visualizer::{Channels, ChannelInterleavement};
use lowpass_filter::simple::sp::apply_lpf_i16_sp;
use minimp3::{Decoder as Mp3Decoder, Frame as Mp3Frame, Error as Mp3Error};
/// This example reads a MP3, applies a digital low pass filter and visualizes
/// the waveform in the end.
fn main() {
let mut path = PathBuf::new();
path.push("test/samples");
path.push("sample_1.mp3");
let mut decoder = Mp3Decoder::new(File::open(path).unwrap());
let mut lrlr_mp3_samples = vec![];
loop {
match decoder.next_frame() {
Ok(Mp3Frame { data: samples_of_frame, .. }) => {
for sample in samples_of_frame {
lrlr_mp3_samples.push(sample);
}
}
Err(Mp3Error::Eof) => break,
Err(e) => panic!("{:?}", e),
}
}
// split into left and right channel
let (mut left, mut right) = Channels::Stereo(ChannelInterleavement::LRLR)
.stereo_interleavement()
.to_channel_data(&lrlr_mp3_samples);
// left: low pass filter with i16 samples in single precision
apply_lpf_i16_sp(&mut left, 44100, 120);
// right: lpf with i16 samples in single precision
apply_lpf_i16_sp(&mut right, 44100, 120);
// visualize audio as waveform in a PNG file
visualize(
&left,
Channels::Mono,
"test/out",
"sample_1_waveform_lowpassed_left.png"
);
visualize(
&right,
Channels::Mono,
"test/out",
"sample_1_waveform_lowpassed_right.png"
);
}
```
## Visual Examples
### #1: Original Waveform of a short sample
![Example 1: Original Waveform of a short sample](sample1_waveform.png "Example 1: Original Waveform of a short sample")
### #1: Lowpassed Waveform
![Example 1: Lowpassed Waveform of a short sample](sample1_waveform_lowpassed.png "Example 1: Lowpassed Original Waveform of a short sample")
### #2: Original Waveform of a song
![Example 1: Original Waveform of a song](song_waveform.png "Example 1: Original Waveform of a song")
### #2: Lowpassed Waveform
![Example 1: Lowpassed Waveform of a song](song_waveform_lowpassed.png "Example 1: Lowpassed Original Waveform of a song")
### #2: 3x Lowpassed Waveform
![Example 1: Lowpassed Waveform of a song 3x](song_waveform_lowpassed_3x.png "Example 1: Lowpassed Original Waveform of a song 3 times")