audiodevlog 004: Creating an example and getting an audio backend to work

Having written a few generators, it was time to put them to practice, since unit tests can't prevent something from sounding bad.

I created an example rising_sine which creates a Sine generator, fills an f32 vector of 44100 samples with that generator, and increases the frequency by 100 Hz over those 44100 samples. The sampling frequency has thoughtfully been set to 44100 samples per second.

// examples/rising_sine.rs

use disperse::{
    backends,
    generators::{generator::Generator, sine::Sine},
};

/**
 * Plays a sine for 1 second, while linearly increasing its frequency from 440 to 540
 */
fn main() {
    let mut frequency = 440.0;
    let mut sine = Sine::new(frequency, 44100);

    let samples = vec![0.0; 44100]
        .iter()
        .map(|_| {
            sine.set_frequency(frequency);

            // Make it rise 100 Hz over the span of 44100 samples
            frequency = frequency + 100.0 / 44100.0;

            sine.get_next_value()
        })
        .collect();

    backends::cpal::play_samples(samples);
}

Running this example with cargo run --example rising_sine satsifyingly plays exactly what I expected.

I did run into an issue with cpal though, where it simply didn't call the provided data_callback, which should write the samples into an audio output buffer.

// src/backends/cpal.rs (partial)

let stream = device
    .build_output_stream(
        config.into(),
        move |data: &mut [f32], _| {
            for frame in data.chunks_mut(channels) {
                let value = next_value();

                for sample in frame.iter_mut() {
                    *sample = value;
                }
            }
        },
        |err| eprintln!("{err}"),
        None,
    );

// Nothing happens here!
stream.unwrap().play().unwrap();

The solution was moving an unwrap() call from the line that tries to play() the stream to the line that creates the stream. The reason is not unwrapping a build_output_stream call immediately drops the handle to the cpal::Stream, which results in stream.play() not doing anything nor erroring.

While I have my default search engine set to DuckDuckGo, it was Google that showed me this 3 year old GitHub issue as its top search result, whereas DuckDuckGo failed to give me the results I needed to fix this (with the same prompt used for both searches). As an EU citizen, I'm all for using software that isn't part of Big Tech™, but every now and then stuff like this happens, making me feel funny on the inside ¯\_(ツ)_/¯.