Add a simple A440 synth example

[ulfmagnetics]

Just taking a crack at solving #18. I'm a Rust newbie, but this was pretty straightforward.

I couldn't figure out a way to specify an oscillator frequency without using the synth::dynamic module... if there's a simpler way I'd love to know about it!

[mitchmindtree]

An alternative to using the synth::dynamic::Oscillator would be to make a synth::Oscillator with constant amplitude and frequency types, i.e.

let waveform = synth::oscillator::waveform::Sine;
let amp = 1.0;
let hz = 440.0;
let freq_warp = ();
let oscillator = synth::Oscillator::new(waveform, amp, hz, freq_warp);

Using synth::dynamic::Oscillator always produces an Oscillator<waveform::Dynamic, amplitude::Dynamic, frequency::Dynamic, freq_warp::Dynamic> - meaning that the type of waveform, amplitude, frequency or freq_warp providers is change-able at run-time in exchange for the slight run-time cost of checking which of each is used.

In the example I've given above however, the type is Oscillator<waveform::Sine, amplitude::Constant, frequency::Hz, ()>, meaning that we know at compile-time the Oscillator will always produce sine waves at some constant amplitude and hz, with no freq_warp. This should allow for slightly greater performance at the cost of less run-time flexibility.

Note that when producing a dynamic Synth (i.e. synth::dynamic::new(), as you've done below), it will always expect a dynamic Oscillator type as well. Using the regular Synth::new() constructor should allow you to pass the Oscillator type in the example I gave above.

By the way, I noticed that you're setting the Oscillator's constant frequency to 440.0hz, while setting the base_pitch to C1 and giving the note_on as C1. This is probably fine for this example, however I think a user might expect the synth to play back at the frequency of the note that was given. Perhaps it might be more intuitive to have the oscillator frequency match the base_pitch frequency, so that the frequency given at note_on always matches the frequency that plays back? I.e. if base_pitch and oscillator frequency were both C1 and note_on was given an A4, the synth should produce a 440hz sine wave.

[ulfmagnetics]

Ah yes, now I see, I missed the impl Amplitude for f32 and impl Frequency for f64 blocks before. I hadn't considered that numeric types could implement traits -- very useful in this case!

[mitchmindtree]

Thanks! 😸

I left a comment inline re- avoiding the synth::dynamic module, hopefully it makes sense and is useful!

BTW, I'll have a PR within the next few days that removes the dynamic module all together in favour of Synth::dynamic and Oscillator::dynamic methods and making better use of rust's impl blocks in general, along with some other slight improvements. I'm happy to merge this though (with those alterations I mentioned in the comment) and then make any necessary adjustments in my PR 👍

[ulfmagnetics]

Thanks for the very clear explanation! Hopefully this'll get the job done now..