WebAudio AudioWorklets in C++: sample rate?

[Luis Torres]

I'm following this nice tutorial to get an AudioWorklet going in C++:

https://emscripten.org/docs/api_reference/wasm_audio_worklets.html

But how do I figure out what the audio sample rate is? If I want to generate a tone in the audio callback, it will need to know the audio sample rate.

[Mehdi Sabwat]

Hi,

You get to set the sampleRate in the audio context you use, for audio worklets samplerate is read only,

```

typedef int EMSCRIPTEN_WEBAUDIO_T;

typedef struct EmscriptenWebAudioCreateAttributes
{
        const char *latencyHint; // Specify one of "balanced", "interactive" or "playback"
        uint32_t sampleRate; // E.g. 44100 or 48000
} EmscriptenWebAudioCreateAttributes;

// Creates a new Web Audio AudioContext, and returns a handle to it.
EMSCRIPTEN_WEBAUDIO_T emscripten_create_audio_context(const EmscriptenWebAudioCreateAttributes *options);

```

from system/include/emscripten/webaudio.h

you can also have it EM_ASM_INT{console.log(sampleRate)} or something equivalent (check out https://emscripten.org/docs/porting/connecting_cpp_and_javascript/Interacting-with-code.html), but you need to be careful and run this in the audio worklet scope not the main one.

Hope it helps,

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[Carl Woffenden]

But how do I figure out what the audio sample rate is?

It's a property of the audio worklet global scope, but you can also query it from the returned context, something like:

emscriptenGetAudioObject(context).sampleRate

I'm going to be proposing API changes so that it's passed to the callback (in a struct with other data).

[Mehdi Sabwat]

I wonder how reliable it would be to do this since it can change, for example you could ask for a sampleRate and get less. What happens if it changes during playback?

As I see it it should be read from the callback

[Mehdi Sabwat]

It shouldn't be passed to the callback because it is readonly (https://developer.mozilla.org/en-US/docs/Web/API/AudioWorkletGlobalScope/sampleRate)

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[Carl Woffenden]

Setting it creation time is only a suggestion, AFAIK, but reading this after creation should be good. You can read the same in the callback too, but the changes I'd like to make report this per callback.

[Carl Woffenden]

Readonly, so you can read the value valid for that audio frame.