网络音频 API - 立体声到单声道

Question

I need to convert an stereo input (channelCount: 2) stream comming from chrome.tabCapture.capture to a mono stream and send it to a server, but keep the original audio unchanged.

I've tried several things but the destination.stream always has 2 channels.

            const context = new AudioContext()

            const splitter = context.createChannelSplitter(1)
            const merger = context.createChannelMerger(1)
            const source = context.createMediaStreamSource(stream)
            const dest = context.createMediaStreamDestination()

            splitter.connect(merger)
            source.connect(splitter)
            source.connect(context.destination) // audio unchanged
            merger.connect(dest) // mono audio sent to "dest"
            
            console.log(dest.stream.getAudioTracks()[0].getSettings()) // channelCount: 2

I've also tried this:

            const context = new AudioContext()

            const merger = context.createChannelMerger(1)
            const source = context.createMediaStreamSource(stream)
            const dest = context.createMediaStreamDestination()

            source.connect(context.destination)
            source.connect(merger)
            merger.connect(dest)
            
            console.log(dest.stream.getAudioTracks()[0].getSettings()) // channelCount: 2

and this:

            const context = new AudioContext()

            const source = context.createMediaStreamSource(stream)
            const dest = context.createMediaStreamDestination({
                channelCount: 1,
                channelCountMode: 'explicit'          
            })

            sourcer.connect(context.destination)
            soruce.connect(dest)
            
            console.log(dest.stream.getAudioTracks()[0].getSettings()) // channelCount: 2

there has to be an easy way to achieve this... thanks!

Answer 1

There is a bug in Chrome which requires the audio to flow before the channelCount property gets updated. It's 2 by default.

The following example assumes that the AudioContext is running. Calling resume() in response to a user action should work in case it's not allowed to run on its own.

const audioContext = new AudioContext();
const sourceNode = new MediaStreamAudioSourceNode(
    audioContext,
    { mediaStream }
);
const destinationNode = new MediaStreamAudioDestinationNode(
    audioContext,
    { channelCount: 1 }
);

sourceNode.connect(destinationNode);

setTimeout(() => {
    console.log(destinationNode.stream.getAudioTracks()[0].getSettings());
}, 100);