Ask HN: How do browsers isolate internal audio from microphone input?

I've noticed an interesting feature in Chrome and Chromium: they seem to isolate internal audio from the microphone input. For instance, when I'm on a Google Meet call in one tab and playing a YouTube video at full volume in another tab, the video’s audio isn’t picked up by Google Meet. This isolation doesn’t happen if I use different browsers for each task (e.g., Google Meet on Chrome and YouTube on Chromium).

Does anyone know how Chrome and Chromium achieve this audio isolation?

Given that Chromium is open source, it would be helpful if someone could point me to the specific part of the codebase that handles this. Any insights or technical details would be greatly appreciated!

This is something that is usually taken care of by the App that's receiving the input from the microphone (Google Meet, Teams, etc). The App breaks the audio into frequencies, and the ones that correspond to human voice ranges are accepted, and anything else is rejected. This is referred to as, for example, voice isolation, and has been turned on by default in all major meeting Apps for a little while now.

Surprised to hear that it doesn't seem to work for you when the audio is generated by a different browser, this shouldn't make a difference.

The way this works (and I'm obviously taking a high level view here) is by comparing what is being played to what is being captured. There is an inherent latency in between what is called the capture stream (the mic) and the reverse stream (what is being output to the speakers, be it people taking or music or whatever), and by finding this latency and comparing, one can cancel the music from the speech captured.

Within a single process, or tree of processes that can cooperate, this is straightforward (modulo the actual audio signal processing which isn't) to do: keep what you're playing for a few hundreds milliseconds around, compare to what you're getting in the microphone, find correlations, cancel.

If the process aren't related there are multiple ways to do this. Either the OS provides a capture API that does the cancellation, this is what happens e.g. on macOS for Firefox and Safari, you can use this. The OS knows what is being output. This is often available on mobile as well.

Sometimes (Linux desktop, Windows) the OS provides a loopback stream: a way to capture the audio that is being played back, and that can similarly be used for cancellation.

If none of this is available, you mix the audio output and perform cancellation yourself, and the behaviour your observe happens.

Source: I do that, but at Mozilla and we unsurprisingly have the same problems and solutions.

Google Meet uses source separation technology to denoise the audio. It's a neural net that's been trained to separate speech from non-speech and ensure that only speech is being piped through. It can even separate different speakers from one another. This technology got really good around 2021 when semi-supervised ways of training the models were developed, and is still improving :)