Tags Tag archive: 'MediaStreamTrack'

November 2, 2016

Changing Firefox MediaStreams to accommodate cloning

Contributed by Andreas Pehrson, Andreas was a software engineer at Telenor Digital 2013-2017.

This article is a re-post of one that I wrote on the Telenor Blog earlier this year. It explains how MediaStreams work in Firefox and the changes I did to them to accommodate cloning.

First of all. What is a MediaStreamTrack, and how can you clone it?

A MediaStreamTrack represents a realtime stream of audio or video data.

It provides a common API to the multiple producers (getUserMedia, WebAudio, Canvas, etc.) and consumers (WebRTC, WebAudio, MediaRecorder, etc.) of MediaStreamTracks.

A MediaStream is simply put a grouping of MediaStreamTracks. A MediaStream also ensures that all tracks contained in it stay synchronized to each other, for instance when it gets played out in a media element.

Cloning a track means that you get a new MediaStreamTrack instance representing the same data as the original, but where the identifier is unique (consumers don’t know it’s a clone) and disabling and stopping works independently across the original and all its clones.

Now, how does all this come together in Firefox? (more…)

October 19, 2016

Warm-up with dummy tracks and replaceTrack

Contributed by Jan-Ivar Bruaroey,

If you’ve looked at the WebRTC spec over the last year, you’ll find a gap between what it says and what browsers have implemented so far. The big difference is the spec talks about senders and receivers of tracks, whereas most browsers still operate with streams.

Firefox is the only browser so far to have pivoted to tracks, the stuff streams are made of. This opens up exciting possibilities like sender.replaceTrack(). It lets you switch out an already-sending track without needing to renegotiate, and still see the change reflected remotely. We’re not fully done with senders and receivers, and we have more to do (like transports), but replaceTrack works.

The spec, however, goes even further, defining transceivers, a pairing of a sender and a receiver. Even Firefox hasn’t implemented that yet. The spec shows an example of connection warm-up using transceivers to set up a connection before media is ready. This means you can send media the instant it becomes available. Quite neat, and a benefit of the spec moving to objects that can exist before media is added.

But it turns out we don’t have to wait for transceivers to do warm-up, which is what we’re going to show here! (more…)