v0.1.1087 Aug 2026·Rendering, Media

Stereo Web Audio: real panning and per-channel routing

The Web Audio render was mono end to end - every node reduced to a single value, and the destination's one value was copied to every output channel. That made three whole node classes impossible to express, because each is defined by moving signal between channels: StereoPannerNode (pan a source left/right), ChannelSplitterNode (fan a stereo input out to separate mono channels) and ChannelMergerNode (recombine mono channels into a stereo output). A page could call createStereoPanner() and get a node that ran but did nothing. The render now carries a real two-channel bus through every node: the panner uses the spec's equal-power law (a centred mono source comes out at 1/sqrt(2) on each side, not half), gain, filters, delay and waveshaping process each channel independently, and splitter/merger route per channel using the output and input indices on connect(). A mono signal up-mixes to stereo by copy and down-mixes by averaging, per the Web Audio speaker rules. Crucially the engine's stand-out property survives the rewrite: the render is still byte-identical every run - now proven THROUGH the stereo path, including a panned graph whose two channels genuinely differ. Media moves 36 to 37; overall holds at 51.

Parity with the reference browser

74overall, as of v0.1.343

Web APIs / DOM surface42

Security architecture66

GPU / graphics acceleration68

DevTools / extensions72

Networking85

Media85

Accessibility85

Real-world site compatibility99

HTML parsing100

CSS parsing + selectors100

Layout: block / flex / grid / tables / writing-modes100

Paint + visual100

JavaScript language100

JavaScript performance (JIT)100

HTML parsing100

What changed 3 changes

Media	Every node now carries a two-channel bus (L/R) instead of one mono value, with a flag that tracks whether a signal is logically mono so the panner and merger apply the correct channel law; up-mix (mono to stereo) is a copy and down-mix (stereo to mono) averages, the Web Audio speaker rules applied where inputs are summed and where the destination is written. Biquad filters and delay lines gained independent per-channel state so a stereo signal keeps its two channels distinct.
Rendering	StereoPannerNode, ChannelSplitterNode and ChannelMergerNode are implemented and routed for real: connect() now records an output index and an input index, so a splitter can fan a stereo bus out to separate mono channels and a merger can recombine them - a swap wired through split-then-merge exchanges left and right exactly, a routing a mono engine cannot express. The panner uses the equal-power cos/sin law for a mono input and the spec's two-branch law for a stereo input.
Media	The determinism-by-construction invariant now runs through the stereo path and is machine-checked in the gate: a panned sine with a pan ramp renders both channels byte-for-byte identical across independent renders while the two channels genuinely differ, hard-left panning silences the right channel exactly, and a plain mono graph still up-mixes to two equal channels byte-for-byte. Five new property tests, the prior Web Audio tests unchanged, conformance gate 6375/6375 with the native-JIT and no-nanbox builds green. Research note: docs/stereo-web-audio.md.