Play to two DACs at once on a Mac

Several outputs, one stereo signal, and no aggregate device to build by hand.

macOS can drive several audio devices at once, but only through an Aggregate Device — a combined device you normally assemble yourself in Audio MIDI Setup, which then sits in your device list forever whether you are using it or not. BitMuse builds one for you instead: tick the outputs you want, and it creates a private aggregate that never appears in Audio MIDI Setup and is destroyed when you quit.

What it is for

Two DACs, one signal. The reasons people want this are ordinary ones: desk speakers and a headphone amp both live, so you can lift the headphones without touching anything; a second system in the next room; a subwoofer on its own converter; or an A/B between two DACs where the only honest way to compare is for both to receive the same bits at the same moment.

How BitMuse builds the aggregate

Select more than one device in the device sheet — from the status bar or from Settings — and BitMuse assembles a CoreAudio aggregate named BitMuse Multi-Output. Three details make it behave:

• It is private. The aggregate is created with the private flag set, which keeps it out of Audio MIDI Setup and out of every other app’s device list, and has CoreAudio tear it down when the process exits. Your system audio configuration is exactly as you left it.
• The first device you pick is the clock. It becomes the master sub-device, and the app marks it in the list so you know which one the others are following.
• Every other device gets drift compensation. Two USB DACs have two independent crystals that will not agree forever; without compensation they slide apart and one eventually drops or repeats a sample. BitMuse enables drift compensation on each non-master sub-device.

Two devices is the minimum; there is no cap in the picker. They do not have to be the same model or run at the same rate — the playable rates become whatever CoreAudio reports for the combined device, and the sheet shows each device’s own rates so you can see where they overlap.

How stereo reaches every DAC

This is the part a hand-built aggregate does not solve for you. An aggregate of two stereo DACs is a four-channel device; three DACs make six channels. Most players see four channels and send your stereo music to the first two, leaving the second DAC silent — which is why people build the aggregate, play something, and hear nothing from the other box.

BitMuse reads the combined channel count when it opens the device and keeps that number where the real-time thread can read it safely. The audio itself stays stereo the whole way through the engine — the ring buffer is two channels and nothing wider is ever decoded or processed. Then, at the very end of the render callback, the stereo pair is copied outward: left into every even channel, right into every odd one. Every DAC in the group gets the same stereo signal, in step.

There is a reason the copy happens last rather than first. The DSP chain’s filters keep separate state for left and right, so handing them six channels would corrupt that state; BitMuse clamps the buffer to stereo for the processing stage and restores the full width only for the copy. One consequence worth knowing: every output gets the same processing. There is no separate EQ for the speakers and the headphone amp while both are live — for that, use the per-device DSP profiles, which keep a chain per output and swap it when you change device, and switch between the two instead.

What multi-output costs you

Multi-output is not a bit-perfect mode. That is the reason not to leave it on. BitMuse does not take exclusive control of an aggregate: engaging multi-output switches exclusive mode, integer mode and hardware volume off and moves volume into software. BitMuse greys the toggles out and says so in the device sheet rather than leaving them switched on and inert.

So the choice is real: one DAC with an integer, exclusive path, or several DACs sharing a software-mixed one. Neither is wrong; they are different jobs. See bit-perfect audio on a Mac for what you are trading away.

Cleaning up after itself

A stray aggregate device left behind in your system is the standard annoyance of doing this by hand. BitMuse removes its own three ways over: it destroys the aggregate explicitly when the app quits, CoreAudio destroys it anyway because it was created private, and at launch BitMuse sweeps up any aggregate of its own left behind by a crash. If one of the sub-devices is unplugged while playing, multi-output switches itself off rather than continuing against a device that is gone. Your selection is remembered and restored on the next launch.

FAQ

Can a Mac play to two DACs at once?

Yes, through an Aggregate Device. You can build one in Audio MIDI Setup, or let BitMuse create a private one that stays out of your system configuration and disappears when you quit.

Do both DACs have to be the same model?

No, and they need not share a clock. The first device you select is the clock master and the others get drift compensation.

Is multi-output still bit-perfect?

No. BitMuse does not take exclusive control of an aggregate: when multi-output is engaged it switches exclusive mode, integer mode and hardware volume off rather than pretending otherwise.

How does stereo reach more than two channels?

BitMuse keeps the audio stereo through the engine, then copies left into every even channel and right into every odd one, so each DAC receives the same pair.

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