Process what your computer plays

The System node puts everything your computer plays onto the board: a music player, a browser tab, a video call, a game. Patch it into a filter, a reverb or a delay network, and listen to the result. Or record it.

It is a feature of the Bleep App. The plugins have your DAW for this, and Bleep Play cannot see other programs.

You don't need a virtual audio driver (BlackHole, Loopback, VB-Cable). Bleep App asks your operating system for the sound directly.

Turn it on

1. Open File → Settings → Audio.
2. Under System audio, tick Capture what this computer plays.
3. Allow the permission if your system asks (macOS does, once).
4. On the board, choose Add → Port → System.

The node has two outputs, L and R, and a Gain dial, like the other Audio In nodes. It stays off until you tick the box: nothing is captured before that.

The settings show what is going on: the sample rate, whether your own sound is left out, and whether sound arrives (signal) or not (silent).

Recipes

• Hear your music player through an effect. Play something in another program, then patch System L/R → Reverb → Out 1/2. You hear the player itself and, a moment later, the effect. Play Bleep App to headphones and the player to speakers (or mute the player's output) to hear the effect alone.
• A filter sweep over a song. System L/R → Filter → Out 1/2, and an LFO on the cutoff.
• Record what you hear. Patch System and your synth into a mixer node and on into a recorder.

Because the captured sound reaches Bleep App a moment after it leaves the other program, the effect is slightly late. See Delay below.

Feedback

If Bleep App plays to your speakers and System captures your speakers, a cable from System to Out 1/2 would feed back: what comes out is captured, played, captured again.

Bleep App prevents this by default: Leave out Bleep App's own sound is on. The capture then holds everything except Bleep App.

Where it can't be left out, the setting is disabled and says so. Play Bleep App to another output than the one you capture (headphones for Bleep App, speakers for everything else), or don't patch System to an output.

Turn the setting off if you want to record Bleep App's own sound together with everything else.

macOS

You need macOS 14.2 or later. The first time you turn the capture on, macOS asks if Bleep App may record system audio. If it doesn't ask, or you said no, System stays silent. Allow it in System Settings → Privacy & Security → Screen & System Audio Recording (the System Audio Recording Only list), then turn the capture off and on again.

If the app is started from a terminal rather than by double-clicking it, macOS may credit the terminal instead of Bleep App, and the capture stays silent. Start the app from Finder or the Dock.

Sound that is protected against copying (some streaming services) arrives as silence. That is the system's rule, not a setting.

On older macOS, use a loopback driver such as BlackHole as an input device instead (File → Settings → Input).

Windows

Windows 10 or later. No permission prompt.

With Windows 10 build 20348 or later (and Windows 11), Bleep App captures everything except itself, whichever output device you use. On older versions it captures the default output device including its own sound.

If you change the default output (plug in headphones), the capture follows within a second.

Linux

You need PulseAudio, or PipeWire with its PulseAudio support (pipewire-pulse, the default on most desktops), and a build of the app with libpulse (install libpulse-dev before building).

Every output has a monitor, a source that carries what that output plays. The settings list them under Output to capture. Default output follows your system's default.

Linux can't leave Bleep App's own sound out of a monitor: use a different output for Bleep App than for the rest, as described under Feedback.

Delay

The captured sound takes about 40 ms to arrive: 30 ms is a buffer that absorbs hiccups, and the system's own capture adds 10 to 20 ms. That doesn't matter for processing, mixing or recording. It does matter for playing live against the captured sound; for that, use a normal audio input (a cable or a loopback driver).

Two sound cards never run at exactly the same speed. Bleep App corrects for that continuously with a tiny pitch adjustment (well below what you can hear) instead of dropping or repeating samples, so long sessions stay clean.

When it doesn't work

Notes for developers

How it is built, the clock-drift handling and the open questions are in the specification, doc/specs/system-audio.md in the repository.