Big sessions and the load meter

Every few milliseconds your Mac must work out the next slice of sound before the speakers need it. That slice is a buffer. When a song has many tracks and plug-ins, it can run out of time, and you hear a click. This chapter shows how to see that coming and what to do.

The load meter

The LOAD meter in the transport bar shows how much of each buffer's time Nebula uses. Under 70 % is comfortable. Over 70 % the bar changes colour, and over 90 % it turns red: there is almost no time left, and you may hear clicks.

The thin mark on the bar is the highest load of the last two seconds. Click the meter for more: the load now and its peak, how many buffers were missed (with Reset), the buffer size and the sample rate.

The overload light

The small light on the meter turns red when a buffer was missed: something was not ready in time. It stays red until you click it, so you know a click happened even if you were not listening closely.

Which track is heavy

Click the load meter and look under Heaviest Channels and Heaviest Plug-ins. They name the tracks and plug-ins that use the most of each buffer while the song plays. The critical path is the longest chain of channels each waiting for the ones that feed it.

That tells you which track to freeze or which plug-in to swap.

Buffer size

The buffer size is in Audio Settings (the speaker button in the transport bar). A bigger buffer gives every slice more time, so fewer clicks, but you hear what you play a little later. Use a small buffer (64 or 128) while recording, and a big one (512 or 1024) while mixing a big song.

Low latency monitoring

Low Latency Monitoring (the hare in the transport bar, or Transport ▸ Low Latency Monitoring) leaves out slow plug-ins on what you are playing and on the Stereo Out, so you hear yourself without delay. The plug-ins stay in the song and in every bounce; they are drawn dashed with a hare while they are left out.

Freeze

Track ▸ Freeze Track (⌃⌘F) renders a track with its instrument and plug-ins into audio, so it costs almost nothing to play. Track ▸ Unfreeze Track (⌃⇧⌘F) brings the plug-ins back to edit them.

To make a frozen track's audio permanent, choose Track ▸ Flatten Frozen Track. The track becomes an audio track playing the render; its instrument, MIDI effects and plug-ins are gone, since their sound is in the audio. The regions it replaced stay in the song's files until you press Remove Unused in File ▸ Audio Files…. Undo takes it back in one step.

Big songs

Nebula is built for big songs: two hundred tracks and thousands of regions play and scroll smoothly, and the screen stays quick. The arrangement and the mixer draw only the tracks and strips you can see, and choosing a region redraws only the tracks it changes.

You can change a song while it plays. Add a track, drop an audio file onto the arrangement, or change the tempo or the time signature, and the song plays on without a gap. A tempo or time signature change lets go of notes that are sounding at that moment.

If a song opens slowly, it is usually reading its audio files and plug-ins. Holding Shift while Nebula starts opens it without plug-ins, if a plug-in stops a song from opening at all.

Several cores

Nebula can render a heavy song's tracks side by side on several of your Mac's cores, so it has more time for each buffer. It is on unless you turn it off.

1. Open Audio Settings (the speaker button in the transport bar).
2. Under Cores, Use several cores is on. Turn it off to render every track on the audio thread alone.
3. Leave At most on Automatic, or choose fewer cores for a Mac that has other work to do.

A light song still renders on one core, and a bounce sounds the same either way. With it off, every track renders on the audio thread alone. Click the load meter to see Cores: how many the last buffers used and how busy the busiest was.

If one plug-in crackles or fails only with several cores on, keep just it on the audio thread: open the Plug-in Manager (Nebula DAW ▸ Plug-in Manager…), choose the plug-in and tick Render on the main audio thread. It and the track it is on stay off the other cores.

A plug-in that stops

If a plug-in gets stuck while it renders (for more than 50 milliseconds), or its process quits, Nebula stops calling it and the rest of the song keeps playing. That insert gives silence, not its dry sound, so a stuck limiter on the Stereo Out can never let through what it was holding. A bar at the top of the arrangement names it: "stopped responding and was bypassed", or "quit unexpectedly and was bypassed". The slot says the same.

• Press Reload to load it again with its last saved settings.
• Press the cross to hide the bar. The plug-in stays bypassed, and its slot still says so.

An Audio Unit v2 normally runs inside Nebula, so a crash in it can take the song down. To keep it apart, open the Plug-in Manager (Nebula DAW ▸ Plug-in Manager…), choose it and tick Run in separate process: if it crashes, only it is bypassed and the song plays on. It takes a little longer to load, and applies the next time it loads. Plug-ins that run apart load side by side, so several that never finish loading cost the song about 15 seconds once, and it opens without them; plug-ins inside Nebula load one after another. A plug-in that will not load in a process of its own (some copy protection will not) is loaded inside Nebula instead, and the box says so. Nebula's own plug-ins (Nebula Kick, Drums, Genesis, EQ) always run inside Nebula, where Claude can reach every setting. Audio Unit v3 plug-ins always run apart.

If Nebula itself quits while a plug-in inside it is loading, playing or showing its window, the next launch says which one it was and runs that plug-in apart from then on, so the same crash does not wait in the song you open next. The box says so; untick it to bring it back in.

Claude can tell you which plug-in stopped and reload it for you: ask it why the reverb went silent.