Render error -50 when AU plugin has multichannel main + sidechain bus

General JUCE discussion
1

Hi all,

I ran into a reproducible issue with AudioUnits when combining:

  • multichannel main bus (e.g. 5.1 / 6 channels)

  • and a second input bus used as external sidechain

This works perfectly as VST3, but breaks in AU.

To isolate the problem I created a minimal test plugin (“BusCheck”) that does almost nothing:
it simply applies a gain of 0.5 to all main output channels and optionally reads the sidechain input for metering.

No DSP complexity, no routing, no latency — just bus configuration.

Observed behaviour

VST3:
Works correctly in all tested configurations.

AudioUnit:

When loading the plugin as a multichannel instance (e.g. 5.1):

  • Logic Pro:

    • UI does not open for the multichannel version

    • instead produces a strange multi-channel hum/noise

    • multi-mono version works

  • Studio One / Fender Studio:

    • plugin instance goes silent

    • render error in debugger:

ioData.mNumberBuffers=2,
ASBD::NumberChannelStreams(output.GetStreamFormat())=6;
kAudio_ParamError
render err: -50

Key observation

If I remove the sidechain bus from the constructor:

.withInput ("Sidechain", juce::AudioChannelSet::stereo(), false)

then multichannel AU works perfectly again.

So the failure only occurs when:

  • main bus = multichannel

  • AND second input bus exists

Minimal bus setup used

AudioProcessor (BusesProperties()
    .withInput  ("Input",  juce::AudioChannelSet::stereo(), true)
    .withOutput ("Output", juce::AudioChannelSet::stereo(), true)
    .withInput  ("Sidechain", juce::AudioChannelSet::stereo(), false)
)

isBusesLayoutSupported() allows matching multichannel layouts for main in/out and stereo sidechain.

The plugin itself just applies gain to all main channels.

Additional notes

The plugin UI correctly reports:

  • Main In: 6 channels

  • Main Out: 6 channels

  • Sidechain: 2 channels

So the layout negotiation appears to succeed.

However during AU rendering there is a mismatch between:

  • number of channel streams expected by ASBD

  • number of buffers provided by host

This results in:

kAudio_ParamError (-50)

Repro project

Minimal repro project (JUCE 8.0.12):

Questions

Is this a known limitation or bug in the AU wrapper when combining:

  • multichannel main bus

  • additional input bus?

Or is there a required configuration step I’m missing for AU specifically?

Any guidance appreciated.

Thanks!