Bug? Mono impulse response makes signal passing through it also mono? And other Convolution Reverb strangeness!

I noticed a strange behavior with the Convolution Reverb in 2.02.07 on Mac:

  • If I drag-and-drop a MONO IR into the wave area, then the resulting Dry signal playback on that audio track is also MONO, even if it was originally a stereo file.
  • It stays mono, even if I BYPASS Convolution Reverb.
  • This is not a behavior I’m accustomed to with other convo verbs, the Wet signal would be mono, but Dry should stay stereo unless I do additional channel processing to collapse it to mono.
  • Disabling Link does nothing to change the sound, unlike with a Stereo IR, where turning it on makes it mono and turning it off makes it stereo. I’m actually kind of confused about why this button affects the Dry signal too?

In short, I don’t expect the IR to affect the Dry signal. Could there be an option to keep the Dry signal as-is, at least?

Another weird thing:

  • If I replace a Mono IR with a Stereo IR, it still sounds Mono
  • UNLESS I right-click on the IR area and Clear Data
  • Then I load in a Stereo IR, and I hear the signal in Stereo again.

And duplicating an event does NOT retain the IR?

  • Add a new event.
  • Add a Convolution Reverb to it, anywhere.
  • Duplicate the event.
  • OBSERVE the IR field is blank (!).
  • I EXPECT the IR to be copied over.

I can’t find more info about expected behavior in the documentation… unlike the API, https://www.fmod.com/docs/2.02/studio/glossary.html#effect doesn’t have links to specific effect docs?

This is intended. The convolution reverb effect downmixes the signal to have the same number of channels as the IR, or to be mono, depending on whether LINK mode is enabled.

This is not unique to the convolution reverb effect. Effects that alter the channel format continue to do so even when bypassed. This is a limitation of effect bypass that exists to allow effects to be bypassed on the fly without having to destroy and recreate the associated mixer connections.

I can see why this might seem confusing. In truth, the “wet” and “dry” signals aren’t truly separate; every effect consumes a single input signal and produces a single output signal, and the “wet” and “dry” levels just determine how the input signal is processed to create the output. In a sense, there is no truly dry signal, only the possibility of a wet signal that’s very similar to the input.
In the case of effects that produce output in a channel format that differs from the channel format of their input signal, this is still true.

If you want the truly dry signal, I recommend inserting a send to the left of the convolution reverb effect. Note that, as sends add a fixed amount of latency to their signals, you may want to add a send to the right of the convolution reverb effect to carry the wet signal, so that the dry and wet signals are perfectly synchronized and do not exhibit phasing when mixed.

This sounds like a bug. I’ll add it to our bug tracker.

This sounds like another bug. I’ll add it to our tracker.

Unfortunately, we’ve yet to write that chapter of the FMOD Studio User Manual. It’s in our backlog of planned improvements, but has not yet been scheduled for writing.

For now, we recommend checking the effect reference chapter of the FMOD Engine User Manual - and of course, if there’s anything that that documentation doesn’t make clear, we’d be happy to answer any questions you have.

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@joseph Thank you for going through these finer points with your signature grace and precision. It really helps me distinguish what’s intended vs. bugs here.

The convolution reverb in FMOD is very useful, but as you explained, works differently than how someone like myself might be accustomed to one working in a DAW like Ableton.

In the meantime, I’ve been using an external wave editor to convert mono IRs to stereo (just doubling the data on a channel), so they don’t behave differently in times where I’m looking for a consistent stereo path.

As always, I hope your answers will help others that come looking with curiosity! :raised_hands: