VCV Rack 2 module bypass conventions: fine points

[gc3 (gc)]

(Moving this over from the Rack 2 announcement thread).

Quick intro: With Rack 2 coming, modules will be able to route inputs to outputs when they are bypassed (or, in probably rare cases, do more complicated things when they are bypassed). I for one am VERY happy about this feature, as I think it’s much more useful behavior in some cases than simply blocking all output, but for this feature to be widely adopted and work predictably and usefully, the right conventions will need to evolve pretty quickly.

Quick history: @Eurikon made an initial feature request and posted here, and @Vortico put forth a modified proposal here which, although nominally open, has happily made it into VCV Rack 2:

@Vortico

[A] bypassed module [in V2] just calls processBypass() instead of process(). The default implementation is to apply bypass routes configured by configBypass(AUDIO_INPUT, AUDIO_OUTPUT), but you can override it if your module bypassing is more complex.

@Vega raised an interesting question over in the V2 announcement thread:

@Vega

does this mean, in theory, a bypassed module could still provide some of its function- like a mixer, when bypassed, just doing a unity sum?

And @Vortico confirmed that it could, but he wouldn’t do it that way for a mixer:

@Vortico

processBypass() could do that, but for that example, I’d just leave it unimplemented since users would most expect silence. Think of bypassing as disabling. When you stomp on a guitar pedal to turn it off, it routes inputs to outputs. When you turn off a mixer, it outputs silence.

I asked:

@gc3

Consider VCV VCA; it seems immediately obvious that the bypass effect would be passing the input to the output at unity. Cool and useful.

So then what about VCV Mixer in bypass? I’d assume that In 1…4 would go to Ch Out 1…4 (since they’re effectively four separate VCAs) and, per your point above, the Mix Out would go silent (rather than becoming a unity mixer). OK.

But what about Audible Quad VCA, then, with (say) inputs in 1 and 2 and output 2 (but not 1) connected? Does bypassing turn off the normalization and mixing, so that In 1…4 go to Out 1…4 but there’s no longer any mixing down?

And @Vortico responded:

@Vortico

@gc3

But what about Audible Quad VCA

I wouldn’t add any bypass routes for this. If the module isn’t an “effect” in a signal chain, the user likely wants to mute/unmute the output with the bypass menu item or key command.

I schlepped this over here in case anyone was interested in following up, since I didn’t want to further extend a side discussion in an announcement thread.

Preface over, next post below:

[gc3 (gc)]

So I think this is a really interesting design question. Mixing aside, if Quad VCA doesn’t pass its individual ins to its individual outs, but instead mutes its outputs because it’s not an “effect”, would a single VCA like VCV VCA also be expected to mute its output, rather than just becoming unity? What’s the line between effect and signal chain? A VCA with a LFO over amplitude is a tremolo and I might well want to bypass the tremolo the same way I’d want to bypass a reverb. And with fancier VCA configurations like Quad VCA, or even VCA Mixer, have both individual VCA and signal chain capability.

I think where I get tripped up is that if I think of bypassing as “removing from the signal chain” it’s not automatically clear that it means “mute” versus “remove, to the extent possible, the particular processing being done by this module so that I can hear something less processed in my output.” I was interested to note that @Eurikon’s post linked above notes that Reason offers separate Bypass and Off modes, which reflects this distinction.

Happily, the API will let module developers try different things, including implementing what I would probably do in my own modules if there were an ambiguity–a right-click configuration that let users pick a more Off-like or a more Remove-Processing-like consequence of bypass. The first would almost always mute outputs, and the second would almost always apply a “neutral” process (bypass for reverbs, unity for VCAs, unity sum for mixers, etc.)

Since bypass status will (I presume) continue to be saved in V2, there is a little bit of a patch-compatibility point here in that changing how a module handles bypassing by default may change some previously saved patches.

[Eurikon (Latif)]

Thank you all for putting an effort in to this