Faust VST/AU plugins - report latency to host?

[matthewjumpsoffbuildings]

I'm very new to Faust, just testing it out currently. One thing I have noticed is if I use the faust2au or faust2vst commands to generate plugins (an amazing feature btw) - there doesn't appear to be a way to report the introduced latency to the host/DAW?

For example, if I am using a lookahead compressor/limiter which introduces n samples delay to the signal, based on the lookahead seconds and the sample rate. There doesn't appear to be a way to get the generated VST/AU plugin to report this to the host? Unless I am missing it in the docs.

[sletz]

I guess this can not be done automatically in the general case. How do VST plugin developers do that in general?

[matthewjumpsoffbuildings]

I had a quick look - in case of the VST3 SDK this seems to do it

https://steinbergmedia.github.io/vst3_doc/vstinterfaces/classSteinberg_1_1Vst_1_1IAudioProcessor.html#af8884671ccefe68e0a86e72413a0fcf8

In the case of AU, kAudioUnitProperty_Latency seems to do it

https://developer.apple.com/library/archive/documentation/MusicAudio/Conceptual/AudioUnitProgrammingGuide/TheAudioUnit/TheAudioUnit.html#//apple_ref/doc/uid/TP40003278-CH12-SW20

[matthewjumpsoffbuildings]

So given that there isnt a solution for this already, perhaps would make sense to have some kind of native function reported_latency(int samples) or something like that, that the compiler could then generate the relevant platform specific code? This would work differently for different targets, eg faust2au, faust2vst, faust2juce etc?

[sletz]

Thanks for investigating. My point is that if the compiler cannot know the latency automatically, there is no point in adding a new function AFAICS. We may better have a new option like -latency <num> in some faust2xx tools to setup the underlying getLatencySamples or kAudioUnitProperty_Latency code depending of the used model.

[matthewjumpsoffbuildings]

Oh the compiler flag option would certainly be the simplest option for plugins that introduce a fixed amount of latency.

However, the reason I suggested using a function is that AU/VST etc support plugins changing their reported latency in response to settings changes, so I would say that you might want to do something like:

d = vslider("Delay",0,0,256,1);
process = de.delay(d,d) : attach(_, reported_latency(d));

[kmatheussen]

I want to chime in that I've thought about Faust needing a "reported_latency" function too, approximately the same way @matthewjumpsoffbuildings described above. In Radium I'm using Faust for various filters and other things, and I've gotten the impression from users that it's somewhat of a problem for certain types of music when latency is not compensated.

[sletz]

We could possibly prototype this using the ffunction primitive way in a very "hackish" way, like in the following code:

import("stdfaust.lib");

reported_latency = ffunction(float reported_latency(float), "latency.h", "");
d = vslider("Delay",0,0,256,1);
process = de.delay(d,d) : attach(_, reported_latency(d));

Then having the architecture file implement the C reported_latency function.

[magnetophon]

Is there any news on this?

I'd very much appreciate if this could move forward. Even if it's "just" the compiler flag that would be great.

[trummerschlunk]

+1 for the 'in-dsp' solution reported_latency(int samples)

I think I saw some plugins even change the reported latency dynamically, depending on settings (e.g. look-ahead in compressors).

[sletz]

Nothing new.