[LAU] LADSPA processing time for audio zeroes

Ken Restivo ken at restivo.org
Sat Mar 28 19:38:11 EDT 2009

On Sat, Mar 28, 2009 at 05:26:23PM +0100, Arnold Krille wrote:
> Hi,
> On Saturday 28 March 2009 16:39:35 Ken Restivo wrote:
> > When a LADSPA plugin is being sent zeroes-- i.e. when the volume of the
> > input data is down to nothing-- is it still sucking up CPU cycles?
> > I understand from the JACK API that anytime a client gets a callback with
> > data, it has to drop down and deal with the data and then return, even if
> > the data is zero. But that could be lightweight, if the data are zero, or
> > is that very compute-intensive for the plugin?
> > I'm trying to build an outboard effects chain in various LADSPA hosts (JACK
> > Rack, ecasound, AMS, others... haven't settled on one yet), and when I've
> > got that plugin's volume MIDI'ed down to zero, I'd like it to not be
> > dominating CPU cycles at that time.
> Actually with double- (or any floating point-) resolution it is rather hard to 
> detect exact 0.0 values. And because of the variable exponent, the cpu usage 
> for numbers near zero is higher then for numbers near 1 (or any other higher 
> value). The keywords to search for are "denormal problems".
> In practice this means that while setting a volume to zero "should" give you 
> the desired effect of less cpu-usage, in fact the opposite is the case in the 
> worst case. When you are already controlling your effects via midi, why not 
> bind the "mute" or "active" (don't remember the current naming in jack-rack) 
> to a midi controller. An in-active effect shouldn't eat cpu cycles when the 
> host-author did his job right...

That's the reason why I was asking.

There doesn't appear to be a way to bind the "Enable" button in Jack-Rack to  a MIDI controller. If there were, I'd just use jack-rack and be done with the problem. But right-clicking on the button does nothing; only the faders appear to support MIDI mapping.

So I was instead binding the Wet/Dry to a controller, but the processing on all the plugins enabled simultaneously was too CPU-bound.

Perhaps the question I should ask is: how hard would it be to hack jack-rack to allow MIDI binding to the Enable button?


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