Bug 105806

I'm not familiar with run_adding(), except for what I just read from ladspa.h, and yes, it looks like it could be used for controlling the wet/dry level, kind of, but it's not very good for that purpose, because the dry signal will always play at 100% volume and the wet signal is just added to it.

I don't know why you don't hear any difference between run() and run_adding(). Maybe the filter that you tried doesn't implement run_adding() correctly?

Now I realized that you don't copy the input data to the output buffers before you call run_adding(). That means that the output is added to silent buffers, which is indistinguishable from using run(). run_adding_gain should have effect on the output volume, though.

I think using run_adding for the wet/dry level works well because merging the input and output is done by the plugin directly. Currently I'm merging them using module-loopback, but there're latency problems and probably redundant resampling: https://bugs.freedesktop.org/show_bug.cgi?id=105504

There's the (probably generated) source code of the lowpass filter: https://pastebin.com/tGynMJ1v
If I understand it correctly, a buffer_write function is defined differently for run and run adding.

I've got the same volume with run_adding_gain set to 0.1 and 10 for any reason.

Created attachment 138473 [details] [review]
attempted wetness

To find out how to copy the input samples to the output, I tried to add a simple "wet" parameter, see the attached patch. I thought after calling run I can simply copy the input arrays contents into the output arrays assuming input_count is the same as output_count and the cth input array belongs to the cth output array. But it didn't work: Regardless the wet value, it always sounded completely wet. Instead of mixing it with input, a time delay was added when playing sound.

How can I copy the input to the output?

Maybe the problem is that u->input and u->output are actually the same array? If LADSPA_IS_INPLACE_BROKEN() returns false, then they are the same array.

I've tested it with the noise.so ladspa plugin: 
pactl load-module module-ladspa-sink sink_master=0 sink_name=myt plugin=/usr/lib/ladspa/noise.so label=noise_white control=1 run_adding_gain=0.0001 && pacmd set-default-sink myt
When using run_adding, it did not sound like white noise but like some noise starting over and over. When playing some music, it is not added to the strange sound. It sounds different if I change the run_adding_gain or play music.

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