Add a DSP script to slowly fade in/out

scripts/a_slow_mute.lua

@@ -0,0 +1,66 @@
+ardour {
+	["type"]    = "dsp",
+	name        = "a-Slow-Mute",
+	category    = "Amplifier",
+	license     = "MIT",
+	author      = "Ardour Team",
+	description = [[Mute button with slow fade in/out]]
+}
+
+function dsp_ioconfig ()
+	-- -1, -1 = any number of channels as long as input and output count matches
+	return { { audio_in = -1, audio_out = -1} }
+end
+
+
+function dsp_params ()
+	return { { ["type"] = "input", name = "Mute", min = 0, max = 1, default = 0, toggled = true } }
+end
+
+local cur_gain = 1
+local lpf = 0.002 -- parameter low-pass filter time-constant
+
+function dsp_init (rate)
+	lpf = 100 / rate -- interpolation time constant
+end
+
+function low_pass_filter_param (old, new, limit)
+	if math.abs (old - new) < limit  then
+		return new
+	else
+		return old + lpf * (new - old)
+	end
+end
+
+-- the DSP callback function
+-- "ins" and "outs" are http://manual.ardour.org/lua-scripting/class_reference/#C:FloatArray
+function dsp_run (ins, outs, n_samples)
+	local ctrl = CtrlPorts:array() -- get control port array
+	local target_gain = ctrl[1] > 0 and 0.0 or 1.0;  -- when muted, target_gain = 0.0; otherwise use 1.0
+	local siz = n_samples -- samples remaining to process
+	local off = 0 -- already processed samples
+	local changed = false
+
+	-- if the target gain changes, process at most 32 samples at a time,
+	-- and interpolate gain until the current settings match the target values
+	if cur_gain ~= target_gain then
+		changed = true
+		siz = 32
+	end
+
+	while n_samples > 0 do
+		if siz > n_samples then siz = n_samples end
+		if changed then
+			cur_gain = low_pass_filter_param (cur_gain, target_gain, 0.001)
+		end
+
+		for c = 1,#ins do -- process all channels
+			if ins[c] ~= outs[c] then
+				ARDOUR.DSP.copy_vector (outs[c]:offset (off), ins[c]:offset (off), siz)
+			end
+			ARDOUR.DSP.apply_gain_to_buffer (outs[c]:offset (off), siz, cur_gain);
+		end
+		n_samples = n_samples - siz
+		off = off + siz
+	end
+end