Prefer using C++ DSP for noise generation in scripts

This significantly reduces the DSP load of the noise
generator (avg. process time is reduced by a factor of 8 or more).

share/scripts/a-noise.lua

@@ -0,0 +1,55 @@
+ardour {
+	["type"]    = "dsp",
+	name        = "a-Noise",
+	category    = "Utility",
+	license     = "MIT",
+	author      = "Ardour Team",
+	description = [[Noise Generator]]
+}
+
+function dsp_ioconfig ()
+	return
+	{
+		-- -1, -1 = any number of channels as long as input and output count matches
+		{ audio_in = -1, audio_out = -1},
+	}
+end
+
+function dsp_params ()
+	return
+	{
+		{ ["type"] = "input", name = "Noise Level", min = -80, max = 0, default = -18, unit="dBFS"},
+		{ ["type"] = "input", name = "Noise Type", min = 0, max = 2, default = 0, enum = true, scalepoints =
+			{
+				["White Noise"] = ARDOUR.DSP.NoiseType.UniformWhiteNoise,
+				["Gaussian White Noise"] = ARDOUR.DSP.NoiseType.GaussianWhiteNoise,
+				["Pink Noise"] = ARDOUR.DSP.NoiseType.PinkNoise,
+			}
+		},
+	}
+end
+
+local cmem  = nil
+local gen   = nil
+local noise = 0
+
+function dsp_init (rate)
+	cmem = ARDOUR.DSP.DspShm (8192)
+	gen = ARDOUR.DSP.Generator ()
+end
+
+function dsp_run (ins, outs, n_samples)
+	local ctrl = CtrlPorts:array()
+	local lvl = ARDOUR.DSP.dB_to_coefficient (ctrl[1])
+	if (noise ~= ctrl[2]) then
+		noise = ctrl[2]
+		gen:set_type (noise)
+	end
+	for c = 1,#ins do
+		if ins[c] ~= outs[c] then
+			ARDOUR.DSP.copy_vector (outs[c], ins[c], n_samples)
+		end
+		gen:run (cmem:to_float(0), n_samples)
+		ARDOUR.DSP.mix_buffers_with_gain (outs[c], cmem:to_float(0), n_samples, lvl)
+	end
+end