complete lua DSP filter with parameter interpolation & comments

scripts/filt.lua

@@ -6,14 +6,13 @@ ardour {
 	author      = "Robin Gareus",
 	email       = "robin@gareus.org",
 	site        = "http://gareus.org",
-	description = [[
+	description = [[Example Ardour Lua DSP Plugin]]
-	An Example DSP Plugin for processing audio, to
-	be used with Ardour's Lua scripting facility.]]
 }
 
 function dsp_ioconfig ()
 	return
 	{
+		-- allow any number of I/O as long as port-count matches
 		{ audio_in = -1, audio_out = -1},
 	}
 end
@@ -37,59 +36,111 @@ function dsp_params ()
 	}
 end
 
--- these globals are *not* shared between DSP and UI
+-- translate type parameter to DSP enum
-local filt -- the filter instance
+-- http://manual.ardour.org/lua-scripting/class_reference/#ARDOUR.DSP.Biquad.Type
-local cur = {0, 0, 0, 0} -- current settings
+function map_type (t)
-
+	if     t == 1 then
-function dsp_init (rate)
+		return ARDOUR.DSP.BiquadType.LowShelf
-	self:shmem ():allocate (1) -- shared mem to tell UI about samplerate
+	elseif t == 2 then
-	local cfg = self:shmem ():to_int (0):array ()
+		return ARDOUR.DSP.BiquadType.HighShelf
-	cfg[1] = rate
+	elseif t == 3 then
-	filt = ARDOUR.DSP.Biquad (rate) -- initialize filter
+		return ARDOUR.DSP.BiquadType.LowPass
+	elseif t == 4 then
+		return ARDOUR.DSP.BiquadType.HighPass
+	else
+		return ARDOUR.DSP.BiquadType.Peaking
+	end
 end
 
+-- these globals are *not* shared between DSP and UI
+local filt -- the biquad filter instance
+local cur = {0, 0, 0, 0} -- current parameters
+local lpf = 0.03 -- parameter low-pass filter time-constant
 
--- apply parameters, re-compute filter coefficients if needed
+function dsp_init (rate)
-function apply_params (ctrl)
+	self:shmem ():allocate (1) -- shared mem to tell the GUI the samplerate
+	local cfg = self:shmem ():to_int (0):array ()
+	cfg[1] = rate
+	-- http://manual.ardour.org/lua-scripting/class_reference/#ARDOUR:DSP:Biquad
+	filt = ARDOUR.DSP.Biquad (rate) -- initialize filter
+	lpf = 13000 / rate -- interpolation time constant
+end
+
+-- helper functions for parameter interpolation
+function param_changed (ctrl)
 	if ctrl[1] == cur[1] and ctrl[2] == cur[2] and ctrl[3] == cur[3] and ctrl[4] == cur[4] then
 		return false
 	end
+	return true
+end
 
-	local ft
+function low_pass_filter_param (old, new, limit)
-	if     ctrl[1] == 1 then
+	if math.abs (old - new) < limit  then
-		ft = ARDOUR.DSP.BiQuadType.LowShelf
+		return new
-	elseif ctrl[1] == 2 then
-		ft = ARDOUR.DSP.BiQuadType.HighShelf
-	elseif ctrl[1] == 3 then
-		ft = ARDOUR.DSP.BiQuadType.LowPass
-	elseif ctrl[1] == 4 then
-		ft = ARDOUR.DSP.BiQuadType.HighPass
 	else
-		ft = ARDOUR.DSP.BiQuadType.Peaking
+		return old + lpf * (new - old)
+	end
+end
+
+-- apply parameters, re-compute filter coefficients if needed
+function apply_params (ctrl)
+	if not param_changed (ctrl) then
+		return
 	end
 
-	-- TODO low-pass filter ctrl values, smooth transition
+	if cur[1] ~= ctrl[1] then
-	filt:compute (ft, ctrl[3], ctrl[4], ctrl[2])
+		-- reset filter state when type changes
-	cur[1] = ctrl[1]
+		filt:reset ()
-	cur[2] = ctrl[2]
+		for k = 1,4 do cur[k] = ctrl[k] end
-	cur[3] = ctrl[3]
+	else
-	cur[4] = ctrl[4]
+		-- low-pass filter ctrl parameter values, smooth transition
-	return true
+		cur[2] = low_pass_filter_param (cur[2], ctrl[2], 0.1) -- gain/dB
+		cur[3] = low_pass_filter_param (cur[3], ctrl[3], 1.0) -- freq/Hz
+		cur[4] = low_pass_filter_param (cur[4], ctrl[4], 0.01) -- quality
+	end
+
+	filt:compute (map_type (cur[1]), cur[3], cur[4], cur[2])
 end
 
 
 -- the actual DSP callback
 function dsp_run (ins, outs, n_samples)
-	if apply_params (CtrlPorts:array ()) then
+	local changed = false
-		self:queue_draw ()
+	local siz = n_samples
+	local off = 0
+
+	-- if a parameter was changed, process at most 64 samples at a time
+	-- and interpolate parameters until the current settings match
+	-- the target values
+	if param_changed (CtrlPorts:array ()) then
+		changed = true
+		siz = 64
 	end
-	for c = 1,#ins do
+
-		if ins[c]:sameinstance (outs[c]) then
+	while n_samples > 0 do
-			filt:run (ins[c], n_samples) -- in-place
+		if changed then apply_params (CtrlPorts:array ()) end
-		else
+		if siz > n_samples then siz = n_samples end
-			ARDOUR.DSP.copy_vector (outs[c], ins[c], n_samples)
+
-			filt:run (outs[c], n_samples)
+		-- process all channels
+		for c = 1,#ins do
+			-- check if output and input buffers for this channel are identical
+			-- http://manual.ardour.org/lua-scripting/class_reference/#C:FloatArray
+			if ins[c]:sameinstance (outs[c]) then
+				filt:run (ins[c]:offset (off), siz) -- in-place processing
+			else
+				-- http://manual.ardour.org/lua-scripting/class_reference/#ARDOUR:DSP
+				ARDOUR.DSP.copy_vector (outs[c]:offset (off), ins[c]:offset (off), siz)
+				filt:run (outs[c]:offset (off), siz)
+			end
 		end
+
+		n_samples = n_samples - siz
+		off = off + siz
+	end
+
+	if changed then
+		-- notify display
+		self:queue_draw ()
 	end
 end
 
@@ -102,20 +153,24 @@ function round (n)
 end
 
 function freq_at_x (x, w)
+	-- x-axis pixel for given freq, power-scale
 	return 20 * 1000 ^ (x / w)
 end
 
 function x_at_freq (f, w)
+	-- frequency at given x-axis pixel
 	return w * math.log (f / 20.0) / math.log (1000.0)
 end
 
 function db_to_y (db, h)
+	-- y-axis gain mapping
 	if db < -20 then db = -20 end
 	if db >  20 then db =  20 end
 	return -.5 + 0.5 * h * (1 - db / 20)
 end
 
 function grid_db (ctx, w, h, db)
+	-- draw horizontal grid line
 	local y = -.5 + round (db_to_y (db, h))
 	ctx:move_to (0, y)
 	ctx:line_to (w, y)
@@ -123,6 +178,7 @@ function grid_db (ctx, w, h, db)
 end
 
 function grid_freq (ctx, w, h, f)
+	-- draw vertical grid line
 	local x = -.5 + round (x_at_freq (f, w))
 	ctx:move_to (x, 0)
 	ctx:line_to (x, h)
@@ -131,32 +187,43 @@ end
 
 function render_inline (ctx, w, max_h)
 	if not filt then
-		-- instantiate filter (to calculate the transfer function)
+		-- the GUI is separate from the DSP, but the GUI needs to know
+		-- the sample-rate that the DSP is using.
 		local shmem = self:shmem () -- get shared memory region
 		local cfg = shmem:to_int (0):array () -- "cast" into lua-table
+		-- instantiate filter (to calculate the transfer function's response)
 		filt = ARDOUR.DSP.Biquad (cfg[1])
 	end
 
-	apply_params (CtrlPorts:array ())
+	-- set filter coefficients if they have changed
+	if param_changed (CtrlPorts:array ()) then
+		local ctrl = CtrlPorts:array ()
+		for k = 1,4 do cur[k] = ctrl[k] end
+		filt:compute (map_type (cur[1]), cur[3], cur[4], cur[2])
+	end
 
 	-- calc height of inline display
-	local h = math.ceil (w * 10 / 16) -- 16:10 aspect
+	local h = math.ceil (w * 10 / 16) -- use 16:10 aspect
-	h = 2 * round (h / 2) -- even number of vertical px
+	h = 2 * round (h / 2) -- with an even number of vertical pixels
-	if (h > max_h) then
+	if (h > max_h) then h = max_h end -- but at most max-height
-		h = max_h
+
-	end
+	-- ctx is a http://cairographics.org/ context
+	-- http://manual.ardour.org/lua-scripting/class_reference/#Cairo:Context
 
 	-- clear background
 	ctx:rectangle (0, 0, w, h)
 	ctx:set_source_rgba (.2, .2, .2, 1.0)
 	ctx:fill ()
 
+	-- set line width: 1px
+	-- Note: a cairo pixel at [1,1]  spans [0.5->1.5 , 0.5->1.5]
+	-- hence the offset -0.5 in various move_to(), line_to() calls
 	ctx:set_line_width (1.0)
 
 	-- draw grid
 	local dash3 = C.DoubleVector ()
 	dash3:add ({1, 3})
-	ctx:set_dash (dash3, 2)
+	ctx:set_dash (dash3, 2) -- dotted line
 	ctx:set_source_rgba (.5, .5, .5, .5)
 	grid_db (ctx, w, h, 0)
 	grid_db (ctx, w, h, 6)
@@ -170,7 +237,7 @@ function render_inline (ctx, w, max_h)
 	grid_freq (ctx, w, h, 10000)
 	ctx:unset_dash ()
 
-	-- draw transfer function
+	-- draw transfer function line
 	ctx:set_source_rgba (.8, .8, .8, 1.0)
 	ctx:move_to (-.5, db_to_y (filt:dB_at_freq (freq_at_x (0, w)), h))
 	for x = 1,w do