blocks/signal/agc: add AGCBlock

resolves #10.

radio/blocks/init.lua

@@ -84,6 +84,7 @@ return {
     --- Miscellaneous
     ThrottleBlock = require('radio.blocks.signal.throttle'),
     PowerSquelchBlock = require('radio.blocks.signal.powersquelch'),
+    AGCBlock = require('radio.blocks.signal.agc'),
 
     -- Protocol Blocks
     --- RDS

radio/blocks/signal/agc.lua

@@ -0,0 +1,117 @@
+---
+-- Apply automatic gain to a real or complex valued signal to maintain a target
+-- power.
+--
+-- $$ y[n] = \text{AGC}(x[n], \text{mode}, \text{target}, \text{threshold}) $$
+--
+-- Implementation note: this is a feedforward AGC. The `power_tau` time
+-- constant controls the moving average of the power estimator. The `gain_tau`
+-- time constant controls the speed of the gain adjustment. The gain has
+-- symmetric attack and decay dynamics.
+--
+-- @category Miscellaneous
+-- @block AGCBlock
+-- @tparam mode Mode, choice of "fast", "slow", "custom"
+-- @tparam[opt=-35] number target Target power in dBFS
+-- @tparam[opt=-75] number threshold Threshold power in dBFS
+-- @tparam[opt={}] table options Additional options, specifying:
+--                               * `gain_tau` (number, default 0.1 seconds for
+--                                  fast and 3.0 seconds for slow)
+--                               * `power_tau` (number, default 1.0 seconds)
+-- @signature in:Float32 > out:Float32
+-- @signature in:ComplexFloat32 > out:ComplexFloat32
+--
+-- @usage
+-- -- Automatic gain control with fast gain
+-- local agc = radio.AGCBlock('fast')
+--
+-- -- Automatic gain control with slow gain, -20 dbFS target
+-- local agc = radio.AGCBlock('slow', -20)
+--
+-- -- Automatic gain control with custom time constant, -30 dbFS target, -100 dbFS threshold
+-- local agc = radio.AGCBlock('custom', -30, -100, {gain_tau = 0.5})
+
+local math = require('math')
+
+local block = require('radio.core.block')
+local types = require('radio.types')
+
+local AGCBlock = block.factory("AGCBlock")
+
+function AGCBlock:instantiate(mode, target, threshold, options)
+    self.mode = assert(mode, "Missing argument #1 (mode), can be \"fast\", \"slow\", or \"custom\"")
+    self.target = target or -35
+    self.threshold = threshold or -75
+    self.options = options or {}
+
+    self.gain_tau = ({fast = 0.1, slow = 3.0})[self.mode] or self.options.gain_tau
+    self.power_tau = self.options.power_tau or 1.0
+
+    assert(self.mode == "fast" or self.mode == "slow" or self.mode == "custom", string.format("Invalid mode \"%s\"", tostring(mode)))
+    assert(self.gain_tau, "Missing gain_tau parameter for \"custom\" mode")
+
+    self:add_type_signature({block.Input("in", types.Float32)}, {block.Output("out", types.Float32)}, self.process_real)
+    self:add_type_signature({block.Input("in", types.ComplexFloat32)}, {block.Output("out", types.ComplexFloat32)}, self.process_complex)
+end
+
+function AGCBlock:initialize()
+    -- Compute normalized alpha for power estimator
+    self.power_alpha = 1/(1 + self.power_tau*self:get_rate())
+    -- Compute normalized alpha for gain filter
+    self.gain_alpha = 1/(1 + self.gain_tau*self:get_rate())
+    -- Initialize average power and gain state
+    self.average_power, self.gain = 0.0, 0.0
+    -- Linearize logarithmic power target
+    self.target = 10^(self.target/10)
+    -- Linearize logarithmic power threshold
+    self.threshold = 10^(self.threshold/10)
+
+    self.out = self:get_input_type().vector()
+end
+
+function AGCBlock:process_real(x)
+    local out = self.out:resize(x.length)
+
+    for i = 0, x.length-1 do
+        -- Estimate average power
+        self.average_power = (1 - self.power_alpha)*self.average_power + self.power_alpha*(x.data[i].value*x.data[i].value)
+
+        if self.average_power >= self.threshold then
+            -- Compute filtered gain
+            self.gain = (1 - self.gain_alpha)*self.gain + self.gain_alpha*(self.target*(1/self.average_power))
+            -- Apply sqrt gain
+            out.data[i].value = math.sqrt(self.gain)*x.data[i].value
+        else
+            -- Pass through without gain
+            out.data[i].value = x.data[i].value
+        end
+    end
+
+    return out
+end
+
+function AGCBlock:process_complex(x)
+    local out = self.out:resize(x.length)
+
+    for i = 0, x.length-1 do
+        -- Estimate average power
+        self.average_power = (1 - self.power_alpha)*self.average_power + self.power_alpha*x.data[i]:abs_squared()
+
+        if self.average_power >= self.threshold then
+            -- Compute filtered gain
+            self.gain = (1 - self.gain_alpha)*self.gain + self.gain_alpha*(self.target*(1/self.average_power))
+            -- Apply sqrt gain
+            local gain = math.sqrt(self.gain)
+            out.data[i].real = gain*x.data[i].real
+            out.data[i].imag = gain*x.data[i].imag
+        else
+            -- Pass through without gain
+            out.data[i].real = x.data[i].real
+            out.data[i].imag = x.data[i].imag
+        end
+    end
+
+    return out
+end
+
+return AGCBlock