zerocrossingclockrecovery.lua

---
-- Generate a real-valued clock signal from zero-crossings in a data signal.
-- This clock signal can then be used to sample the data signal with a
-- [`SamplerBlock`](#samplerblock).
--
-- $$ y[n] = \text{ZC}(x[n], \text{baudrate}, \text{threshold}) $$
--
-- @category Carrier and Clock Recovery
-- @block ZeroCrossingClockRecoveryBlock
-- @tparam number baudrate Baudrate in symbols per second
-- @tparam[opt=0.0] number threshold Zero-crossing threshold
--
-- @signature in:Float32 > out:Float32
--
-- @usage
-- -- Zero-crossing clock recovery of 1200 baudrate data signal
-- local clock_recoverer = radio.ZeroCrossingClockRecoveryBlock(1200)
-- top:connect(src, clock_recoverer)
-- top:connect(src, 'out', sampler, 'data')
-- top:connect(clock_recoverer, 'out', sampler, 'clock')
-- top:connect(sampler, snk)

local block = require('radio.core.block')
local types = require('radio.types')

local ZeroCrossingClockRecoveryBlock = block.factory("ZeroCrossingClockRecoveryBlock")

function ZeroCrossingClockRecoveryBlock:instantiate(baudrate, threshold)
    self.baudrate = assert(baudrate, "Missing argument #1 (baudrate)")
    self.threshold = threshold or 0.0

    self:add_type_signature({block.Input("in", types.Float32)}, {block.Output("out", types.Float32)})
end

function ZeroCrossingClockRecoveryBlock:initialize()
    self.hysteresis = false
    self.symbol_period = self:get_rate() / self.baudrate
    self.sample_offset = self.symbol_period

    self.out = types.Float32.vector()
end

function ZeroCrossingClockRecoveryBlock:process(x)
    local out = self.out:resize(x.length)

    for i = 0, x.length-1 do
        -- If we detect a zero crossing, adjust our sample offset to half of a
        -- symbol period
        if self.hysteresis == false and x.data[i].value > self.threshold then
            self.hysteresis = true
            self.sample_offset = self.symbol_period/2
        elseif self.hysteresis == true and x.data[i].value < self.threshold then
            self.hysteresis = false
            self.sample_offset = self.symbol_period/2
        end

        -- Count down to our sample offset
        self.sample_offset = self.sample_offset - 1

        -- If we've reached our sample point, generate a +1 pulse
        if self.sample_offset < 1 then
            out.data[i].value = 1

            -- Increase our sample offset by a symbol period
            self.sample_offset = self.sample_offset + self.symbol_period
        else
            -- Revert to -1 pulse
            out.data[i].value = -1
        end
    end

    return out
end

return ZeroCrossingClockRecoveryBlock