MITIGATING DISTORTION IN CODED LIGHT
FIG17 is a schematically illustrates the timing of example set actions that generate an LED current  FIG18 is a schematic block diagram of a control scheme of an output converter with coded light modulator
In the presently-described example use-case, the communication speed of coded light is 2000 symbols per second. Every symbol should be set for a period of 500 microseconds. The coded light modulator 23, 24 is activated every 250 microseconds to modulate the next half coded light symbol onto the LED current ILED. Two set-points are required to generate one coded light symbol (the Xs in FIG17). In this example implementation a symbol is defined by two LED current target values which should be set with an interval of 250µs. A coded light symbol is sampled twice (the dots in FIG17). A set-action is followed by a measurement-action. The measured LED current ILEDf contains the coded light symbol that was last set. Note however that this two-set point example is not limiting and other embodiments may be implemented based on only a single set-point per symbol, or indeed more set-points. With one point there is only information on the level of a given symbol. With two points there is information about the slope, while three or more points provide information on curvature. In other words, more points gives more feedback information for keeping the current level steady. Two points per symbol (level) gives good enough results. Increasing the number of points improves results but at the cost of increasing the rate that the coded Light task scheduler, i.e. sampler, has to run.  FIG18 shows the solution in the control scheme of the output converter according to embodiments of the present disclosure. The coded light injector 24 (including the encoder and scaler 60) generates the LED current level Idata that belongs to the coded light symbol to be set. The adaptive filter comprises a "residue controller" 70 which uses this information to generate a symbol level ILED' that is expected to be present in the measured LED current ILEDf. The adaptive filter 34 further comprises a (second) subtractor 65 arranged to subtract this generated signal ILED' from the measured LED current ILEDf before it is fed back to the (first) subtractor 33 in the main current control feedback loop.
schematic, schematic block diagram
22
controller
62
actuator
64
block
250
is activated every
70
residue controller residue controllers
500
about
26
power circuitry
33
subtractor
65
subtractor
23
adder
32
filter
['1. Apparatus (16) for controlling a current flowing through an illumination source, the apparatus (16) comprising: current measurement circuitry (28) arranged to take a current measurement (I LEDf ) measuring the current flowing through the illumination source (I LED ); a current controller (22) configured to output a current control signal (I ctrl ) to control the current flowing through the illumination source, and thereby control an amount of illumination emitted by the illumination source; power circuitry (26) arranged to receive the current control signal (I ctrl ) and in dependence thereon to drive the current flowing through the illumination source, the current controller (22) being configured to adjust the current control signal (I ctrl ) based on an error signal (I err ) representing a comparison between an input set-point (I ref ) and the current measurement fed back from the current measurement circuitry (28), order to thereby keep said current flowing through the illumination source on average substantially steady at a level specified by said input set-point (I ref );  characterized in that  the apparatus (16) further comprises: a coded light injector (24) arranged to modulate the current control signal (I ctrl ) output by the current controller (22) in order to cause a sequence of data symbols to be modulated into the current flowing through the illumination source, and to thereby cause the sequence of data symbols to be embedded in the emitted illumination; and a compensation filter (34) arranged to receive an indication of a presently transmitted data symbol (I data ) and apply a compensation (I af ) to the error signal (I err ) based thereon, to at least partially remove an effect of a distortion of the current measurement caused by the modulation of the data symbols into said current flowing through the illumination source (I LED ) by means of the compensation (I af ).']