MITIGATING DISTORTION IN CODED LIGHT
FIG16 is a schematic illustration of a feed forward solution to suppress a coded light pulse in the measured LED current
To solve this problem the coded light symbols in the measured LED current ILEDf should be suppressed. The following describes an example feed forward solution to implement this. As shown in FIG16, the idea is to generate a coded light symbol ILED' that is similar in shape to the coded light symbol in the measured LED current ILEDf and subtract this from the measured LED current. The shape of a measured coded light pulse is not rectangular as shown in FIG16 but deformed due to hardware filters and delays in the signal path, other disturbances, and because the feedback loop contains a low pass hardware filter 32 (e.g. 1592 Hz). The solid lines in the circle on the upper left-hand side of FIG16 represent a coded light symbol in the measured LED current when the LED current is not filtered and before it is sampled. The dotted lines represent a coded light symbol when the LED current is filtered but not yet sampled. In embodiments the LED current will be filtered. The two variants shown by the dotted and solid lines are shown to indicate that the coded light symbol in the measured LED current is deformed (dotted line) and, as a result, it is not easy to generate a replica (lower left side circle, which shows the coded light LED current only, so without the DC component in LED current).
schematic
24
coded light injector
22
controller
62
actuator
64
block
20
software domain modulator
34
filter
56
data source
16
driver
26
power circuitry
65
subtractor
23
adder
60
multiplier scaler
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 ).', '13. The apparatus (16) of any preceding claim, wherein the current controller (22) and coded light injector (24) are implemented in software arranged to run on one or more processing units of an LED driver, whilst said power circuitry (26) is implemented in dedicated hardware circuitry.']