Abstract: A touch sensing apparatus is controlled to determine the position of one or more objects (7) that interact with a touch surface (1). The apparatus includes a group of emitters (2) arranged to emit light to illuminate at least part of the touch surface (1), a light detector (4) arranged to receive light from the group of emitters (2), and a processing element (7). Each emitter (2) is controlled to transmit a code by way of the emitted light such that the code identifies the respective emitter (2). The codes may at least partly be transmitted concurrently. The codes may be selected such that a value of an autocorrelation of each code is significantly higher than a value of a cross-correlation between any two codes of different emitters (2).