An optical measurement apparatus comprising a data acquisition system and arranged to measure, when in use, a physiological characteristic of a part of a body, the data acquisition system comprising: a signal sample trigger apparatus comprising: an input; a processing resource coupled to the input; and an output coupled to the processing resource; wherein the processing resource is arranged to generate, when in use, a trigger signal in response to location increment information, the location increment information comprising digital pulses and each digital pulse has a rising edge and a falling edge; the location increment information corresponds to a repeating predetermined travel distance and is received via the input; the output is arranged to provide the trigger signal for triggering execution of a sample of a measurement signal, the trigger signal comprising a repeating series of pulses associated with the location increment information, including a first pulse and a second pulse; the processing resource is arranged to generate the first pulse and the second pulse in response to the rising edge of the each digital pulse and the falling edge of the each digital pulse; and a data sampling module coupled to the output, the data sample module being arranged to sample the measurement signal in response to a characteristic of the trigger signal. a signal sample trigger apparatus comprising: an input; a processing resource coupled to the input; and an output coupled to the processing resource; wherein the processing resource is arranged to generate, when in use, a trigger signal in response to location increment information, the location increment information comprising digital pulses and each digital pulse has a rising edge and a falling edge; the location increment information corresponds to a repeating predetermined travel distance and is received via the input; the output is arranged to provide the trigger signal for triggering execution of a sample of a measurement signal, the trigger signal comprising a repeating series of pulses associated with the location increment information, including a first pulse and a second pulse; the processing resource is arranged to generate the first pulse and the second pulse in response to the rising edge of the each digital pulse and the falling edge of the each digital pulse; and an input; a processing resource coupled to the input; and an output coupled to the processing resource; wherein the processing resource is arranged to generate, when in use, a trigger signal in response to location increment information, the location increment information comprising digital pulses and each digital pulse has a rising edge and a falling edge; the location increment information corresponds to a repeating predetermined travel distance and is received via the input; the output is arranged to provide the trigger signal for triggering execution of a sample of a measurement signal, the trigger signal comprising a repeating series of pulses associated with the location increment information, including a first pulse and a second pulse; the processing resource is arranged to generate the first pulse and the second pulse in response to the rising edge of the each digital pulse and the falling edge of the each digital pulse; and