Scintillator-based gamma-ray detectors convert gamma-ray photons into a burst of scintillation photons, and then into a pulse-shaped electrical signal. By digitizing the pulse waveform, analyses that require information about the shape of the pulse can be performed, such as pulse-shape discrimination, pile-up detection and maximum-likelihood event-parameter estimation of position, energy and time. We have developed an analog-to-digital conversion (ADC) method that hugely reduces the complexity of the data-acquisition (DAQ) system while retaining pulse-shape information, and increases the amount of information that can be extracted from detected gamma rays compared to analog methods. The new DAQ system is based on a modified 2-bit sigma-delta modulator (SDM), in which the possible outputs (00, 01, 10 and 11) are decoded in such a way that they don't necessarily maintain a linear relationship between them. This makes it possible to optimize the SDM algorithm for different characteristic pulse shapes in order to extract as much information as possible. The optimization method that we present in this work is scintillation-crystal specific, but the use of the ADC method is not limited to gamma-ray detection.