The invention relates to a method and system for use in assessing the quality and integrity of a data transmission path or link between a data transmitting location and at least one receiving location at which a broadcast data receiver is located with means to allow an error rate calculation to be made with respect to a known data signal sequence which is inserted into the transmitted signal. On the basis of the calculations made and sent to the transmitting location a particular data modulation scheme is adopted to provide the most efficient data transmission method for each receiving location. The invention is of particular use in cable data transmission networks of the type which for example allow television channels and other services to be provided to a plurality of receiving locations.