1. Receiver-side apparatus for a transmission link, the link using different transport formats for blocks of data, and using convolutional coding with additional error detection code information, for transmitting the blocks to the receiver side, the apparatus having: a decoder for performing a trellis decode and tracebacks of one of the received coded blocks, to produce decoded data candidates, an error detection element for carrying out one or more speculative error detections on the candidates, assuming part of each candidate is the additional error detection code, and part is the block of data, and a format detector for determining a transport format based on a result of the speculative error detections, the apparatus being arranged to determine a probability of a candidate being valid, and prioritise carrying out the tracebacks or the speculative error detections according to the probabilities.