A typical radio frame (300) comprises A, B, and C vocoded bits (304). At the end of each frame (300) A and B bits (305) are inserted from a previous frame. Thus, each frame not only comprises A, B, and C bits (304) for that frame, but also comprises those A and B bits (305) originally transmitted in a prior frame. Thus, each frame comprises high and low priority vocoded bits (304) from the current vocoder frame, and those higher priority bits from a preceding frame (305). By placing an inner CRC (302, 303) around the most important bits of the vocoded frame, even though a frame is erased (e.g. its outer CRC (301) failed) it can still be verified that the most important bits in the vocoded frame are correct. Since the class B and C bits can tolerate some errors, the vocoded frame can then play out if its inner CRC passes.