This invention uses multiple codecs to efficiently achieve the right balance between compression and coverage for a given design. This application illustrates a simple example using two codecs including a high compression codec and a low compression codec. The test engineer generates a first set of test patterns using the high compression codec. If this high compression results in unacceptable fault coverage loss, the top-up patterns for additional coverage are generated using the low compression codec. The invention may use multiple codecs serially one after the other. The codecs can be of different types or parameters (such as compression ratio, debug tolerance and combinational codec versus sequential codec).