Signature based error detection techniques (e.g. the application of watchdog processors) can be easily extended to support software debugging. The run-time sequence of signatures is stored in an extension of the traditional checker. As the signatures identify the states of the program, a trace of the statements executed by the checked processor is available. The signature buffer can be efficiently utilized if the signature sequence is compressed. In the paper, two realtime compression methods are presented and compared. The general method uses predefined dictionaries, while the other one utilizes the structural information encoded in the signatures.