For the past few years, there has been an increased interest in the detection of ST-segment changes in ambulatory patients. This interest has broadened the scope of application of long-term Holter ambulatory recording to include the routine monitoring of patients for episodes of asymptomatic (silent) ischemia. ST measurements derived from tape-recorded data, however, have been viewed with some skepticism, based on the poor fidelity of earlier Holter recorders and playback systems. The author focuses on the technical issues regarding accurate recording and playback of ST-segment data and demonstrates that a cassette-based recorder and playback system using "direct recording" can accurately reproduce the ST-segment. This discussion is based on the use of the impulse response (rather than frequency response) to characterize and improve the fidelity of the recorder/playback system for ST-segment changes.