Presenting Signals to the Analyzer

Two distinctly different types of tests are possible. If the device can accept digital inputs, the reference signals can be supplied in that form (perhaps after resampling with xform to convert the digitized samples to the expected sampling frequency and numerical range, and possibly with additional digital signal processing to simulate the signal conditioning normally performed by the device's front-end data acquisition hardware). The primary advantage of testing in the digital domain is that the test is (or should be) strictly reproducible, since no noise or additional quantization error can be introduced in this way. This method usually avoids the issue of synchronization of the test annotations with the reference signals discussed below.

Testing in the analog domain requires that analog signals be recreated from the digital signals. (It should be noted that even the analog versions of the MIT and AHA databases that have been available in the past were recreated from the digitized signals by the database developers.) The advantage of this approach is that it exercises the entire system, including the front-end data acquisition hardware. It is often difficult, however, to establish synchronization between the signal source and the analyzer, needed in order to permit comparisons of annotations. One way of dealing with this problem is to arrange for the analyzer's sampling clock to trigger the digital-to-analog converter used to recreate the analog signals, or to arrange for an external clock to trigger both D/A conversion in the playback system and A/D conversion in the analyzer. Another method is to begin and end the signal generation process by delivering signals from the analyzer to the playback device, and recording the analyzer's clock time at the times of the signals; assuming that both the analyzer and the playback device have stable clocks, event times in the analyzer's frame of reference can be converted to database sample numbers by linear interpolation. The WFDB software package includes a program (sample) that uses a Microstar DAP 2400-series analog interface board4and an MS-DOS PC to recreate analog signals from digital database records on CD-ROMs or magnetic disk files.

Up: WFDB Applications Guide


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