Example 6: A Differentiator

The program below inverts and differentiates the signals read by getvec and writes the results with putvec. The output is readable as record `dif'. A wide variety of simple digital filters can be modelled on this example; see section Example 7: A General-Purpose FIR Filter, for a more general approach.

1 #include <stdio.h> 2 #include <wfdb/wfdb.h> 3 4 main(argc, argv) 5 int argc; 6 char *argv[]; 7 { 8 WFDB_Siginfo *s; 9 int i, nsig, nsamp=1000; 10 WFDB_Sample *vin, *vout; 11 12 if (argc < 2) { 13 fprintf(stderr, "usage: %s record\n", argv[0]); exit(1); 14 } 15 if ((nsig = isigopen(argv[1], NULL, 0)) <= 0) exit(2); 16 s = (WFDB_Siginfo *)malloc(nsig * sizeof(WFDB_Siginfo)); 17 vin = (WFDB_Sample *)malloc(nsig * sizeof(WFDB_Sample)); 18 vout = (WFDB_Sample *)malloc(nsig * sizeof(WFDB_Sample)); 19 if (s == NULL || vin == NULL || vout == NULL) { 20 fprintf(stderr, "insufficient memory\n"); 21 exit(3); 22 } 23 if (isigopen(argv[1], s, nsig) != nsig) exit(2); 24 if (osigopen("8l", s, nsig) <= 0) exit(3); 25 while (nsamp-- > 0 && getvec(vin) > 0) { 26 for (i = 0; i < nsig; i++) 27 vout[i] -= vin[i]; 28 if (putvec(vout) < 0) break; 29 for (i = 0; i < nsig; i++) 30 vout[i] = vin[i]; 31 } 32 (void)newheader("dif"); 33 wfdbquit(); 34 exit(0); 35 }

(See http://www.physionet.org/physiotools/wfdb/examples/example6.c for a copy of this program.)

Notes:

Line 24:

Here we attempt to open as many output signals as there are input signals; if we cannot do so, the program exits after osigopen prints an error message.

Line 25:

The main loop of the program begins here. If 1000 samples can be read from each signal, the loop will end normally; if getvec fails before 1000 samples have been read, the loop ends prematurely.

Lines 26--27:

For each signal, we compute the negated first difference by subtracting the new sample from the previous sample.

Line 28:

One sample of each output signal is written here.

Lines 29--30:

The new input samples are copied into the output sample vector in preparation for the next iteration.

Line 32:

This step is optional. It creates a header file for a new record to be called `dif', which we can then open with another program if we want to read the signals that this program has written. Since the record argument for osigopen was `8l', we can also read these files using record `8l'; one reason for making a new `hea' file here is that the `hea' file for `8l' may not necessarily indicate the proper sampling frequency for these signals.

Line 33:

Since the program writes output signals, it must invoke wfdbquit to close the files properly.