PC_SIGNAL_GENERATOR

Published:2009/7/12 22:20:00 Author:May | From:SeekIC

This circuit will produce audio from your PC. The trick is U-the ASCⅡ character U, that is.The hexadecimal value of U is 55, which in binary is 01010101 (with 8 data bits and no parity, or 7 data bits and even parity). The RS232 protocol specifies that the bits of an ASCII character are tran-mitted from least to most significant, preceded by a start bit (always 0) and followed by a stop bit (always 1). So, after adding the requisite start and stop bits, the result is 1010101010. Now, suppose a string of us is generated at the serial port at some steady rate. The result is a continuous series of alternating is and 0s-a square wave. The frequency of the signal will be half the baud rate, which by definition is the number of transitions per second. Each cycle of a square wave comprises two transitions, so, for example, a 9600-bps baud rate produces a 4800-Hz square wave. In practical terms, just about any computer should be able to deliver frequencies of 55, 150, 300, 600, 1200, 2400, and 4800 Hz, corresponding to the standard baud rates from 110 to 9600. The output of a serial port is nominally 24 V p-p, which is much too high a voltage to feed to the input of an audio amplifier. The circuit attenuates the signal to a more useful level, a variable 2-V p-p. The circuit also protects the computer from static electricity and voltage surges. Capacitor C1, a nonpolar unit, blocks dc because the serial port, when idling, outputs approximately -12 V.



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