So what’s happening here? When he says that the DC offset is being slowly “cancelled” out by the sound card, he actually means that the output capacitor I mentioned above was being charged by the oscilloscope input impedance. The easy, brute force (but often cheapest) way of solving this problem is to throw the biggest, cheapest output capacitor in series on the output which won’t filter out too much of the low end of the audio spectrum when you form the RC high pass filter with the impedance of your headphones (or audio load). It all boils down to having a significant DC offset on the output, which can damage headphones and other loads connected to it. Typically your el-cheapo sound card will probably use cheap high voltage offset op amps (which turns into something significant). Going to be honest.if you knew anything about sound card outputs, you’d realize that he didn’t quite figure out what was going on. Posted in digital audio hacks, Tool Hacks Tagged DC offset, signal generator Post navigation also notes that though Audacity is a cheap free way to generate simple signals, any number of complex signals can be generated using MATLAB if you happen to own a copy. A schematic for his offset circuit is available on his site, should you wish to build one of your own. Using a LM358 as a non-inverting summing amplifier, he was able to apply a steady DC offset and generate usable signals for his micro controller projects. Upon plugging his scope into his sound card’s audio jack, he observed that the PC had good frequency fidelity, though it required an additional DC offset as most cards are built to remove that offset from the waveform. He found that the tone generator built into Audacity was quite useful, at least for generating waveforms at less than 20 KHz or so. He found that the easiest and cheapest way to get clean, reliable signaling was by using something that was already sitting on his desk – his PC. Needed a simple signal generator for a project he was working on, but didn’t have one handy.
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