> This means the frequencies emitted are very high (5 samples per period is 19.2 kHz) > and it seems the audio output is being low pass filtered resulting in si

> This means the frequencies emitted are very high (5 samples per period is 19.2 kHz) > and it seems the audio output is being low pass filtered resulting in silly wobbly lines. Sound Card output is low pass filtered. With a 22KHz cutoff filter frequency, a digital square wave at 22KHz would optimally render as sine of the same period, because that's the base frequency. Resonant frequencies are filtered because they are not audible. That's the principle of digital-analog conversion obeying the sampling theorem.