Laptop soundcard square/sawtooth waves?

I lately attached a home-made oscilloscope (LED matrix display) to the headphone jack of my computer. I was able to use Audacity to generate sine, square and sawtooth waves of different amplitude and frequency on the computer, and output them to the oscilloscope, where the oscilloscope drew the waveforms perfectly. However, when i tried this on a laptop computer, a very different result occurred. While sine waves were easily generated and displayed by the oscilloscope, the oscilloscope drew instead of a square wave, drew a waveform that looked distantly like a square wave, but looked more line a v=tan(t)) waveform, with spikes appearing in the middle of the top and bottom of the waveform. A similar result occur ed when i tried to use a sawtooth wave, large spikes appearing at the top of the sawtooth, not a perfect linear ramp. What i am asking, is arr the sound cards found in laptop computers incapable of creating anything other than a sine wave? I know that no device can create a perfect square or sawtooth wave (thy have finite slew rates), but do laptop sound cards tend to be more noisy than PC sound cards to the extent of producing that sort of waveform?