adcaudiosignalvoltage
When I record audio from the line-in jack on my laptop, what function of voltage produces recorded amplitude?
followup/example:
Since I've been told "voltage", unqualified: I apply a voltage square wave to a mono cable. (tip-sleeve switches between 0V and +5V every second). Since the voltage as a function of time is a square wave, why is the waveform of the recorded audio not a square wave?
And here's a recording of the same signal at 40Hz:
The top trace is a perfect output for a differentiated square-wave:
The \$C\$ is your coupling capacitor. This is a high-pass filter with
\$ f_c = \dfrac{1}{2 \pi RC}\$
So the higher RC the lower the cutoff frequency. If \$f_c\$ is lower than the signal frequency the signal will pass without much distortion, that's the rightmost waveform. As RC becomes lower the cutoff frequency will move to the right, and ultimately only the signal's highest frequency components will pass the filter. That's the leftmost waveform, only the edges will pass.
So to get a good reconstruction of your signal you'll have to increase RC. I'm not quite sure where exactly you measured this signal, so I don't know the components' values, but you may increase \$R\$ by a factor 10 to start with.
It looks as though the DC wander is caused by the android phone output, which is switching from 0V when the signal is off to some mixture of 0V and some negative voltage when the signal is on. The mean value when the signal is on is therefore a negative voltage (roughly half the peak voltage), which constitutes a DC offset from the no-signal value.
If you can change the android phone output, try making the signal symmetric (off bits at -1V, on bits at 1V, for example), which will remove this offset.
It looks as though you have an additional problem, which is that your biasing network sets the DC point of the android output to the same (Vcc/2) as the other input of the comparator; thus noise is causing unwanted transitions. Looking at your first scope trace, note that if the comparator threshold was instead set to the value of the thicker horizontal grid line across the centre of the screen (0V?), it would detect every transition without needing to modify the android code or use a Schmitt trigger.
Best Answer
You are recording voltage (1V p-p for line level) over time. The line signal is high pass filtered before the voltage sampling by the ac coupling capacitor. Ideally it is also low pass filtered by an anti aliasing filter.
If your square wave had higher frequency (say, 440 Hz) then it would show the voltage sampling better without the high pass filtering getting in the way.