Audio Output: An audio signal is a complex AC waveform. The diagram below shows 2 sine waves. These are much less complex than music. If you play a pure tone from a test disc and monitor the output with an oscilloscope, it would look similar to one of the sine waves below. Many of the aftermarket car stereo head units have 2 different types of outputs. One type of output (the preamp output) is designed to drive the preamp level input of an amplifier, crossover or equalizer. The other type of output (speaker level output) is designed to drive a set of speakers and may also be used to drive the speaker level input of an amplifier. The main difference between the preamp signal and the speaker output signal is the current capability of the signal. The preamp level signal is a very low noise, low current output. The speaker output signal is capable of supplying much more current than the preamp output signal. The output voltage levels from the preamp outputs and speaker outputs MAY be approximately the same but the speaker drive voltage is usually higher. When the volume is low, the voltage out is low. To increase the volume, you must increase the output voltage. You are controlling the output voltage level when you set the volume level. The yellow waveform shows the same sine wave output at a lower volume setting than the white waveform. This is one example of what audio might look like on an oscilloscope. The tallest part of the waveform is a sharp/strong beat of a snare drum. Not all such strikes would look like this but you can see that the strong hit has a fairly high amplitude. This sample is a fraction of a second long.