# Sound card output

I posted this in Audio section and didn't get a reliable answer, so I'm posting it here. Does the output of the sound card of a pc or any device like ipod etc have any voltage component which is negative?? I mean below zero. Thanks.

sreedevk: I posted this in Audio section and didn't get a reliable answer, so I'm posting it here. Does the output of the sound card of a pc or any device like ipod etc have any voltage component which is negative?? I mean below zero. Thanks.

Yes.

Audio is normally AC coupled so when attaching it to a system half the signal is below ground and the other half above it.

As mentioned, yes, audio is AC, which swings around a “central point” voltage.

Typically this voltage is taken to be “ground”, but that is not necessarily the case. “Ground” in the context of small electronics like these is just a common reference voltage that all other voltages are measured against - remember, voltage is also known as “Potential Difference” - that difference being the difference between a voltage and the reference voltage (ground). Ground is not 0V in the sense of absolute zero (in the same way that 0°C is not the same as absolute zero [0°K, -375°C], but just a convenient reference point to gauge other temperatures against), but 0V when the difference between itself and itself is measured.

So yes, an audio signal has negative components with respect to it’s central reference voltage, also known as the DC offset. Said DC offset may well be 0V - i.e., “ground”, but it doesn’t need to be. For instance, with a 1.5V DC offset, and a 2VP-P signal the signal voltage v would be +/- 1V, but the absolute voltage V with respect to ground, would be 0.5V-2.5V.

Adding and removing DC offsets is really simple. A series capacitor blocks DC, so just adding a series capacitor (22µF is very common) removes the DC offset from the signal, setting that DC offset effectively to ground.

If there is no DC offset one can be added by simply superimposing a DC voltage over the signal - the simplest way is with a pair of resistors to form a voltage divider.

So you can see, “negative” voltages are purely relative.

But in the context of this question the output of a sound card or iPod or in fact any audio equipment you are likely to purchase the output has no DC offset on it. And therefore always is symmetrical around ground.