Capacitor in series with audio input

[Yasserbn]

I have attached a photo for a part of a circuit taken from a research paper.
The circuit connects the output of the mobile phone audio (headphone) with a micrcontroller.
The output of the headphone is AC 500mv P-P and they couple it with Vcc/2 as seen in the picture.

My question is why they are putting a 0.1uF capacitor in series with the audio signal. This will block the DC signals but there is no DC signal. Any idea?


View attachment 67693

 

[ronsimpson]

but there is no DC signal. Any idea?

1)You don't know if there is DC. Probably no DC but there are many different audio products. Some don't have a capacitor on the output.
2)AC coupled audio is most likely centered around ground but you need a signal centered around 2.5V.

 

[MikeMl]

For the PIC circuit to work right, it needs the blocking capacitor, because it is detecting something centered at 2.5V. You do not know for sure if there is a capacitor in the device that sources the audio.

 

[Yasserbn]

I think this capacitor blocks the Vcc/2 from going back to the audio output. So it is to protect the output.

 

[Nigel Goodwin]

I think this capacitor blocks the Vcc/2 from going back to the audio output. So it is to protect the output.

It's to protect both essentially - but mainly to protect itself, you can't assume what might be on the other end.

Many modern power amplifiers (almost all of them?) use split supplies and DC coupled outputs. If you connected one of those to your input it would short the input to ground, and disrupt the biasing on it's input (Vcc/2) and stop it working.