View source for Audio analog connection

Connecting a simple stereo headphone and a seprate microphone to the '''APF27_Dev''' requires no special precaution.

But there are some more complex configurations which may cause the audio output to refuse functioning with no visible cause, which are described below. 
{{Note| The problems described here do not occur with the '''APF51_Dev''', which has no audio virtual ground.}}
== Electrical background ==
The output of an audio amplifier powered by a single positive voltage source presents normally a quiescent DC voltage of approximately half the power supply.

This DC voltage should never be applied to a loudspeaker, so the classical wiring involves a series capacitor to block this DC voltage (two capacitors for a stereo amplifier).

The capacitance value has to be large enough to preserve the lowest limit of the audio bandwidth (example : 220 uF for a 16 Ohm loudspeaker).

Another solution, avoiding the need of large capacitors, consists of creating a '''virtual ground''' for the speaker, the virtual ground is a DC voltage source with the same value as the quiescent voltage of the amplifier output.

The APF27_Dev board is designed to use the virtual ground available on the TS2101 audio chip.

The consequence is that the ground terminal of the audio output jack is connected to a 1.5V DC source (TS2101 VGND pin) instead of the board real ground.

{{Warning| It is not permitted to connect this virtual ground to the real ground, this would short-circuit the 1.5V source and cause the TS2101 to enter the "short-circuit protection mode" and stop audio output operation (see P. 28 of the TS2101 data sheet).}}

== Ground loop via mains earth ==

Here is an example of virtual ground short-circuit caused by the mains earth connection :
* the audio output of the APF27_dev is connected to a home stereo amplifier or any audio other audio equipement having a mains plug with earth connection
* the APF27_Dev has its ground connected to some equipement having a mains plug with earth connection, for example :
** the development PC, through the RS232 cable
** the power supply (AC adaptor) itself

==== suggested workarounds : ====
* unplug the RS232 cable
* use a power supply with no earth connection

{{Note| the Ethernet cable causes no ground loop, because it is isolated by transformers at both ends.}}

== Ground loop via the microphone jack ==

The audio input connector (microphone jack) of the APF27_Dev has its ground connected to the board's real ground.

Connecting together the grounds of both audio jacks causes a virtual ground short-circuit.

Examples :
* connecting the audio output and input to the same audio equipement, like an audio analog mixer
* connecting the audio output and input to a headset with microphone with common ground

==== suggested workaround : ====
* when both jacks are used, connect only the ground of the input jack, leaving the ground of the output jack unconnected
* then insert 2 capacitors in series with the audio outputs