change directory structure to match index

Signed-off-by: Michel-FK <michel.stempin@funkey-project.com>

docs/developer_guide/hardware_reference/audio.md

@@ -0,0 +1,84 @@
+Playing audio is absolutely part of the gaming experience!
+
+So for a retro gaming console like the **FunKey S**, having a decent
+audio playback is a requirement, despite its lilliputian size.
+
+We discarded the solution of using a piezo-electric buzzer: these can
+get a loud sound in a small volume, but only at their resonance
+frequency, so the sound quality is extremely poor.
+
+Turning back to the solutions used in modern smartphones as an
+example, there are 2 paths to consider:
+
+ - playing audio internally by the mean of speaker(s)
+
+ - playing audio externally by using headphones, with or without a cord
+
+The speakers used in today's smartphones are rather sophisticated and
+achieve very good performance. However, these are using
+made-to-measure speakers and cavities, such that they cannot be found
+and reused as standard parts in a design like ours.
+
+As for the external audio solution, the **FunKey S** is so small that
+it is not possible to integrate an audio jack on the PCB to connect
+headphones! And despite our search, there is no simple and small
+enough way to integrate Bluetooth to output audio to cordless
+headphones either.
+
+![Speaker](/assets/images/Speaker.png){: align=left }
+
+The best solution we have found consists in using a single tiny [CUI
+CDM-10008][1] speaker, that is able to output 72 dB spl @ 1m from a
+0.3W input power, with relatively modest dimensions: 10 mm diameter
+and only a 2.9 mm thickness, out of which 1.4 mm can be inserted into
+a PCB hole, thus only having a height above PCB of only 1.5 mm.
+
+Connections are not easy though, since this speaker is meant to have
+wires soldered to its pads, but we used 2x [castellated][2]
+(half-round plated holes) pads with a placement just over the speaker
+pads that enables manual soldering between the speaker and the PCB
+with a solder blob.
+
+## Schematic
+
+The audio schematic is simple, as the Allwinner V3s already
+contains an analog stereo audio codec (coder/decoder): we only have to
+take one of its headphone output channel (left or right) and feed it
+to a mono audio amplifier.
+
+We chose the [Diodes Inc. PAM8301][3] chip because of its cheap price,
+good availability, its more than sufficient output power of 1.5W and
+its filterless operation, meaning that no bulky series capacitor is
+required to drive the speaker.
+
+Here is the corresponding schematic:
+
+![Audio Schematics](/assets/images/Audio_Schematics.png){.lightbox}
+
+We chose the right headphone channel HPOUTR that is fed to the audio
+amplifier **U2** through a coupling capacitor **C3**.
+
+The audio amplifier /SD shutdown input is driven by one V3s GPIO (PF7
+on pin 100), with a pull-down resistor **R2** to disable the amplifier
+by default.
+
+The audio amplifier power supply is filtered using a ferrite bead
+**L1** in order to eliminate high-frequency digital noise, and
+decoupled by 2 capacitors **C4** and **C5**, as recommended in the
+device datasheet.
+
+The speaker **SP1** is driven in differential mode in order to get the
+maximum voltage swing and thus the maximum power available for a given
+output current.
+
+Two ESD protection TVS diodes **D13** and **D14** are added, since the
+speaker may be accessible to the user through the enclosure grid in
+front of the speaker.
+
+[1]: https://www.cui.com/product/resource/cdm-10008.pdf
+[2]: https://www.pcbdirectory.com/community/what-are-castellated-holes-on-a-pcb
+[3]: https://www.diodes.com/assets/Datasheets/PAM8301.pdf
+
+--8<--
+includes/glossary.md
+--8<--