Add audio notes

audio-video/audio/basics.md

@@ -0,0 +1,40 @@
+---
+title: Audio Basics
+toc_enable: yes
+breadcrumbs:
+- title: Home
+  url: /
+- title: Audio & Video
+  url: /audio-video/
+- title: Audio
+---
+{% include header.md %}
+
+## Notes
+
+- Line levels:
+  - -10dBV: Consumer equipment. 0.316V RMS and 0.447V peak.
+  - +4dBu: Professional equipment. 1.228V RMS and 1.736V peak.
+  - dBv
+  - dBm
+  - Not to be confused with SPL dB.
+- Balance mode:
+  - Unbalanced: Ground and signal.
+  - Balanced: Ground and hot and cold signal with equal impedance. The cold signal is 0V but not (directly) connected to ground.
+  - Differential: Balanced but the cold signal is the opposite voltage of the hot signal instead of 0V.
+  - Balanced and unbalanced mono plugs/sockets can generally be connected together (with the loss of the balanced signal), but don't connect e.g. a stereo unbalanced TRS to a mono balanced TRS. It'll sound weird.
+- Ground loops:
+  - When there exists physical loop in the ground wires. Typically when devices are connected to different grounded power outlets.
+  - Different potentials in the loop will cause undesired current flow.
+  - Can be heard as a 50Hz/60Hz hum in the audio signal.
+  - Solutions:
+    - Use balanced signals.
+    - Connect all equipment to a single grounding point, i.e. a single power outlet.
+    - Break the shielding on one cable to break the loop. Different boxes, like DI units, may have this as a feature known as a ground lift. However, make sure all shields are connected at one end. Don't break the shielding/earthing on devices that needs it for safety reasons!
+    - Use a ground loop isolation transformer.
+    - Group the ground cables together so no currents get induced into the cables.
+    - Use a resistor and/or a ferrite bead to limit AC current.
+- Phantom power:
+  - Applies 48V to XLR3 (or similar) mic inputs. Applying this to devices which aren't made for it can break them.
+
+{% include footer.md %}

audio-video/index.md

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+---
+title: Audio & Video
+breadcrumbs:
+- title: Home
+  url: /
+---
+{% include header.md %}
+
+## Audio
+
+- [Basics](audio/basics/)
+
+{% include footer.md %}

index.md

@@ -2,6 +2,7 @@
 
 ### Areas
 
+- [Audio & Video](audio-video/)
 - [Configuration Notes](config/)
 
 {% include footer.md %}