Update audio-basics.md

media/audio-basics.md

@@ -10,12 +10,24 @@ breadcrumbs:
     - Low midrange (ca. 100Hz-1kHz)
     - High midrange (ca. 1kHz-10kHz)
     - Highs (ca. 10kHz-20kHz)
+- Desi-Bels (dBs):
+    - Formula: `S = 10 log(S_1 / S_2)`
+    - For comparing signal levels.
+    - The base formula and unit is purely for comparisons/changes.
+    - Volt is often used as the quantity measured. In such cases, +6dB represents roughly a doubling in signal, 3dB a doubling in power and 10dB a doubling in perceived loudness.
+    - Since a doubling in signal level results in a 6dB SPL change, a 16-bit digital sound representation gives 96dB of dynamic range.
+- Derivative dB-variants:
+    - Specific dB-variants exist for measurements in specific use cases, e.g. dBu, dBFS, dB SPL etc.
+    - dB SPL: For physical sound pressure level (SPL). Specifically `L_p = 20 log(p / p_0)` for RMS sound pressure `p` (in pascals) and reference sound pressure `p_0 = 20µPa`. Depends on the distance from the sound source (typically 1 meter), unless room noise is measured.
+    - dBu (aka +4dBu): For electrical audio levels in professional equipment, e.g. professional mixers.
+    - dBV (aka -10dBV): For electrical audio levels in consumer equipment, e.g. built-in sound cards. Not to be confused with dB**v** (lower-case). -10dBV is lower than +4dBu.
+    - dBFS: For digital audio levels, relative to full scale. 0dBFS is the maximum possible level and positive levels would cause overflow clipping.
 - Signal levels:
     - (Note) This is the voltage (and somewhat impedance) inside cables/equipment.
     - Mic level: Output from a microphone. Very weak, requires a preamp.
     - Instrument level: Output from e.g. a guitar. Like mic level but slightly stronger.
     - "Professional" line level (+4dBu): E.g. for professional audio mixers.
-    - "Consumer" line level (-10dBV): E.g. for consumer-grade sound cards in PCs. Lower than +4dBu. Not to be confused with dB**v**.
+    - "Consumer" line level (-10dBV): E.g. for consumer-grade sound cards in PCs.
     - Speaker level: High-power signal going from an amplifier to a (passive) speaker.
     - Phono: Old, for turntables etc. Much lower voltage than line level. Typically needs a phono preamp/stage with RIAA equalization.
     - (Note) DI boxes may be used to convert unbalanced non-+4dBu input signals to balanced +4dBu signals.
@@ -24,6 +36,25 @@ breadcrumbs:
     - 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 due to the signal mismatch.
+- Gain:
+    - For adjusting the input level of electrical signal sources, to make it more normalized wrt. other inputs and the supported signal range of the equipment.
+    - "Set and forget".
+    - For digital mixers, -18dBFS RMS may be used as an appropriate gain target.
+    - A similar function is *trim*, which is used more internally in systems to modify the signal level.
+- Equalization (EQ):
+    - Used to cut or boost the signal level of certain frequency ranges. Typically in the form of graphic or parametric EQs.
+    - Typical usages:
+        - To account for the acoustics in a room, applied to the PA/main mix.
+        - To make an instrument or microphone sound better.
+        - To cut certain frequency ranges on microphones to prevent feedback.
+    - Should rarely include more than a few points and more than a few dBs.
+- High-pass filtering (HPF) (aka low cut):
+    - For cutting frequencies below a set point, typically as a slope rather than a hard cut-off.
+    - Should very often be used with most inputs to avoid noise and similar in frequencies that input is not supposed to use.
+    - Also used to limit low frequencies sent to the PA which might cause speaker damage.
+    - Typical values:
+        - 40Hz at the output for typical PAs, or 20Hz if the subs support it. Alternatively set on inputs where no higher HPF is set.
+        - 100Hz for typical vocals and instruments without lows.
 - 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.
@@ -76,7 +107,7 @@ breadcrumbs:
     - Unlike level panning, which changes the left-right level balance, Haas panning adds a few milliseconds of delay to either channel to give the sound a feeling of directionality.
 - Pan rule (or pan law):
     - Determines how much louder either side of a stereo channel should get when panned all the way to that side, so that the perceived loudness stays the same.
-    - For an ideal loudspeaker system and ideal room acoustics, the loudness increase from playing an identical signal in both stereo channels should be around 6.02dB SPL (why?), although in reality it's typically closer to 3dB.
+    - For an ideal loudspeaker system and ideal room acoustics, the loudness increase from playing an identical signal in both stereo channels should be around 6dB SPL, although in reality it's typically closer to 3dB to account for more realistic room acoustics.
     - Most mixers use a pan rule of 3dB, to account for most realistic situations.
 - Devices:
     - DI boxes (direct input): Used to convert some unbalanced non-+4dBu input signal (e.g. microphone level, instrument level, consumer-grade line level, or speaker level) to a balanced +4dBu signal. Typically phantom powered, sometimes battery powered. Passive versions also exist, consisting mainly of a transformer to scale the output voltage.