Merge pull request mixxxdj#37 from Be-ing/gain

replace "volume" with "gain" and explain how to adjust gain properly

source/chapters/djing_with_mixxx.rst

@@ -303,6 +303,96 @@ and click the :guilabel:`Play` button next to the waveform.
 To display the Preview deck, press :kbd:`CTRL` + :kbd:`4` (Windows/Linux) or
 :kbd:`CMD` + :kbd:`4` (Mac).
 
+.. _djing-gain-staging:
+
+Setting your levels properly (gain staging)
+===========================================
+.. sectionauthor::
+   Be <be.0@gmx.com>
+
+.. figure:: ../_static/level-meter-green.png
+   :align: left
+   :alt: A level meter in Mixxx with the gain set properly for the loudest part of a track
+   :figclass: pretty-figures
+
+Setting your levels properly, also known as gain staging, is essential for
+getting the best sound quality out of the equipment you are using. At every link
+in your signal chain, from Mixxx's channel gains to the power amplifier, the
+level should be well above the noise floor, but lower than the maximum level
+before the signal clips. The :term:`level meters <level meter>` should mostly be
+around the top of their green region. The level meter pictured to the left shows
+where Mixxx's level meters should average at the loudest parts of tracks. The
+average level should not be in the yellow region. Use the yellow region to leave
+headroom, or available level above the average before the signal clips. The
+loudest parts of the music (the transients) should briefly go into the yellow
+region.
+
+.. figure:: ../_static/waveform-good.png
+   :align: center
+   :alt: A waveform at a good level
+   :figclass: pretty-figures
+
+   A waveform at a good level. Note that the example waveforms in this section
+   were made by adjusting the visual gain of the waveform display in Mixxx to
+   illustrate the concepts. Adjusting the visual gain of the waveform display
+   does not change the level of the audio.
+
+.. figure:: ../_static/level-meter-red.png
+   :align: right
+   :alt: A level meter in Mixxx indicating clipping. The gain should be turned down!
+   :figclass: pretty-figures
+
+**If a level meter is in its red region, the signal is clipping and the gain
+should be turned down.** Some equipment doesn't have a level meter and only has
+an LED that turns on when the signal clips. Clipping means that the peaks of the
+waveform are flattened because the equipment has reached the maximum level that
+it can amplify the signal to. This distorts sound in an unpleasant way and can
+damage equipment by driving it with more power than it is designed to handle.
+Increasing the gain past the point where clipping begins (further into the red
+on a meter) will distort the signal more. If you want to make the sound louder
+when every part of the signal chain is at its maximum without clipping, use more
+speakers or use speakers that are more sensitive and convert electrical energy
+into acoustic energy more efficiently.
+
+.. figure:: ../_static/waveform-clipping.png
+   :align: center
+   :alt: A clipping waveform
+   :figclass: pretty-figures
+
+   A clipping waveform
+
+.. figure:: ../_static/level-meter-too-low.png
+   :align: left
+   :alt: A level meter in Mixxx with the gain set too low
+   :figclass: pretty-figures
+
+On the other hand, the signal should not be too low. This is because every audio
+device generates a little noise at a level referred to as its noise floor.
+Additionally, analog signals pick up noise as they travel along wires. The signal
+measured by the meter on the left is relatively close to the noise floor. When a
+device is turned up, the noise floor does not go up; only the signal does.
+However, every time the signal is amplified by the gain of another piece
+of equipment, both the noise and the signal from previous devices in the signal
+chain are amplified. For example, if your sound card is turned down and you turn
+the gain up on your mixer to compensate, the signal-to-noise ratio (SNR) of the
+sound card output will be low and the mixer's gain will amplify the signal and
+the noise from the sound card, plus the noise picked up along the wire. The end
+result will have more noise than if the output of the sound card was turned up
+before the signal reached the mixer and the mixer's gain did not have to be
+turned up.
+
+.. figure:: ../_static/waveform-too-low.png
+   :align: center
+   :alt: A waveform at too low of a level
+   :figclass: pretty-figures
+
+   A waveform that is too close to the noise floor
+
+   .. note:: To adjust the output volume of a sound system while maintaing a
+          high signal-to-noise ratio, the gain should be adjusted as close to
+          the speakers as possible. Refer to the
+          :ref:`Gain Knob <interface-gain-knob>` section for details.
+
 .. _djing-edit-metadata:
 
 Edit metadata of audio files

source/chapters/user_interface.rst

@@ -530,22 +530,23 @@ The Mixer Section
 The mixer section of the :ref:`Mixxx user interface <interface-overview>` allows
 you to control how the different decks and samplers are mixed together.
 
-.. _interface-vu-meter:
+.. _interface-level-meter:
 
-Channel Faders and VU Meters
-----------------------------
+Channel Faders and Level Meters
+-------------------------------
 
-**VU meters**
-  In the center of the mixer section are 4 :term:`VU meters <vu meter>`. The
-  two outer-most VU meters are for each deck, while the 2 inner-most VU meters
-  are the left and right VU meters for the master output.
-
-  The light at the top of the VU meter indicates when the audio in the selected
-  channel is clipping. If this light goes on, reduce the gain or EQs for this
-  deck to eliminate distortion.
+**Level meters**
+  In the center of the mixer section are 4 :term:`level meters <level meter>`. The
+  two outer-most level meters are for each deck, while the 2 inner-most level
+  meters are the left and right level meters for the master output. These should
+  stay at the top of the green region, with the loudest parts of the music
+  (transients) briefly going into the yellow region. They should never be in
+  the red region. Refer to
+  :ref:`Setting your levels properly (gain staging) <djing-gain-staging>`
+  for more details.
 
 **Line faders**
-  The two large faders on either side of the VU meters are the deck volume
+  The two large faders on either side of the level meters are the deck volume
   faders, also known as Channel- or Line-faders. Adjusting these controls the
   volume of each deck.
 
@@ -566,7 +567,7 @@ Section Expansion Buttons
 
    The section expansion buttons
 
-Above the VU meters in the mixer are the optional section expansion buttons.
+Above the level meters in the mixer are the optional section expansion buttons.
 
 If you click on either :guilabel:`MIC`, :guilabel:`SAMPLER`, or
 :guilabel:`VINYL` then you will enable control sections for interacting with:
@@ -597,37 +598,64 @@ Headphone and Master Mix Controls
    Headphone split cue option
 
 **Head-Mix Knob**
-  Allows you to customize how much of the master output you hear in your
+  Allows you to control how much of the master output you hear in your
   headphones. It works like a crossfader between the stereo Master and stereo
   Cueing signal. If the knob is set to the left, you only hear the cueing signal
   which can be useful for prelistening tracks.
 
   .. note:: Don't forget to activate the :guilabel:`PFL` button on the deck you
             want to listen to in your headphones.
 
-**Head volume Knob**
-  This button adjusts your headphone volume. You can adjust the volume of a
-  single deck's signal you are listening to in the headphones with the
-  :ref:`Gain <interface-eq-gain>` knob.
-
 **Head-Split Button**
-  If activated, two mono signals are send to the headphone output instead of one
+  If activated, two mono signals are sent to the headphone output instead of one
   stereo signal. The master signal plays in the right channel, while the cueing
-  signal plays in the left channel. You can still adjust the head-mix so in the
-  left channel is not the pure cueing signal.
+  signal plays in the left channel. You can still adjust the
+  :guilabel:`Head-Mix Knob` to control the mix of the master and cue signals in
+  the left channel.
 
 **Balance Knob**
   This knob allows you to adjust the :term:`balance` (left/right orientation) of
   the master output.
 
-**Volume Knob**
-  The Volume Knob controls the overall volume of the master output. Adjust
-  this knob so that the :ref:`Master VU meters <interface-vu-meter>` are just
-  at the peak.
-
-  .. hint:: If the Peak indicator on top of the Master VU meter flashes, the
-            master output signal is clipping (too loud). Lower the volume with
-            using the volume knob.
+.. _interface-gain-knob:
+
+**Gain Knob**
+  The Gain Knob controls the gain applied to the Master output signal that
+  Mixxx sends to your sound card, Internet broadcasting servers, and recorded
+  mixes. Generally, this knob should not be adjusted. To change the output
+  volume, a gain control should be adjusted as close to the speakers as possible
+  for the best signal-to-noise ratio throughout your signal chain. Ideally, the
+  volume would only be adjusted by adjusting the gain on the power amplifier.
+  On active speakers, including computer speakers, studio monitors, and PA
+  systems that do not require a separate amplifier, this means adjusting the
+  gain on the speakers (which may be labeled "Volume").
+
+  As a DJ, you may not have access to the power amplifier. In that case,
+  adjust the volume by adjusting the gain on the equipment closest to the power
+  amplifier that you have access to. Typically, this is a DJ mixer. If you are
+  not going through a mixer, adjust the gain of your sound card. If your sound
+  card does not have any controls on it (for example, for sound cards built into
+  computers), adjust the sound card settings with your :term:`OS <operating system>`
+  mixer program. The Master Gain Knob in Mixxx should be a last resort for
+  adjusting volume.
+
+  .. warning:: **In no case should any part of the signal chain be clipping,
+               indicated by a level meter going into its red region
+               or an LED labeled "clipping" turning on.**
+
+  .. seealso:: For an explanation of why you should set your gains this way, see
+               :ref:`Setting your levels properly (gain staging) <djing-gain-staging>`.
+
+**Head Gain Knob**
+  This knob controls the gain applied to the headphone output signal that Mixxx
+  sends to your sound card. Generally, this knob should not be adjusted. To
+  adjust the volume in your headphones, adjust your sound card's gain on the
+  headphone output. Most external sound cards have a gain knob specifically for
+  the headphone jack. If you use an onboard sound card for your headphones,
+  adjust the gain control in your :term:`OS <operating system>` mixer program.
+  If your sound card only has one gain knob that controls both the main output
+  and the headphone output, then adjust the :guilabel:`Head Gain Knob` in Mixxx
+  to control the volume of your headphones without affecting your main output.
 
 .. _interface-pfl:
 
@@ -649,7 +677,7 @@ will play next in your headphones before your audience hears the track
 (headphone cueing). You can select more than one PFL button simultaneously.
 
 .. note:: Headphone cueing is only available if you have configured a
-             Headphone Output in :menuselection:`Preferences --> Sound Hardware`.
+          Headphone Output in :menuselection:`Preferences --> Sound Hardware`.
 
 .. _interface-eq-gain:
 
@@ -666,15 +694,34 @@ Equalizers and Gain Knobs
    EQ Controls
 
 **Gain Knob**
-  Above these knobs, the gain knob allows you to adjust the gain of the deck. In
-  general, you should adjust this knob so that at full-volume the deck's audio
-  is just at the peak of the center VU meters. This is so you can achieve the
-  widest dynamic range for your track.
+  Above these knobs, the gain knob allows you to adjust the gain applied to the deck. 
+  Use this to compensate for the differences in recording levels between tracks.
+  In general, you should adjust this knob so that the track's :term:`level meter`
+  stays around the top of the green region with the loudest parts of the track
+  (the transients) briefly going into the yellow region. **Do NOT turn the
+  Gain Knob up so much that the level meter is in the red region. At this point
+  the track is clipping, which sounds bad and could damage equipment.**
+
+  .. note:: By default, Mixxx automatically applies an additional ReplayGain so
+            tracks have approximately equal loudness at unity gain. Your tracks
+            must be :ref:`analyzed <library-analyze>` to take advantage of this
+            feature. When an unanalyzed track is loaded, Mixxx calculates its
+            ReplayGain value, but will not apply a newly calculated ReplayGain
+            value to a track after it has already started playing (to avoid a
+            sudden change in the gain of a playing track).
+
+  .. seealso:: For an explanation of why you should set your gains this way, see
+               :ref:`Setting your levels properly (gain staging) <djing-gain-staging>`.
 
 **EQ Knobs**
   The low, mid, and high knobs allow you to change the filters of the audio.
   This allows you to selectively reduce or boost certain frequency ranges of
   the audio.
+
+  .. warning:: Be careful not to raise the EQs so much that the signal clips,
+               indicated by a :term:`level meter` being in the red region. See
+               :ref:`Setting your levels properly (gain staging) <djing-gain-staging>`
+               for a more detailed explanation.
 
 **Kill Switches**
   The small boxes next to each EQ knob are called :term:`kills <kill switch>`.
@@ -922,9 +969,9 @@ Sample Deck Controls
   listen to the track in your headphones.
 
 **Gain Knob**
-  Allows you to adjust the volume of the track.
+  Allows you to adjust the gain applied to the track.
 
-**VU Meter**
+**Level Meter**
   Shows the level of the signal and can be adjusted with the Gain knob.
 
 **Pitch/Rate slider**

source/glossary.rst

@@ -180,10 +180,12 @@ Glossary of Terms
      i.e. treble, mid and bass. Useful for effects where the DJ drops a track
      out for a period or creates room for a transition.
 
-   vu meter
-     The volume unit (VU) meter is used to show the relative levels of audio
-     signals and is subdivided across channels. Basically it represents how
-     'loud' a sound from a channel is.
+   level meter
+     The level meter is used to show the average levels of audio
+     signals. The level should average around the top of the green region, with
+     the loudest parts of the music (the transients) briefly going into the
+     yellow region. If the level meter is in the red, the signal is clipping and
+     the gain should be turned down.
 
    balance
      A balance control on a mixer allows you to adjust the balance between the
@@ -195,7 +197,8 @@ Glossary of Terms
 
    volume
      A term that refers to the degree of sound intensity or audibility; loudness.
-     A volume control is used to adjust the output gain setting.
+     Volume is determined by people's perception and does not directly correspond
+     to any physical property of sound.
 
    reverse
      Reverse plays a track backwards.