190 lines
8.0 KiB
HTML
190 lines
8.0 KiB
HTML
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<h2>Introduction</h2>
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<p>
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An engineer reading and using audio level meters compares to a musician
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reading or writing sheet-music. Just like there are virtuoso musicians
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who can't read a single note, there are great sound-engineers who just
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go by their ears and produce great mixes and masters without ever looking
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at a single meter.
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</p>
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<p>
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Yet, in order to work in or with the broadcast industry, it is
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usually unavoidable to use meters.
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</p>
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<p>
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Audio level meters are very powerful tools that are useful in every
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part of the entire production chain:
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</p>
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<ul>
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<li>When tracking, meters are used to ensure that the input
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signal does not <dfn>overload</dfn> and maintains reasonable
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<dfn>headroom</dfn>.</li>
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<li>Meters offer a <dfn>quick visual indication</dfn> of an
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activity when working with a large number of tracks.</li>
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<li>During mixing, meters provide an rough estimate of the
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<dfn>loudness</dfn> of each track.</li>
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<li>At the mastering stage, meters are used to check
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compliance with upstream <dfn>level</dfn> and <dfn>loudness
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standards</dfn> and to optimize the <dfn>loudness range</dfn>
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for a given medium.</li>
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</ul>
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<h2>Meter Types</h2>
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<p>
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A general treatise on metering is beyond the scope of this
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manual. It is a complex subject with a history…
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For background information and further reading we recommend:
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</p>
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<ul>
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<li><a href="http://www.digido.com/how-to-make-better-recordings-part-2.html">How To Make Better Recordings in the 21st Century—An Integrated Approach to Metering, Monitoring, and Leveling Practices</a> by Bob Katz. Has a good historic overview of meters and motivates the K-meter</li>
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<li><a href="https://en.wikipedia.org/wiki/Peak_programme_meter#Table_of_characteristics">Wikipedia: Peak programme meter</a>—overview of meter types.</li>
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<li>"Audio Metering: Measurements, Standards and Practice: Measurements, Standards and Practice", by Eddy Brixen. ISBN: 0240814673</li>
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<li>"Art of Digital Audio", by John Watkinson. ISBN: 0240515870</li>
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</ul>
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<p>
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There are different metering standards, most of which are available in Ardour. In short:
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</p>
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<table class="dl">
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<tr><th>Digital peak-meter</th>
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<td>A <dfn>Digital Peak Meter</dfn> displays the absolute maximum signal
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of the raw audio PCM signal (for a given time). It is commonly used when
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tracking to make sure the recorded audio never clips. To that end, DPMs
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are always calibrated to 0 <abbr title="DeciBel Full
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Scale">dBFS</abbr>, or the maximum level that can be represented digitally
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in a given system. This value has no musical reason whatsoever and depends
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only on the properties of the signal chain or target medium. There are
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conventions for <dfn>fall-off-time</dfn> and <dfn>peak-hold</dfn>, but no
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exact specifications.
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<p>
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Various conventions for DPM fall-off times and dBFS line-up level can be
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chosen in <kbd class="menu">Edit > Preferences > Metering</kbd>.
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</p>
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</td></tr>
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<tr><th>RMS meters</th>
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<td>An <dfn><abbr title="Root Mean Square">RMS</abbr>-type meter</dfn>
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is an averaging meter that looks at the energy in the signal. It
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provides a general indication of loudness as perceived by humans. Ardour
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features three RMS meters, all of which offer additional peak indication.
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<ul>
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<li><dfn>K20</dfn>: A meter according to the K-system introduced by Bob
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Katz, scale aligned to -20 dBFS, rise/fall times and color schema
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according to spec.</li>
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<li><dfn>K14</dfn>: Same as K20 with scale aligned to -14 dBFS.</li>
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<li><dfn>K12</dfn>: Same as K20 with scale aligned to -12 dBFS (since 3.5.143).</li>
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<li><dfn>Peak + RMS</dfn>: standard RMS, customizable via
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<kbd class="menu">Edit > Preferences > Metering</kbd></li>
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</ul>
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</td></tr>
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<tr><th>IEC PPMs</th>
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<td><dfn><abbr title="International Electrontechnical Commission">IEC</abbr>-type
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<abbr title="Peak Programme Meters">PPM</abbr>s</dfn> are a mix between DPMs and
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RMS meters, created mainly for the purpose of
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interoperability. Many national and institutional varieties exist (<abbr
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title="European Broadcasting Union">EBU</abbr>, <abbr title="British Broadcasting
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Corporation">BBC</abbr>, <abbr title="Deutsche Industrie-Norm">DIN</abbr>).
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<p>
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These loudness and metering standards provide a common point of
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reference which is used by broadcasters in particular so that the
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interchange of material is uniform across their sphere of influence,
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regardless of the equipment used to play it back.
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</p>
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<p>
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For home recording, there is no real need for this level of
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interoperability, and these meters are only strictly required when
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working in or with the broadcast industry. However, IEC-type meters have
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certain characteristics (rise-time, ballistics) that make them useful
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outside the context of broadcast.
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</p>
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<p>
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Their specification is very exact, and consequently, there are no
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customizable parameters.
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</p>
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</td></tr>
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<tr><th>VU meters</th>
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<td><dfn><abbr title="Volume Unit">VU</abbr> meters</dfn> are the dinosaurs (1939)
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amongst the meters. They react very slowly, averaging out peaks.
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Their specification is very strict (300ms rise-time, 1–1.5% overshoot,
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flat frequency response). Ardour's VU meter adheres to that spec, but for
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visual consistency it is displayed as a bar-graph rather than needle-style
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(more below).
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</td></tr>
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</table>
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<h2>Ardour Specifics</h2>
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<figure class="right">
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<img src="/images/mixer-meter-context-menu.png" alt="mixer strip meter context menu">
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<figcaption>
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Mixer strip meter context menu
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</figcaption>
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</figure>
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<p>
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Meters are available in various places in Ardour:
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</p>
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<ul>
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<li>The mixer window features fixed height meters for each <dfn>channel strip</dfn>.</li>
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<li>There are small (narrow) meters on each <dfn>track-header</dfn> in the editor window.</li>
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<li>There are variable height meters in the <dfn>meterbridge window</dfn>.</li>
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<li>Optionally, a fixed-size <dfn>master meter</dfn> can be displayed in the main toolbar.</li>
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<li>Various other locations (<dfn>file import</dfn>, <dfn>sends</dfn>) have level-meters.</li>
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</ul>
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<p>
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They all share the same configuration and color-theme which is available in
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preferences and the theme-manager. Settings for the Peak and RMS+Peak meters
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as well as VU meter standards are found in
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<kbd class="menu">Edit > Preferences > Metering</kbd>.
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</p>
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<p>
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The type of meter and the <dfn>metering point</dfn> (the place in the signal chain
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where the meter taps the signal) are configurable in the context menu of each meter.
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Depending on the <kbd class="menu">Edit > Preferences > Mixer</kbd> settings,
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the metering point is also accessible via a button in each Mixer strip.
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</p>
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<p>
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Regardless of meter type and standard the meter display will highlight red if
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the signal on the given channel exceeds the configured peak threshold.
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</p>
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<p>
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<kbd class="mouse">Left</kbd> clicking on the peak-indicator button resets the
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<dfn>peak-hold indicator</dfn> of a single channel.<br>
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<kbd class="mod1 mouse">Left</kbd> clicking resets a whole <dfn>group</dfn>, and<br/>
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<kbd class="mod13 mouse">Left</kbd> clicking resets all meters.
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</p>
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<h2>Overview of meter types</h2>
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<figure class="left">
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<img class="mini" src="/images/meter-types-18.png" alt="Bar-graph meters in Ardour">
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<figcaption>
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Bar-graph meters in Ardour
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</figcaption>
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</figure>
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<figure class="right">
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<img class="mini" src="/images/needle-meters-18.png" alt="Needle-style meters as external LV2 plugins">
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<figcaption>
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Needle-style meters as external LV2 plugins
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</figcaption>
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</figure>
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<p>
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The figure on the left shows all available meter-types in Ardour when fed with a
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-18 dBFS 1 kHz sine wave.
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</p>
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<p>
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Due to layout concerns and consistent look and feel all meters available in
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Ardour itself are bar-graph type meters. Corresponding needle-style
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meters—which take up more visual screen space—are available as
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LV2 plugins (see image on the right):
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<a href="https://github.com/x42/meters.lv2/">meters.lv2</a>.
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</p>
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