118 lines
5.4 KiB
Markdown
118 lines
5.4 KiB
Markdown
+++
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title = "What is digital audio?"
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description = "What is digital audio?"
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chapter = false
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weight = 3
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#pre = "<b>1. </b>"
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+++
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**Ardour** is a digital audio workstation (DAW). Beforing using it to record and
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edit sound, it might be useful to review how digital audio works.
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<!-- {{<mermaid align="center">}}
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graph TD;
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A(fa:fa-microphone Analog input) --> B(Analog to digital conversion)
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B --> | digital numeric data, samples | C(Digital system)
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C --> D(Digital to analog conversion)
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D --> E(fa:fa-headphones Analog output)
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{{< /mermaid >}} -->
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The diagram above shows how sound travels to and from your computer. The
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"Analogue to Digital Conversion" (ADC) and the "Digital to Analogue
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Conversion" (DAC) are done by the sound card or audio interface. The digital
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system in this case is your computer running Ardour.
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## Frequency and Gain
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Imagine a loudspeaker. To move the air in front of it and make sound,
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the membrane of the speaker must vibrate from its center position (at
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rest) backwards and forwards.
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The number of times the membrane vibrates each second determines the
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_frequency_ (the note, or _pitch_) of the sound you hear. The distance the
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membrane travels from its resting point determines the _amplitude_ (the volume,
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or _loudness_) of the sound. Normally, we measure frequency in _Hertz_ (Hz) and
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amplitude in _decibels_ (dB).
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Check out the great animation on this page illustrating this process:
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{{< youtube RxdFP31QYAg >}}
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A microphone works like a loudspeaker in reverse: vibrations in the air cause
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its membrane to vibrate. The microphone turns these acoustic vibrations into
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an electrical current. If you plug this microphone into a computer's sound
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card and start recording, the sound card makes thousands of measurements of
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this electric current per second and records them as numbers. The number of
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_samples_ (i.e. measurements) made per second is called the _sample rate_, and
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the number of possible values each sample can have is called the _bit depth_.
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The combination of sample rate and bit depth indicates how closely the digital
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signal can reproduce the sound it has recorded.
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## Peaks and Clipping
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When Ardour displays the samples which have been recorded, they appear as the
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_waveform_ we see below. The center horizontal line indicates the membrane of
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the speaker at rest, and the _peaks_ of the waveform indicate the maximum
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_amplitude_.
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If we take a waveform and increase its amplitude a lot, some of the peaks may now fall outside the range that the computer can represent digitally. The computer's inability to represent peaks outside the range of amplitude is called _clipping_, which results in a permanent loss of digital information,
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as well as a change in the sound quality which is recognizable as
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_distortion_. Ardour marks clipped peaks with the color red, as can be seen in
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the image below.
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In the image above, one can also see the _mixer strip_ on the far left,
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which gives a running measurement of the peaks, as well as an indication
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at the top of the _peak meters_ showing the maximum peak so far. The red number indicates clipping has occurred.
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{{% notice tip %}}
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Clipping often can happen at the time of recording if you set your microphone levels too high.
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{{% /notice %}}
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The range of decibels between the region's maximum peak and the clipping point
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is commonly referred to as _headroom_, and common recording practice is to
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keep approximately 3 to 6 decibels of headroom between the maximum of your
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signal and the clipping point, with the clipping point itself being
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represented as 0 dB (zero decibels). In other words, an audio region with a
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comfortable amount of Headroom would have its maximum peaks between −6 dB and
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−3 dB.
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Also, because the peaks of audio signals add together, care must be taken when
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_mixing_ several sources together to keep the combined signals from clipping.
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## Sample Rate and Bit Depth
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To make audio playable on a compact disc, for example, the computer must
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generate 44,100 samples per second. The sample rate determines the highest
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frequency which can be recorded or played back by the computer. A sampling
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rate of 44.1 kHz means that the highest frequency which can be represented is
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just under 22.05 kHz. Since normal human hearing lies within the range of
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approximately 20 Hz to 20 kHz, this is commonly accepted as a reasonable
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sample rate. Other commonly used sample rates include 48 kHz (e.g.
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multi-effects pedals) or 96 kHz (DVD audio).
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Each sample is recorded as a 16-bit number. One _bit_ is a piece of
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information which is either 0 or 1. If there are 16 bits together to make one
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sample, then there are 2^16 (65,536) possible values for each sample.
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Thus, we can say that CD-quality audio has a sample rate of 44.1 kHz and
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a _bit depth_ of 16 bits. Professional music recordings are usually mixed
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using 24 bits to preserve the highest amount of detail before being mixed down
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to 16 bits for CD. Older computer games have a distinctively rough sound,
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using only 8 bits. By increasing the sample rate, we are able to record higher
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sonic frequencies, and by increasing the bit depth, we are able to use a
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greater _dynamic range_ (the difference between the quietest and the loudest
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sounds possible to record and play).
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Here is a great video tutorial explaining sampling rate and bit depth in a lot
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more detail:
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{{< youtube zC5KFnSUPNo >}}
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