120 lines
5.5 KiB
Markdown
120 lines
5.5 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 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. The number of times the membrane vibrates
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each second determines the **Frequency** (the note, or *pitch*) of the
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sound you hear. The distance the membrane travels from its resting point
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determines the **Amplitude** (the volume, or *loudness*) of the sound.
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Normally, we measure Frequency in **Hertz** (Hz) and Amplitude in
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**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
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cause its membrane to vibrate. The microphone turns these acoustic
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vibrations into an electrical current. If you plug this microphone into
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a computer's sound card and start recording, the sound card makes
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thousands of measurements of this electric current per second and
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records them as numbers. The number of **Samples** (i.e. measurements)
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made per second is called the **Sample Rate**, and the number of
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possible values each Sample can have is called the **Bit Depth**. The
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combination of Sample Rate and Bit Depth indicates how closely the
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digital 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
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as the **Waveform** we see below. The center horizontal line indicates
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the membrane of the speaker at rest, and the **Peaks** of the Waveform
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indicate the maximum **Amplitude.**
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If we take a Waveform and increase its the 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
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outside the range of Amplitude is called **Clipping**, which results in
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a permanent loss of digital information, as well as a change in the
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sound quality which is recognizable as **Distortion**. Ardour marks
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clipped Peaks with the color red, as can be seen in 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
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point is commonly referred to as **Headroom**, and common recording
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practice is to keep approximately three to six Decibels of Headroom
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between the maximum of your signal and the clipping point, with the
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clipping point itself being represented as 0 dB (zero Decibels). In
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other words, an audio region with a comfortable amount of Headroom would
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have its maximum peaks between −6 dB and −3 dB.
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Also, because the Peaks of audio signals add together, care must be
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taken when **Mixing** several sources together to keep the combined
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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
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highest frequency which can be recorded or played back by the computer.
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A sampling rate of 44.1 kHz means that the highest frequency which can
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be represented is just under 22.05 kHz. Since normal human hearing lies
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within the range of approximately 20 Hz to 20 kHz, this is commonly
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accepted as a reasonable Sample Rate. Other commonly used Sample Rates
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include 48 kHz (DAT recorders) 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
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make one sample, then there are 2^16 (65,536) possible values for each
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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
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mixed using 24 bits to preserve the highest amount of detail before
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being mixed down to 16 bits for CD. Older computer games have a
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distinctively rough sound, using only 8 bits. By increasing the Sample
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Rate, we are able to record higher sonic frequencies, and by increasing
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the Bit Depth, we are able to use a greater **Dynamic Range** (the
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difference between the quietest and the loudest sounds possible to
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record and play).
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Here is a great video tutorial explaining sampling rate and bit depth in a lot more detail.
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{{< youtube zC5KFnSUPNo >}}
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