knowledge-kitchen / course-notes

class: center, middle # Digital Audio recording and emitting sounds that someone might want to listen to --- # Agenda 1. [Overview](#overview) 1. [Analog vs. Digital](#analog-vs-digital) 1. [Audacity](#audacity) 1. [Conclusions](#conclusions) --- name: overview # Overview -- ## Concept We will take a look at the nature of sound, how it exists in the physical world, and how it is recorded and emitted by contemporary computers. --- template: overview ## What Is Sound Sound is the perceptible sensation that animals have in response to cyclical vibrations of the air or other medium surrounding them. -- > If a tree falls in a forest and no one is around to hear it, does it make a sound? > > -paraphrased from George Berkeley's [Treatise Concerning Human Knowledge](https://en.wikipedia.org/wiki/A_Treatise_Concerning_the_Principles_of_Human_Knowledge), 1710 --- template: overview ## Good vibrations The vibration of air (or other transmission medium) radiates outward from the source, creating waves of changes to the pressure and density of air particles. -- - The air particles do not travel with the sound... they simply vibrate in place, causing those particles next to them to vibrate, and so on. --  --- template: overview ## Perception In humans and other [tetrapods](https://en.wikipedia.org/wiki/Tetrapod), the vibrating air waves are channeled into the inner parts of the ear, where they cause a vibration of the eardrum, a.k.a. **tympanic membrane**, which in turn causes the vibration of the **cochlea**. --  --- template: overview ## Frequency and pitch Different parts of the tympanic membrane vibrate in response to different frequencies of air vibration. -- - Vibrations from each part of the tympanic membrane are _transduced_ into electrical signals, and sent along neurons to the brain. -- - The patterns of these signals are interpreted and possibly recognized by the brain, leading to perception of different **pitches** and other of sound. -- - watch [a video explaining how sound is perceived by the inner ear and brain](https://upload.wikimedia.org/wikipedia/commons/7/72/Journey_of_Sound_to_the_Brain.ogv). --- template: overview ## The limits of perception The average human is capable of hearing frequencies betweeen 20Hz and 20,000Hz. -- - Lower frequencies are perceived as lower pitch. -- - Higher frequencies are perceived as higher pitch. -- - Frequencies use the unit, **Hertz**, which represents how many times per second a waveform oscillates back and forth. -- - When it comes to sound waves, for example, 100Hz would mean that the air density and pressure increase and decrease cyclically 100 times per second. -- - Give yourself [a quick-and-dirty hearing test](http://onlinetonegenerator.com/) (by no means conclusive) by playing sounds at different frequencies to see what you can perceive. --- name: analog-vs-digital # Analog vs. Digital -- ## Analog If one were to map the pressure variations in air of a single frequency of sound over time, it might look something like this:  --- template: analog-vs-digital ## Digital A computer or other digital device has finite memory and storage, and can only do so many operations per second. -- - For these reasons, a digital device can only record a certain number of **samples** of the amplitude of the actual physical sound each second. -- - There is endless debate among audiophiles about whether humans can tell the difference between an analog signal and its well-sampled digital equivalent. --- template: analog-vs-digital ## ADC and DAC The conversion of an analog to digital signal is termed **ADC**, while the reverse is **DAC**. --- template: analog-vs-digital ## Fidelity (lo-fi) A low **sample rate** in ADC leads to a poor quality representation of the original waveform.  -- - draw a line connecting the dots and compare it to the original analog signal --- template: analog-vs-digital ## Fidelity (medium-fi) A medium **sample rate** leads to an ok, but not great, quality representation of the original waveform.  -- - draw a line connecting the dots and compare it to the original analog signal --- template: analog-vs-digital ## Fidelity (hi-fi) A high **sample rate** leads to a good quality representation of the original waveform.  -- - draw a line connecting the dots and compare it to the original analog signal --- template: analog-vs-digital ## Nyquist Shannon Theorum Engineers named **Harry Nyquist** and **Claude Shannon** theorized and proved in 1928 and 1949, respectively, that in order to faithful reproduce a waveform, it had to be sampled at a rate at least twice its frequency. -- - So, for example, a 20Hz wave would need to be sampled no less than 40 times per second. -- - A 20,000Hz wave would need to be sampled no less than 40,000 times per second. -- - If you recall, human hearing falls into the range of 20 - 20kHz. -- - So the max sample rate a sound humans can hear would need would be about 40,000 samples per second. -- - It will therefore not surprise you to learn that **[44.1kHz](https://en.wikipedia.org/wiki/44%2C100_Hz)** is a common sample rate for digital audio, with 96kHz currently considered very high resolution. --- template: analog-vs-digital ## Bit depth In addition to sample rate, another factor in the faithful reproduction of an analog audio signal is the bit depth. -- - **Bit depth** refers to how many bits of data are used to represent each sample. -- - For example, a bit depth of 2 bits per sample would allow one of 2² = 4 different values in any given sample. -- - A bit depth of 16 bits per sample would allow one of 2¹⁶ = 65536 different values in each sample. etc. -- - Today, 32 bits per sample is considered high quality. --- template: analog-vs-digital ## Bit depth (continued) Fitting the amplitude of the analog signal into the available bit depth of a sample is termed **quantization**. -- - Higher bit depths allow a more faithful reproduction of the original analog signal. --  --- name: audacity # Audacity -- ## Audio recording software **[Audacity](https://www.audacityteam.org/)** is a free, cross-platform, open source multi-track audio recording software. -- - Audacity allows you to record and manipulate audio data down to the level of an individual sample. --- template: audacity ## The lame MP3 format While editing audio tracks, Audacity and other multi-track audio tools, will save in their own proprietary "layered" formats. -- - However, most people prefer to share audio tracks in flat **MP3** format, which are playable on most computers and devices. -- - In order to export as MP3, a proprietary _non-free_ format, Audacity includes an MP3 encoding tool called **LAME**. -- - The LAME project has created from scratch a clone of an MP3 encoder much the same way that GNU created from scratch a clone of the UNIX operating system. -- - LAME is a recursive acronym that stands for, "_LAME Ain't an MP3 Encoder_", in clear homage to GNU. --- template: audacity ## MacOS Catalina (10.15) problems Audacity currently will not be able to record audio if run on Mac's Catalina OS (OSX 10.15). Fortunately, there is an easy workaround. -- - Download Audacity and place it into your `Applications` folder. -- - Place the following code into a text file named `audacity.command` stored somewhere convenient, such as your `Desktop`. ```bash #!/usr/bin/env bash # open Audacity from bash... otherwise it doesn't work on Mac Catalina open /Applications/Audacity.app/Contents/MacOS/Audacity ``` -- - Give everyone execute permissions on this file from the command line: ```bash chmod a+x audacity.command ``` -- - Whenever you want to launch Audacity, just double click this `audacity.command` file, rather than launching Audacity directly. --- name: conclusions # Conclusions -- We have now seen some of the conceptual details of how audio is stored and manipulated in a contemporary computer. -- - An early 1980's television show named Bits and Bytes explained many of the concepts of digital audio very well in their [episode on Computer Music](https://www.youtube.com/watch?v=Ho8niBS1nrU). While hardware may have evolved since that time, the fundamental concepts behind digital audio have not. -- - There are many Audacity tutorials available online, such as [this one](https://www.youtube.com/watch?v=aCisC3sHneM). -- - Thank you. Bye.