Ubuntu下音频可视化实战：ThMAD实时光效渲染教程

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音频可视化利用ThMAD实时图形渲染在Ubuntu Linux中的应用是一本由Peter Späth编著的专业技术书籍。该书主要探讨如何通过ThMAD（Thermal Microphone Array Display）技术，将音频信号转换为视觉表现，实现在Ubuntu Linux操作系统环境下的实时图形渲染。ThMAD是一种创新的方法，它允许用户直观地理解和体验音频数据，尤其适用于音乐节拍分析、声音环境监测或声音艺术创作等领域。书中详细介绍了ThMAD的工作原理，包括音频信号采集、处理、以及如何通过图形界面展示频率、强度和时间信息。作者可能涉及了实时信号处理算法，如FFT（快速傅立叶变换），以及图形库或框架，如OpenCV或WebGL，用于动态生成可视化效果。此外，由于本书是针对Ubuntu Linux开发，因此可能会强调跨平台兼容性和系统集成的挑战及解决方案。对于开发者和音频技术爱好者来说，这本书提供了宝贵的实践指导，教他们如何利用开源工具和编程语言（如Python、C++或JavaScript）来开发自己的音频可视化应用程序。书中可能包含示例代码和项目案例，以便读者能够跟随学习和应用到实际项目中。值得注意的是，这本书提供两种版本：纸质版(ISBN-13: 978-1-4842-3167-8)和电子版(ISBN-13: 978-1-4842-3168-5)，并且已经获得了美国图书馆学会的版权许可。如果你在美国亚马逊官网搜索此书，可以获取更详细的信息，如目录、定价、用户评价以及购买链接。 "Audio Visualization Using ThMAD Realtime Graphics Rendering for Ubuntu Linux"是一本深入研究音频与视觉交互的实用指南，适合那些对音频技术有深厚兴趣，并希望将其应用于创意表达或科学研究的读者。通过阅读这本书，读者不仅能提升专业技能，还能扩展在实时图形渲染和音频信号处理领域的视野。

■ INTRODUCTION

xviii

Installation

The installation process of ThMAD is discussed in Chapter 2, “Visualization Basics”.

Conventions Used in This Text

Working with ThMAD involves using its modules, which are organized in a tree-like

structure.

Modules are usually named maths → converters → 4float_to_float4. Or in short with

fixed width font, 4float_to_float4 if the module position inside the module tree is clear

from the context.

State is the common notion for a rendering pipeline while constructing it. Finished

states are also called visuals. References to sample states, including associated code

provided with the installation, as well as informational hints in general, are highlighted

like so:

■ Note This sample is as a source available under A-3.2.1_Visualization_basics_

basic_samples_basic_2d_sample

inside the TheArtOfAudioVisualization folder.

With the “Note” label replaced with “Tip” for informational hints. By folder in this

context I mean a folder as showing up in the module lister or browser. Important notes

and pitfalls are marked as follows:

■ Caution Due to the backfeeding it might easily happen… …

… … …

Code and script snippets, as well as terminal input and output, usually show up in

monospace font like here:

apt-get install libc6 libfreetype6 libgcc1 libpulse0 libstdc++6 libglfw3

Very small code snippets appear directly inside the text. If a longer line does not fit

into a line, a trailing ┐ at each line of code signifies that while actually writing it, the ┐

must be removed and the subsequent line break must be discarded. For example,

echo "cmd [...] rectangle ┐

abc [...]"

should be entered as

echo "cmd [...] rectangle abc [...]"

P. Späth, Audio Visualization Using ThMAD, https://doi.org/10.1007/978-1-4842-3168-5_1

CHAPTER 1

Sound Input

In this chapter, we deepen the knowledge of how the computer can be prepared to capture

incoming sound or produced sound, how the sound is represented internally, and how the

data arrives at ThMAD. We distinguish between the obvious air pressure elongation versus

time representation, and the power versus frequency representation, or spectrum.

Preparing for Sound Input

After you purchase a PC or laptop with Ubuntu Linux, or after you have Ubuntu Linux

installed on your PC, you have basically two options—you can use external audio sources

or you can let the sound play from the computer. As for the latter, you could use the CD

player of your PC, you could play some files from a USB stick or your hard disk, or you

could stream audio files via the Internet or some other means.

Note that other Linux distributions might work as well. Give it a try—chances are

good that you’ll find similar programs, tools, and settings to accomplish the same thing.

If you want to use external audio sources, you need a microphone or a sound card with

a line-in to connect to. Especially for laptops, the built-in microphones are not of the highest

quality, but they might be enough for your purposes. You actually don’t want to accurately

reproduce the sound, but react to it, and for this aim, having perfectly linear input curves is

not too important. On the other hand, if you don’t want to lose important impulses from the

basses, which can happen with cheap microphones, getting yourself a decent microphone

might help you avoid surprises. Also, bear in mind that audio visualizations might be brittle

to the structure of the incoming sound under certain circumstances.

Usually you want to avoid that and the overall outcome should be interesting for

any kind of music input. This is easy enough to check with different recordings. But if,

for example, the basses never make their way through the audio hardware to a suitable

extent, because your microphone misses the basses, your rendering pipeline might lack

reactiveness to an important part of the incoming sound. If instead for external sound

input you just connect some audio source to the line-in jack of your computer, you are

automatically on the safe side.

If you want to play CDs using your computer’s CD player, or play audio files or

streamed audio contents, e.g., using your browser, chances are good you don’t have

to do anything but start suitable programs or let the operating system do it for you

automatically. For larger sound file collections, a program for administering them might

be handy. RhythmBox, which is preinstalled on Ubuntu, is a good option.

Chapter 1 ■ Sound Input

Time-Elongation Representation

On a diagram with the x-axis denoting the time and the y-axis denoting the pressure, the

time-elongation representation might, for a sine wave, look like Figure1-2.

In computer systems, we need a digital representation for this sine wave. The idea

is as follows: we divide the time into small time steps, say 44,100 steps per second, and

for each time step, we write down the current air pressure, or y-value, and save it inside

an array. This is sometimes called analog-to-digital conversion (ADC). Note that 44,100

is a widely adopted industry standard, for example, it’s used with music CDs. Because

we have two ears and like stereo, we do that conversion twice, for the left ear and the

right ear. By that means, we end up with 88,200 numbers, which digitally represent

one second of stereo sound. For the pressure or y-value representation, we use integer

values (-32,768 up to 32,767), with the lower value representing a negative pressure

offset, so maybe 0.997 bars, and the higher value representing a positive pressure offset,

say 1.003 bars.

According to a scaling we can freely define, these could be mapped to 0.997 bars →

y=-1000 and 1.003 bars → y=+1000. All the other numbers are mean pressures between

these values. Of course we could use number ranges other than -32,768 … 32,767 , but the

range we chose here is internally represented by exactly two bytes of data, and computers

like that very much. It is also a trade-off: fewer different values means less resolution and

poorer quality, and more different values means higher storage need.

Of course, in reality music is stored in lots of different formats, including MP3, Ogg

Vorbis and others, mainly for reasons of saving space. The 88,200 numbers per second

add up quite rapidly. But in case of letting an application like ThMAD access PulseAudio

data, it will receive the data in an uncompressed and untransformed, raw format. This is

nice, since then ThMAD doesn’t need extra logic to handle different sound formats.

Frequency-Power Representation

A practically less obvious representation of sound consists of writing down the frequency

distribution at each instant of time.

Consider the time range [10s;10.1s] when listening to some music. Instead of

reporting the air pressure amplitudes at each instant, e.g., 10.000s, 10.001s, 10.002s,

Figure 1-2. A sine wave

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