引言 Intro
这里我们主要介绍三种录制音乐的的技术：留声机、磁带录音机和数字音频存储。
留声机 The phonograph
唱片播放器由三个主要部分组成：转盘、唱臂和唱针。
A record player consists of three main parts: the turntable, tonearm and stylus.
唱片是如何工作的？
- 当唱片旋转时，槽壁中的微小压痕移动唱针。
The principle is that as the record is rotated, tiny indentations in the walls of the groove move the stylus. - 唱针运动在电磁场中产生波动，从而在与其相连的线圈中感应电流。
Movements of the stylus create a fluctuation in the electromagnetic field that induces an electric current in a coil attached to it. - 该电流被送到放大器，放大器再现原始声音。
This current is sent to the amplifier, which reproduces the original sound.
黑胶唱片的问题有哪些？Vinyl Records Problems
- 很难保持唱片无划痕，划痕会产生爆裂噪声。
It was difficult to keep records scratch free which produced popping noises. - 另一种现象是槽锁，导致片段反复重复。
Another phenomenon was groove lock which caused sections to repeat over and over again. - 如果唱片靠近热源放置，可能会变形，使其无法播放。
Records could warp if left close to a heat source making them unplayable.
磁带录音机 The Tape Recorder
磁带的缺点有哪些？
- 窄轨道和慢录音速度影响了保真度。
The narrow tracks and slow recording speed did impact the fidelity. - 容易受损（AI）：磁带易受潮，受温度影响，易老化，导致数据损坏。
Tapes can get damp, are sensitive to temperature, and age quickly, which can ruin your data.
磁带的类型有哪些？
- 原始氧化铁磁带，如今很少见。
The original ferric-oxide tape. It is very rarely seen these days. - 标准氧化铁磁带，也称为“普通偏置”。
The standard ferric-oxide tape, also referred to as “normal bias.” - “铬”或 CrO2 磁带，氧化铁颗粒与二氧化铬混合。
The “chrome” or CrO2 tape. The ferric oxide particles are mixed with chromium dioxide. - “金属”磁带，使用金属颗粒而不是金属氧化物颗粒。
The “metal” tape. Metallic particles rather than metal-oxide particles are used in the tape.
从上往下，音质会提高，金属磁带音质最好。
Sound quality improves as you go from one type to the next, with metal tapes having the best sound quality.
存储数字音频 Storing digital audio
数字音频可存储在 CD、硬盘和固态存储器上。
Digital audio can be stored on CDs, hard drives, and solid-state memory.
完全灵活，因为无论使用哪种格式，文件都相同。
Completely flexible as the file is the same, no matter which format is used.
问题：CD 会被划伤，硬盘会故障，记忆棒容易丢失。
problems – CDs get scratched, Hard drives can fail; and memory sticks can be easily lost.
从模拟输入到数字输出 From analog input to digital outpout
声卡 Soundcards
声卡从模拟输入到数字输出操作的流程如下：
- →模拟输入 Analog Input
- →抗混叠滤波器 Anti-Aliasing filter
- →采样 Sampling
- →量化 Quantization
- →数字值 Digital values
- →数字到模拟重建 Digital to Analog Reconstruction
- →预放大（增益值）Pre-Amplification (Gain value)
- →模拟信号到扬声器 Analog signal to speaker
录音 Recording
选择声卡并决定采样率和位数。
Choose the sound card and decide on the sample rate and number of bits.
录音时注意声卡输入电平，不要让输入过强，否则信号波形顶部会发生削波，导致失真。
Be careful with input levels to the soundcard while recording, don’t let the input be too strong, or clipping of the top of the signal waveform will occur, resulting in distortion.
削波 Clipping
基本上，削波是幅度失真，因为信号顶部被截断而改变的现象。
Basically, it is an amplitude distortion as the top of the signal is altered by being cut off.
改变波形形状会改变其音色和感知质量。
Changing the shape of a waveform will alter its timbre and perceptual quality.
总共有两种削波方式：硬削波和软削波
1. 硬削波与数字系统相关，被认为会给数字录音带来刺耳、不愉快的特性。
Hard Clipping is associated with digital systems and is thought to introduce a harsh, unpleasant quality to the digital recording.
2. 饱和形式的软削波与模拟录音设备相关。
Soft clipping in the form of saturation is associated with Analog recording equipment.
采样率 Samplerate
这是声卡设备中模数转换器每秒对音频信号采样的次数。
This is the number of samples of the audio signal taken every second by the analog-to-digital converter in the sound card device.
选择采样率值时，我们可以参考香农采样定理
香农采样定理 Shannon’s Sampling Theorem
如果信号中存在的最高频率（fmax）不大于采样频率（Fs）的一半，则任何信号都可以从其样本中精确重建。
Any signal can be reconstructed exactly from its samples if the highest frequency present in the signal (fmax) is no greater than half the sampling frequency (Fs)
Fs/2 的值也称为奈奎斯特频率！
The value Fs/2 is also called the Nyquist frequency!
抗混叠滤波器 Anti-Aliasing Filter
抗混叠滤波器始终用于抑制正在录制的模拟信号中高于奈奎斯特频率的能量。
An anti-aliasing filter is always used to suppress the energy at frequencies above the Nyquist frequency from the analog signal being recorded.
关于采样率… About samplerate
采样点越多 = 数据量越大 = 所需存储空间越大 = 处理的数据越多=音频质量也更好
more samples=more data = more storage space required = more data to proces=better audio quality