你是否过于关注视频而忽略了音频?

本文由 Epiphan系统.

When people think about video streaming, video is usually the first thing they think of. 但是音频也非常重要, and it plays a huge role in helping you to convey the message of your video. This a primer for audio to get you speaking the right lingo to the sound guys, and to help you understand what an audio waveform looks like and how to treat it properly. This will help ensure that you get your audio into your video in a clean and efficient manner.

It really does make a big difference for your audience when you have pleasing audio. Generally, when we refer to “pleasing audio,” we mean audio that’s nice and clear with no distortion. 不太安静, 所以他们真的必须努力去听, 而且变化不大, 非常安静的部分接着是非常吵闹的部分. Good audio allows your viewers to enjoy an easy listening volume that plays throughout the whole presentation.

理解音频信号

捕捉悦耳的声音, you need to understand a little bit about audio signals to know how to treat them, 如何正确地记录它们而不扭曲它们, 这样你就能得到一个很好的, 干净的信号. 图1(下面) shows a waveform of an audio clip that I took from one of our videos promoting one of our products. 它看起来很混乱，但实际上很有规律. The peaks you see represent the points where the voice is actually speaking. The quiet parts are the little bits in between that really don't make any sound.

图1. 带有良好音频录制的视频的典型波形. 单击图像以查看其全尺寸.

When you speak about audio with sound engineers, it's important to use the language that they use. Let’s begin by talking about decibels (dBs), the unit used to measure sound. 很多电路工程师, when they’re dealing with amplifiers and such will refer to voltage, 但总的来说, 人们谈论分贝. The reason we use decibels to talk about audio is that they more accurately reflect the perception of audio in the human ear. It’s a logarithmic scale that can manage very, very quiet sounds and very, very loud sounds.

线性单位，比如伏特，就不能这么简单. 音频处理中的正常范围, 在分贝, 是- 60dbu到+ 30dbu(或- 60db到+30dB)——一个不错的, 易于管理的范围与简单的数字. 在电压上，相同的范围是 .0007伏到25伏. Decibels are a logarithmic scale rather than a linear one like volts, 功率比的表示功率比而不是具体数量的. 如果我把音量增加6db, 再增加6db, 一次又一次, 一次又一次, 这被认为是一次又一次的相同数量的变化. 相比之下, 如果我用线性刻度，比如伏特, 当我提高信号电平时, I need to increase it more in successive times to get the same perceived value. 如果我增加一伏特，那就是感知值. 如果再增加1伏特, 我好像没有增加同样的量, 所以分贝更准确地反映了你听到声音的方式, 这样一来，讨论声音就容易多了.

Decibels are the unit that most engineers will use when they're talking about audio. 这个度量单位有两种使用方法. The first is an absolute reference, so this tells you, literally, what the sound level is. 你可以拿它和规格进行比较. 你可以用它来测量. Line level on record outs is -10 dBV, with DBV representing decibels referenced to a volt. A bolt is a very specific number, and that tells you what the level is.

同样，如果我说录音机输入的最大额定值是12.峰值3伏，这是一个绝对值. 如果我说混频器的标称电平是+ 4dbu, 这是一个可以测量的绝对值, 但他们也用分贝作为相对的东西. 当我说我需要安静点的时候, 或者我说, “我们有18分贝的净空,这就暗示了参考是相对于我要剪辑的地方的.

The clipping point (the point at which the sound becomes overdriven and distorted) is the implied reference. 这不是一个绝对的数字. 这个信号可能会以各种不同的水平进入, 这取决于我使用的设备, 并且离终点18分贝. 它是相对于一些隐含的参考. 类似的, 如果我说, “我需要10db的衰减,“这并不是指信号处于什么水平, 或者我需要什么电平的信号. 它只是说我需要它比现在高10db.

这些只是分贝如何被使用的一些例子, both in an absolute reference and in a relative reference to describe the signals. Even though we have absolute numbers measured 在分贝 to describe a signal, 随着时间的推移，信号变化很大.

作为一个人在说话, 或者在一段音乐中, 如果您查看如图1所示的音频波形, you’ll see the audio level go up and down drastically from time to time. 赋一个奇异值是不可能的, “这个音频信号是4db,并完整地描述信号的性质, or tell you everything you need to know about the signal in order to record it and not have it clip.

所以我们把它分成两个量. 第一个测量是平均值. 它是均方根，或均方根值. 它告诉你信号的相对平均电平. 这大致告诉你人耳听到的声音有多大. 然后我们寻找峰值振幅, which describes how much above that average value the signal will get at its highest point. 两者之间的差异被称为动态范围.

Dynamic range is important because it tells you how much the signal will vary. 平均水平很容易测量, 而且很容易处理, 但这个平均值只是一个平均值. 波形也会有峰值, 哪个会比平均值高得多, 这就是你真正有可能切断信号的地方. 你要尽可能地避免信号被截断.