Are You So Focused On Video That You’re Neglecting Audio?

This article is sponsored by Epiphan系统.

当人们想到视频流媒体时，视频通常是他们首先想到的. But audio is also very important, 它在帮助你传达视频信息方面起着巨大的作用. 这是一个入门的音频，让你说正确的行话音响人员, 帮助你们理解音频波形是什么样的以及如何正确处理它. 这将有助于确保您以干净有效的方式将音频转换为视频.

当你拥有令人愉悦的音频时，它确实会给你的听众带来很大的不同. 一般来说，当我们提到“令人愉悦的音频”时，我们指的是声音清晰，没有失真. 不太安静, so they really have to strain to listen to it, and it doesn't vary too much, with very quiet sections followed by very loud sections. 好的音频可以让你的观众在整个演讲过程中享受轻松的音量.

Understanding Audio Signals

To capture pleasing audio, 你需要对音频信号有一点了解，才能知道如何处理它们, how to record them properly without distorting them, so that you get a nice, 干净的信号. 图1(下面) 显示了一个音频剪辑的波形，这是我从我们的一个产品推广视频中截取的. It looks very chaotic, but in fact it's very regular. 你看到的峰值代表了声音实际说话的点. 安静的部分是介于两者之间的一小部分，它们真的不会发出任何声音.

图1. A typical waveform from a video recorded with good audio. Click the image to see it at full size.

当你与音效工程师谈论音频时，使用他们使用的语言是很重要的. 让我们从分贝(db)开始，分贝是用来测量声音的单位. A lot of circuit engineers, when they’re dealing with amplifiers and such will refer to voltage, 但总的来说, people talk about decibels. 我们用分贝来谈论音频的原因是它们更准确地反映了人耳对音频的感知. 这是一个对数尺度，可以控制非常非常小的声音和非常非常大的声音.

Linear units, like volts, can't do that very easily. The normal range in audio processing, 在分贝, is -60 dBu to +30 dBu (or -60 dB to +30dB)--a nice, manageable range with easy numbers. In voltage, that same range is .0007 volts to 25 volts. 分贝是一个对数刻度，而不是像伏特那样的线性刻度, expressing a power ratio rather than a specific amount. If I increase volume by 6 dB, and I increase it by 6 dB again, 一次又一次, 一次又一次, that is perceived to be the same amount of change over and over. 相比之下, if I use a linear scale like volts, as I increase the signal level, 我需要在连续的时间里增加它来获得相同的感知值. If I'm increasing by one volt, that's a perceived value. Then if I increase by one volt again, it doesn't seem like I increased it by the same amount, 所以分贝更准确地反映了你听到声音的方式, and thus it make it a lot easier to talk about sound.

分贝是大多数工程师在谈论音频时使用的单位. There are two ways to use this unit of measure. 第一个是绝对参考，它告诉你，字面上，声级是多少. You can take that and compare it to specifications. You can use it in order to meter. 记录输出的线路电平为-10 dBV, dBV表示以伏特为单位的分贝. 螺栓是一个非常具体的数字，它告诉你水平是多少.

类似的, 如果我说 the maximum rating on a recorder input is 12.3 volts peak, that's an absolute number. If I say the nominal level on a mixer is +4 dBu, that is an absolute number that you can measure, but they also use decibels to be something that's relative. When I say I need something quieter, 或者我说, "We've got 18 decibels of headroom," it's implied that the reference is relative to where I will clip.

剪切点(声音变得过度驱动和扭曲的点)是隐含的参考. This is not really an absolute number. That signal could be coming in at all kinds of different levels, depending on the equipment I'm using, and be 18 decibels away from the clipping point. It's relative to some implied reference. 类似的, 如果我说, "I need 10 dB of attenuation," that doesn't refer to what level the signal is at, or what level I need the signal to be. It just says I need it to be 10 dB different than what it is now.

These are just some examples of how decibels are used, 无论是在绝对参考还是在相对参考中描述信号. 尽管我们用分贝来描述一个信号, signals vary a lot over time.

As a person is speaking, or during a musical passage, if you look at an audio waveform such as the one in 图1, 你会看到音频水平不时地急剧上升和下降. It’s not possible to assign a singular value and say, “This audio signal is at 4 dB,” and completely describe the nature of the signal, 或者告诉你你需要知道的关于信号的一切以便记录它，而不是让它被剪辑.

So we break this into two measurements. The first measurement is an average value. It's a root mean square, or RMS value. It tells you the relative average level of that signal. That tells you roughly how loud it seems to the human ear. Then we look for the peak amplitude, 哪个描述了信号在最高点时比平均值高出多少. The difference between those two is called the dynamic range.

动态范围很重要，因为它告诉你信号变化的程度. The average level is very easy to measure, and very easy to deal with, but that average is just that--an average. The waveform will also have peaks, which will come in much higher than that average value, and that’s where you have a real risk of clipping the signal. You want to avoid clipping the signal as much as possible.