如何消除现场活动制作中的地环路嗡嗡声

To maximize the quality of your audio when you’re producing a live event, it’s important to assess your signal carefully all the way through the chain, 正如我在我的 May/June 2015 article 如何掌握混音 for Live Production and Streaming. 使用模拟视频流的另一个挑战, 实时音频, 甚至是录音, 是处理地面环路嗡嗡声在您的记录或广播信号. 它可能是一种低沉的嗡嗡声，也可能是一种听得见的令人讨厌的嗡嗡声. 地环路嗡嗡声可以以不同的方式产生, 但不管它听起来像什么，也不管它来自哪里, 最重要的是你得干掉它.

The key reason for eliminating audio hum (other than it’s annoying to listen to), 是因为它改变了信噪比吗. 虽然嗡嗡声在你的信号中可能不是特别响亮, it raises the noise floor and creates a perceived lowering of the dynamic range of your program. If your program content—typically the people who are speaking—happens against a very quiet aural background, 这样更容易理解说话者所说的一切, 包括他们声音最安静的部分. 然而, 如果信号有嗡嗡声或嗡嗡声, 说话者的声音变得难以听清或理解.

最常, a ground loop hum comes from different parts of the audio chain being plugged into different places, 或者有不同的接地路径或电位. 这就是为什么他们称之为地环路嗡嗡声. The interference results from excess energy trying to level itself out, 通过传递音频路径来实现. I won’t specifically go any deeper into the physical and electrical principles that create this phenomenon because were more interested in solving it and getting good production audio. There are several different ways that you can eradicate the hum — both electrical, 在音频路径本身.

电方法

If you have an audio mixer on the stage and an analog snake or long audio cables going to the back of the room where there’s a second audio board for your production or video village—or even just a camera—you’ve got the potential for audio hum. 如果这大致描述了你的设置, 先试试这个:如果可能的话, 将音频路径的两端插入同一电源. 这样你就可以消除嗡嗡声了. 然而, you don't want to run the AC cable right next to the audio cables because that, 本身, 会导致你的音频嗡嗡声吗.

如果这不起作用或不可能与您的设置或场所, the next step to try eliminate the grounding issue is to use a ground lifter like the one shown in 图1(下面) to lift the ground on one or the other side of the audio signal path. 说实话, this is a quick-and-dirty solution that is not the best long-term way to solve the problem; it’s a simple Band-Aid to let you get on to other things that you need to do before your event begins. Then you can come back and revisit the grounding issue and solve it in a better way.

图1. 一个Pro Co声音GLX直列桶地面升降机

Here’s another common ground-loop hum scenario cause by electrical problems: The onstage mixer is plugged into a power strip and that power strip plugged into a ground lift adapter. The problem with this is it opens up the potential for other electrical issues because now your audio gear is no longer grounded properly. This creates the potential for equipment that’s not working properly to pass an electrical signal into the audio path itself.

虽然你可能从来没有做过, there are times when a singer’s lips may touch a microphone and it feels like there’s a tingle. That’s because there's extra voltage on the line going into the microphone and actually being felt by the singer as their lips touch the microphone. This won't be an issue if all of your presenters are wearing lavalier microphones on their jacket or blouse, 但这仍然是你不希望发生的事情. You really do want to keep things grounded properly throughout the entire electrical system.

A better and more permanent solution is to use transformers in the electrical path to isolate the ground from the house to your equipment, 但仍能提供额外电压的安全分散(下面的图2). These devices were often used in analog video days when the video itself would have a visible “hum bar.” You could use an isolation transformer for an entire rack of gear and ensure that there were no grounding issues between that rack of year and something else elsewhere in the studio.

图2. Tripp-Lite隔离变压器，前(左)和后(右)

These transformers tend to be fairly big and heavy because they use large transformers of heavy copper to pass 120 V AC on through to the equipment. 相比, 你可以拿起一个音频转换器, 这也能很好地隔离你的电, 同时还能让你传递信号. Audio transformers are considerably smaller and lighter than power transformers. 我将在下一节中介绍它们.