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背景噪音可以帮助我们抓住犯罪分子

The hidden background noise that can catch criminals

The alternating current running through these power lines changes direction constantly.
流经这些电线的交流电不断变换着方向
A full cycle back and forth, 50 times a second.
电流每秒钟来回变化50次
60 in some of the parts of the world, but here in Europe, 50 times a second.
某些地方交流电频率是60Hz 欧洲则是50Hz
But that transmission isn’t perfectly efficient.
但这种传输效率并非100%
There are imperfections and stray electromagnetic fields.
由于存在缺陷和杂散电磁场
And a little bit of that power
一部分电力
becomes vibration, which makes sound at that frequency of 50 hertz.
转化成振动 并以50Hz的频率发出声响
Or 100, or other multiples of 50.
也可能是100Hz 总之是50的倍数
That’s the “mains hum” you can sometimes hear coming from power lines,
有时你会听到“嗡嗡声” 这是电源线
or maybe from speakers that aren’t quite plugged in properly.
或没插好的扬声器发出的
Also, that hum isn’t exactly 50 cycles a second.
而且 那嗡嗡声不一定是每秒50个周期
The grid frequency shifts slightly and constantly over time
电网频率会随着时间不断发生细微的变化
as the engineers in charge balance supply and demand.
因为主管工程师要平衡电力供需
It’s only a tiny fraction of a shift, but it’s there.
改变很微小 但依然存在
The whole grid runs at the same,
整个电网以相同的
slightly wobbling, almost-50 hertz,
大约50Hz的频率运行 其波动范围很小
no matter where you are.
全世界都是如此
That mains hum can be really annoying for people recording audio.
对录制音频的人来说 电源嗡嗡声真的很烦
You can hear it in the background sometimes.
有时你可以听到背景杂音
But if you were to log what that wobbling grid frequency was,
但如果你记录下每天每秒电网的波动频率
every second of every day, keep all that in a database,
并储存在一个数据库里
and then compare it to recordings with mains hum on them,
然后与背景有电源嗡嗡声的录音比对
then in theory, you’d have a perfect watermark
理论上 你就拥有了完美的“水印”
to prove the time when anything was recorded.
用来证明录音的时间
I talked to one of the forensic experts who works with it,
我和其中一个研究这方面的法医专家交谈后
and also set them a challenge.
也给他们设置了一个挑战
Historically, what’s had to happen is:
历史上一个不可避免的事情是
we’ve had to record the grid frequency data ourselves
我们必须把录音设备插入电源
by literally plugging a recording device into the mains.
来记录电网频率数据
The National Grid have recently released their data,
国家电网最近公布了他们的数据
seven years recorded right at the source.
七年来都是在电源处记录的
It’s fairly simple, actually, signal processing.
实际上 信号处理相当简单
You’re looking to see if there is a component around 50 hertz
你要找的是一个50Hz左右的分量
or one of the harmonics.
或者是其中一个谐波
I would hesitate against advising anyone to
我不太建议
present themselves to court as a forensic expert,
法医专家们亲自出庭
but this type of evidence has been used in court to date recordings.
但这类证据一直在法庭上用于确定录音日期
The longer recording you have, the better.
录音的时间越长越好
As an absolute minimum, I think about a minute.
最短时长 我觉得大概也要1分钟
If it’s a noisy recording, or if the ENF isn’t particularly clear,
如果录音有噪音 或者ENF不是特别清楚
then the longer recording you have, the better.
那么录音时长越长越好
So, find the mains hum frequency in the footage and then write code
找出视频中嗡嗡声的频率 进行编码
to match against the millions of possibilities in the National Grid logs.
与国家电网日志中的数百万个数据进行匹配
Does it work?
这可行吗?
Well to find out, I asked some friends of mine who make videos
为了找出答案 我请一些做视频的朋友
to send me over some of their raw footage.
把他们的原始视频发给我
Straight out of a camera, unprocessed, unfiltered.
原片直出 未经处理 未经过滤
And I asked the forensic team: can you timestamp it?
我拿去问法医团队:你们能确定拍摄时间吗?
First up, footage from Taha,
第一个视频 来自塔哈
one of the team behind educational channel Answer in Progress.
教育频道Answer in Progress背后的团队之一
There’s a quite strong component here,
这里有一个相当强的分量
and you can see it’s got a bit of a wobble.
你可以看到它有些波动
We use a signal processing algorithm to extract the peak,
我们用信号处理算法来提取峰值
and we constrain the algorithm
我们对算法进行了限制
so that it can’t suddenly jump up here.
这样轨迹就不会突然出现跳跃
So even if there is a higher amplitude, something up here,
所以即使上面有一个很高的振幅
it will stick to this curve.
它也不会超出这条曲线
Then compare,
然后进行比对
if we know that a recording was taken on a certain day,
如果我们知道录音是哪一天录的
then we just need to give it a day’s worth of National Grid data.
那么我们就只拿某一天的国家电网数据对比
If we’re not sure when it was taken, we’ll throw everything at it,
如果我们不确定是哪一天录的 就全部比对一遍
and it will take a bit longer to process, but it’s not prohibitive.
处理时间会更长 但也不是不行
Spot on. Got it to the second.
完全正确 精准到秒
Although it’s important to say that the team never said they were certain about the timing,
很重要的一点是 团队从未笃定他们的答案
only that they had a very high chance of being correct.
只是说正确的概率很大
And yes, they were.
确实如此
Next up, footage from Hannah Witton, sex and relationships educator.
接下来是性与人际关系教育家 汉娜·威顿的视频
There wasn’t a component at around 50 hertz.
没有50Hz左右的分量
So here we have the axis, 49 hertz to 50 hertz,
这里有49到50Hz的坐标轴
and we can see no apparent ENF trace.
我们看不到明显的ENF轨迹
So I’m going to change the axes up to around 100.
我要把坐标轴改到100左右
And then we’ll be able to see the harmonic component around 100 hertz.
我们就能看到100Hz附近的谐波分量
Because it’s quite strong
因为它很强
and seems to be behaving in a way that we’d expect ENF to behave,
而且和我们预期的ENF的表现一致
we can be fairly confident that this is an ENF harmonic.
可以很有把握地说这是ENF的谐波
What’s interesting about this one is: it sort of appears halfway through the recording.
有趣的是 这个谐波在录音差不多到一半才出现
Maybe there’s a little bit of something earlier on, but…
也许之前有点什么 不过呢
So there’s a possibility that maybe Hannah was moving around when she made this recording,
汉娜录制这段录音时可能在四处走动
getting closer or further away from mains powered equipment.
所以离电源供电设备时远时近
Maybe the mains hum was coming from something on a timer or a thermostat
也许电源嗡嗡声来自背景中定时器或恒温器上
in the background that just switched off, but the timestamp was accurate.
瞬间关掉的什么东西 时间戳是正确的
So the next test was Steve Mold from his science channel.
下一个测试来自史蒂夫·莫德的科学频道
And I thought that this one would be easy because he sent B-roll.
我原本以为这会很简单 因为他给的是花絮
He’s not talking, it’s just ambient noise.
视频没有他的声音 只有环境噪音
And also: he’s holding a fan that’s plugged into the mains!
还有 他手里拿着一台插在电源上的风扇!
There’s no strong component at all for ENF.
这段音频根本没有ENF的强分量
So the next step would be to look at the
所以下一步我们要看看
other frequency regions to see if we can identify any harmonics.
其他频段 是否能识别出谐波
How loud the person is often doesn’t have much of an effect.
人声大小通常不会产生太大影响
It’s often environmental noise, that would be lower to mask ENF.
而通常是通过降低环境噪音来掩盖ENF
There is possibly something here,
这里可能有些线索
but because we haven’t got a strong trace,
但由于我们没得到较强的轨迹
it’s sort-of fading in and out,
它有点弱且不稳定
we wouldn’t be confident to use this for ENF analysis.
我们没有信心拿来做ENF分析
Sometimes it just doesn’t work.
有时候就是行不通
As a last test, I sent over some of my old footage
作为最后一次测试 我寄了一些去年
from when I was filming against green screen in my tiny flat last year.
在我的小公寓里对着绿幕拍摄的旧胶片
I was sneaky, though, and it hid an edit in there.
不过 我很狡猾 藏了一段剪辑在里头
What could happen with an edit is: you have a sudden jump in ENF frequency.
剪辑时可能发生的情况是:ENF频率突然跳跃
In that case, it’s quite likely to be an edit.
见到这种情况 很可能就是一段剪辑
It’s possible that the ENF trace would just sort of stop.
ENF的轨迹有可能突然终止
The reason for that could be
原因可能是
because it’s got an edit in, which in this case we believe it has,
中间有一段剪辑 在这个视频中我们认为是有的
but could also be because you walked away from the source of the ENF energy.
但也有可能是因为你远离了ENF能量的源头
Like any forensic analysis,
如同其他法医分析方法
this isn’t some magic tool that always works.
这不是一个总能奏效的神奇工具
Compression can stop it working.
它无法分析压缩录音
You need minutes of uninterrupted audio to have enough confidence in the result.
需要连续几分钟的音频才能得到有信心的结果
And sometimes there might be no background mains hum at all.
有时可能根本没有背景电源嗡嗡声
But I love this because it’s an accident of technology
但我还是很爱它 因为这是个科技的意外
that’s created a tool that would sound like science fiction 20 years ago.
创造了一种仿佛20年前科幻小说里的工具

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视频概述

电网频率分析,即ENF分析: 匹配背景噪声与国家电网日志。我给研究人员设置了一个挑战。

听录译者

收集自网络

翻译译者

彭译喵

审核员

审核员ZIYU

视频来源

https://www.youtube.com/watch?v=e0elNU0iOMY

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