AM / FM如何准确地承载音高和响度？

21

几乎所有有关AM / FM调制的教程都将调制信号显示为简单音调或连续正弦波。现在，这很容易，对于AM，您只需将调制信号叠加在载波上作为包络和叠加，对于FM，您就可以连续不断地改变频率。但是似乎没有人指出这个明显的问题……语音既有音调，即频率，又有响度，它们是两个独立的模拟数据流。我没有看到教程或解释，然后采取了非常必要的下一个步骤，来解释如何通过无线电方案传输这两个方面，而无线电方案显然只能采用一个程度的变化，即AM的幅度或FM的频率。

TL; DR：

每个都只有一个可调节变量的AM或FM调制如何承载语音的音高和响度，它们至少是两个不同的模拟数据流？
为什么绝对没有人似乎在任何有关无线电调制的教程/视频/文章中解决这个明显的问题？

modulation fm amplitude-modulation

— aAaa aAaa
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19

您了解信号是如何调制的，对吗？因此它具有频率（大致来说是音调）和振幅（即“响度”）。这些不是不同的流。这些都是相同的“波”，这是“信封”中的部件，比方说AM调制信号..

— 尤金嘘。

2

尽管电台确实使用音频压缩来避免过度调制，而过度调制会导致严重的失真和边带噪声，但这两种调制方案都使用音频信号的所有方面来调制载波幅度或频率。

— Sparky256 '18

12

frequency, and loudness, which are two separate analog data streams...这是不正确的....这只是一个模拟数据流

— jsotola

41

语音具有音调（即频率和响度），它们是两个独立的模拟数据流。

不会。语音最初是作为声压波的一个模拟“流”传输的，其中气压变化幅度对应于音量（在该时刻），变化率给出音高。

没有教程……说明如何通过无线电方案传输这两个方面，而无线电方案显然只能进行一个程度的变化，...

AM和FM调制方案是模拟的，之所以称为模拟，是因为调制类似于原始信号（语音或音乐）是类似的（形容词，在某些方面具有可比性，通常以更清晰的方式比较）。

但是我也很好奇，为什么下一个显而易见的问题在进行这些教程和解释的人们看来从来没有出现过，而且由于我一直在徒劳地寻找，所以在这里也很难找到答案。

当您找出答案时，也许那里会为您提供机会。

这些教程使用正弦信号演示了结果，因为否则将无法在图中以合理的比例看到复杂信号的调制。

图1. 来自Wikipedia 的标准AM的简化分析有点描述了您的要求。

请注意，图中的波形不是正弦波，而是任意波形。还要注意，幅度调制正好跟随信号波形。没什么更多的了。麦克风会将语音转换成模拟电信号，而调制器也将类似地调制载波。

— 晶体管
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12

啊啊我知道了我觉得有点愚蠢...虽然，当然，我没有看过针对第二部分的教程，没有显示第二部分如何显示复杂波，但是我完全错过了关于瞬时振幅与振幅变化率的关系的部分。实际频率变化。真是的这些年来，我一直没有。

— aAaa aAaa

3

@ Sparky256：AM广播早于1950年代-Wiki说广泛的广播始于1920年代。FM发明于1933年与实验广播在1934年

— 彼得·贝内特

3

这是一个很好的答案！@aAaaaAaa; 使我理解这一点的一件事是，当我意识到载波与正在传输的音频相比具有惊人的速度时。

— bitsmack

7

@bits：让我惊讶的一件事是我意识到某些AM频率并不那么高。欧洲的LW（长波）频带始于148.5 kHz，大约仅是其要传输的最高音频频率的十倍。（也许您甚至不能在LW无线电上传输10 kHz音频？）

— 晶体管

2

@Transistor nyquist会告诉您，您只需要比AM的最高频率高2倍的载波。

— 棘轮怪胎

26

忘了无线电吧-您如何认为语音是通过仅具有“电压”的电线传输的-还是一个变量？

关键是，“音高”和“幅度”是时间的单值函数的抽象参数。实际上，您可以在一条电线上以不同的频率叠加许多不同的信号。如此复杂的波形的每个分量都有自己的频率，相位和幅度，但是我们仍然可以将它们区分开。

可以在AM发送器中将电压转换为幅度，在FM发送器中将其转换为频率。在这两种情况下，接收器都可以将信号转换回具有相同电压波形的副本，该副本首先会产生调制。

因此，如果您认为语音（和音乐）可以通过电线传输，则是将其作为无线电信号传输的简单扩展。

— 戴夫·特威德
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6

实际上，您甚至可以忘记电线上的电压。声音在同一个房间中如何从一个人的嘴巴传到另一个人的耳朵？同样，它是一个瞬时空气压力值，随时间变化。

@besmirched：公平的观点，但这是一个EE网站，所以我需要将回答保持在主题：-)

— Dave Tweed

也许是由纤毛产生的微小压力响应压力计数的变化？

12

声音只是一维时变信号。麦克风实质上是连续不断地跟踪气压的变化。在任何时间点，这都是一个值。该值被“调制”到载波上。

该一维时变信号既包含响度信息，又包含音调信息。实际上，它可以在此单个时变值中同时包含许多不同语音的响度和音调信息，或同时包含许多乐器的响度和音调信息。

— 亚历克斯·弗兰西奇
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6

语音具有音调（即频率和响度），它们是两个独立的模拟数据流。

有两个以上，完全取决于您对轨道的感知/分析方式以及发生了什么。有可能是数百个在我的血腥情人节的歌，流有流和他们去11。

如果我们强迫它们都适合一个数据流怎么办？

因为当这些东西全部进入空气介质时，这正是发生的情况，空气是所有声音的固有介质。它只能处理一个数据流，因此必须进行压缩。

当我们将麦克风放在空气中并获得波形时，我们得到的是一个数据流。在堆栈中的其他16个效果踏板中，将Bilinda Butcher的呼吸颤音与合唱中的MP-41相位压缩器（尤其是吉他）分开的声音分开了……这是不可能的。因为在压缩到单个流中丢失了太多的唯一性。

然而，这就是音乐，我们喜欢它。

这一可发音流是在AM或FM上进行编码的东西。那就是你所缺少的。

我忽略了立体声，这是其本身的问题。

— 哈珀-恢复莫妮卡
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5

在简单的AM系统中，传输的信号类似于

X （ Ť ） = 一种 （ 1个 + 米 （ Ť ） ） 罪 ω_{C} Ť

$x(t) = A\left(1+m(t)\right) \sin\omega_c t$

和 $m(t)$ 被称为消息信号。

在AM收音机中，消息信号基本上只是说明在每个时间点推动扬声器锥孔有多困难。如果音频信号是单音， $m(t)$ 本身就是正弦曲线。

如果您想要更大的音调，可以增加 $m(t)$ 。如果您想要更高的频率音调，可以增加 $m(t)$ 。

如果您想获得音乐音频信号，则可以将具有不同频率和幅度的多个音调加在一起，然后以旋律的方式改变它们。

— The Photon
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2

Voice has both pitch, i.e. frequency, and loudness, which are two separate analog data streams.

"Pitch"/"frequency", "loudness"/"amplitude". Those words belong to a model that we construct to understand sound/voice/music and human hearing. But many phenomena can be modeled and understood on different levels--sometimes, on many levels.

Another way to describe sound is with a single quantity, sound pressure, that varies with time. (See Dave Tweed's answer). Sound pressure is a concept that belongs to a lower level/more primitive model. It also is the quantity that AM or FM radio modulation conveys.

为什么绝对没有人似乎可以解决这个明显的问题...？

国际海事组织，作者和教育者通常都专注于教授某种现象的特定模型，而他们却没有意识到存在其他模型和其他理解水平的事实。与对无线电设计感兴趣的人相比，最主要的兴趣是了解人的大脑如何处理语音或音乐，从而可以对“实际上是”声音有完全不同的理解。而且，如果他们两个都足够封闭，那么他们可以就其中哪个是“正确的”争论不休。

他们俩都不对。声音实际上不是他们两个人所说的。声音就是它的本质，他们有不同的理解方式。

0

It's been pointed out that the instantaneous signal level is just a one-dimensional time-varying variable. So why bother with sine signals? Because both AM and FM are used for transmitting a band-limited signal through a higher-frequency carrier signal, and the simplest band-limited signal is a sine signal as it has only a single frequency. AM is pretty straightforward regarding its frequency spread (and you can double the capacity by using sideband modulation) whereas FM is quite more fuzzy and involves Rice distributions, with the frequency spread partly depending on modulation depth.

Either way, the simplest signal for analysing the combination of a carrier frequency and a band-limited signal remains a sine signal.

I think they meant to ask why more arbitrary waveforms aren't used more often as the signal to be sent in examples. I don't think they were asking why the carrier wave is a sine wave.

— Kyle A

0

Not yet mentioned is how FM does this. The amount of frequency deviation from the carrier frequency corresponds to amplitude. Higher frequency is positive amplitude, lower frequency is negative amplitude. The rate of change of the FM signal corresponds to the frequency.

Wiki article includes a moving image for both AM and FM.

https://en.wikipedia.org/wiki/Frequency_modulation

— rcgldr
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Sure, but that does not in any way address the question, or the fundamental misconception driving it. Answers need to either answer the question, or explain why it is mistaken, not make tangential commentary.

— Chris Stratton

@ChrisStratton - the OP asked how frequency and loudness information is transmitted. My answer was specific to FM, since there are already other answers for AM. I assumed that noting amplitude is related to frequency would explain how loudness information is transmitted, and that that the rate of change in that amplitude the amplitude information would explain how frequency information is transmitted. The animated image in the wiki article shows this fairly well.

— rcgldr

0

In addition to the existing answers which point out the fundamental misconception about signals in general, let me point out something. You write:

Almost every tutorial on AM/FM modulation shows the modulating signal as something like a simple tone or continuous sine wave

Yes, and that's perfectly fine without loss of generalization thanks to Fourier's theorem, according to which most signals we care about can be expressed as a sum of sines.

The (quasi) linearity of our devices then makes it admissible to reason about simple sines guaranteeing that things will work out even in the presence of more complex signals - linearity essentially means that feeding a sum of sines to a device is the same as summing the results of feeding n sines to n devices.

— Tobia Tesan
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I had considered adding some comment on Fourier in my answer but decided that it only covered periodic signals and general music and voice would not fit into that category.

— Transistor

This is not really my field and I don't think that going too much in depth will help OP, so I think some handwaving is okay, but as I understand it a non-periodic signal such as speech is simply taken to be piecewise periodic in order to leverage Fourier's theorem. And lo, we can still get MP3s of Milli Vanilli.

— Tobia Tesan

0

I agree with you that there are two separate information components of sound waves, pitch (frequency) and volume (amplitude).

As shown in fig 1 of Transistor's answer, not only does the sound wave varies in amplitude, it also varies in frequency. The amplitude of the sound, modulates the amplitude of the carrier, while its frequency modulates the frequency of the carrier. So the carrier also has both information components of the sound wave. After the carrier is demodulated, both information components of the original sound wave are recovered.
Hopefully this clarifies your misunderstanding of the capabilities of the carrier, and makes it clear that it has two (not one) degrees of variability.

— Guill
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Have another look at my Figure 1. You can see that the AM frequency is constant. There is only one degree of variability - the amplitude. You are missing something in your understanding of modulation.

— Transistor

You're making the same mistake as the poster - amplitude is not really separable from frequency, you only have the strengths (and phases) of frequency components, or to put it another way, a frequency is present only if it has a non-zero magnitude. To really understand the original mistake, consider how timbre is conveyed, ie how we hear a trumpet as distinct from a clarinet. Is that a third degree of freedom? No. It's just a different mix of frequency component strengths (even overtones are missing on a clarinet). The same goes for multiple instruments or multiple people talking at once.

— Chris Stratton

But then Transistor is also wrong - the frequency of an AM signal is neither constant nor singular, if it were there would be no information content. The information content is all in sidebands displaced in frequency from the central or carrier frequency component. All the carrier does is serve as a reference permitting simpler detectors, vs needing to manually or algorithmically tune the local oscillator feeding the product detector that would be needed if the waste power in the constant frequency carrier component were removed (as long routine outside of legacy settings)

— Chris Stratton

@Transistor: The frequency I am referring to is the sound. You can clearly see that the left side of the wave has a higher frequency than the right half. Sound does not have a constant frequency (or amplitude).

— Guill

@Guill: But that's not quite what you said. "The amplitude of the sound, modulates the amplitude of the carrier, while its frequency modulates the frequency of the carrier."

— Transistor