Phase (相位)

[takasehuang]

這麼快就急著要答案，果然是有錢人

我都自己來
1. 先弄清楚何謂相位
2. 相位對聲音的影響
3. 如何量測或聽出相位問題
4. 如何解決相位問題

其實我不太清楚大家是希望怎麼去"解決"相位問題....
這篇老paper可能幫得上忙吧，至少就第1.2.3點來講

www.bksv.com/doc/17-198.pdf

[Leo Yeh]

這段就說到，即使是Phase Linear，所有聲波的頻率都以一個固定的延遲時間通過人器材，在相對的基礎下我們還是可以查覺

舉例來說，你比較靠一邊的喇叭聽兩隻喇叭的聲音，即使你將在那個位置的聲音調成左右聲道一樣，你還是可以感覺到聽到的聲音跟坐在喇叭正中間是不同的
另外一個例子是我們將音樂延遲後混到正常沒有延遲的版本，若是一點點延遲的混音，會使頻率相互取消、若長一點的延遲就會變成echo

若將你的一隻喇叭反過來，將訊號反相，就可達到phase cancellation (一般人run喇叭的方法)，在mono訊號與低頻的部份特別容易發現，喇叭的距離就比較沒關係了


Relative phase

Now, even if the phase is linear (simply an overall delay), we can easily detect a phase difference if we have a reference. For instance, you can get closer to one of your stereo speakers than the other; even if you use the stereo balance control to even the relative loudness between speakers, it won’t sound the same as being equidistance between them.
Another obvious case is when we have a direct reference to compare to. When you delay music and mix it with the un-delayed version, for instance, it’s easy to hear the effect; short delays cause frequency-dependent cancellation between the two signals, while longer delays result in an obvious echo.
If you connect one of your stereo speakers up backwards, inverting the signal, you’ll get phase cancellation between many harmonic components simultaneously as they cancel in the air. This is particularly noticeable with mono input and at low frequencies, where the distance between the speakers has less effect.

[Leo Yeh]

其實我覺得相位問題一但發生很難解決
不過我們至少要能知道與瞭解相位問題

[999369]

資枓~~~~~~~~~~~~~~

[chsiang]

考慮到邊界造成的效果, 問題又複雜起來......

[Leo Yeh]

在來看看一般的Case，不是Linear Phase，而是frequency-dependent delay，跟頻率有關的延遲
我們能聽出相同訊號，但是相位不同的差異嗎？
答案是No and Yes
相位錯誤(Phase Error)，在真實社會，這個通常是指固定的，且影響一群的頻率。

NO => 人耳對聲音固定相位的改變是不敏感的，你聽不出來聲波只有相位的不同
Yes => 如果是動態的改變，那就會影響聲音了，音樂正是動態的聲波形式，其影響程度視相位的角度 (phase error)與其所影響的範圍、音樂本身來決定

The general case
Having dispensed with linear phase, let’s look at the more general case of phase as a frequency-dependent delay.
Does it seem likely that we could hear the difference between a music signal and the same signal with altered phase?
First, I should point out that phase error, in the real world, is typically constant and affects a group of frequencies, usually by progressive amounts. By “constant”, I mean that the phase error is not moving around, as in the effect a phase shifter device is designed to produce. By “group of frequencies”, I mean that it’s typically not a signal frequency that’s shifted, or unrelated frequencies; phase shift typically “smears” an area of the music spectrum.
Back to the question: Does it seem likely that we could hear the difference between an audio signal and the same signal with altered phase? The answer is… No… and ultimately Yes.
No: The human ear is insensitive to a constant relative phase change in a static waveform. For instance, you cannot here the difference between a steady sawtooth wave (which contains all harmonic frequencies) and a waveform that contains the same harmonic content but with the phase of the harmonics delayed by various (but constant) amounts. The second waveform would not look like a sawtooth on an oscilloscope, but you would not be able to hear the difference. And this is true no matter how ridiculous you get with the phase shifting.
Yes: Dynamically changing waveforms are a different matter. In particular, it’s not only reasonable, but easy to demonstrate (at least under artificially produced conditions) that musical transients (pluck, ding, tap) can be severely damaged by phase shift. Many frequencies of short duration combine to produce a transient, and phase shift smears their time relationship, turning a “tock!” into a “thwock!”.
Because music is a dynamic waveform, the answer has to be “yes”—phase shift can indeed affect the sound. The second part is “how much?” Certainly, that is a tougher question. It depends on the degree or phase error, the area of the spectrum it occupies, and the music itself. Clearly we can tolerate phase shift to a degree. All forms of analog equalization—such as on mixing consoles—impart significant phase shift. It’s probably wise, though, to minimize phase shift where we can.

[Leo Yeh]

不要將極性(polarity)與相位(Phase)搞混喔...............

[chsiang]

polarity我是真的聽不太出來, 測試片那種很刻意的訊號可以

但是如果是cd player切換正反向 以及 小葉你給的那個網址 我是真的聽不出來

[小佐]

因為聽不出差異啊。如果心理毛毛的，把左右兩邊喇叭線一起紅黑反接再聽聽看。如果聽不出來就別管了，如果覺得有聽得出來的差異，那就挑自己聽起來較舒服的方式來接。

我就是這樣~~~~:*:P

[999369]

Phase~~~~~~~~~~~~