[转+原创]Color Definitions in Spectral Phase Display and Spectral Pan Display in Audition

The following article is transmitted from part of an introduction to Adobe Audition 2.0, which makes a brief presentation of newly added Spectral Phase Display and Spectral Pan Display.

以下文字将介绍在Audition中,“频谱-相位显示”与“频谱-立体声位置”图像对颜色的定义。第一部分为转载,第二部分为原创。

from Adobe Audition 2.0 – A major transformation for this audio recording and editing program

http://digitalproducer.digitalmedianet.com/articles/viewarticle.jsp?id=36828-2


The new Spectral Pan and Phase displays allow audio to be viewed as a plot of pan position or phase over time. In the Spectral Pan Display, you can visually analyze the L-R position of every frequency by looking at the stereo plane. The horizontal x-axis measures time and the vertical y-axis measures pan position. Signals that are panned full left are at the top of the graph, and full right panned signals are at the bottom. If you think something’s off in your stereo field, this is an excellent way to see exactly what’s going on. Then you can fix it, using tools such as Audition’s Center Channel Extractor and Pan/Expand effect.

The Spectral Phase display shows the phase difference between left and right channels. Frequencies that are 180 degrees out of phase are indicated by bright patches adjacent to the +/-180� markers. The Histogram Phase Meter augments the existing phase meters, which already included a standard x-y graph, mid/side graph, a spin graph (animated phase display) and a phase wheel. What the Histogram Meter provides that the others don’t is a wedge-shaped view of the L-R phase relationship. If you can’t figure out a stereo phase problem with these tools, maybe you’re in mono!

Spectral Controls panel at bottom with topographic color bars

Histogram phase analysis at left


 

The next portion is written by myself.

Through above we can see the Spectral image is plotted based on a function with multiple values and two-dimension axis. The horizontal axis maps to timer shaft and the vertical pan or phase depending on which plot is selected by you.

But my question is how the values of the function is defined and how they are displayed on the plot. It really occurs to me, after searching the Internet for “spectral phase display spectral pan display”, to make another google search on keywords “audition spectral phase display color definition”, partly because of a forum post complaining about poor explaination of the two spectral images in Adobe’s official documents.

It took me several minutes to perform some preliminary experiments about how they work. I used “generate” function, which can be found collected as a menu in the main window, to make some seconds of single-frequence tone (major A, f=440Hz) and pink noise on -6dBw. Later I made another period of tones, a little more complicated, which contains a base freq at 440Hz (A again) and four harmonic frequencies 880, 1760, … with a calculated gain based upon fourier series (to make the sound a little prettier and thus not so harsh! ). Pan sweeping was also applied as 360 degrees per cycle, 0.5 cycle per second. After (confusing) configurations, the plot turned out to be sort of interesting. For a period of pink noise, all of the three Spectral Displays are bright, even and harsh, showing the noise has a frequency spectrum on every frequence, while the pan and phase are changing rapidly and randomly.

Then we come to the “melodic A” tones. In Spectral  Frequency Display, five bright lines on the bottom of the plot indicates the tones generated. In Spectral Pan Display, a straight horizontal line stands for a constant pan position (determined by the configuration you have made when generating the signal). In Spectral Phase Display, which has diverted me, contains a series of slope. It’s easy to understand – the phase is, due to the configuration, sweeping linearly with respect to time. Hearing the sound, you can feel that the source of the signal keeps moving around you, and perhaps you can even tell it’s circling your head – that’s what phase really means!

The “Spectral Controls” panel can be called out by clicking the “View” menu, and it was until I got the text at the top I haven’t realized this additional control panel was hidden by default. It shows how the colors on the Spectral Displays are defined. Virtually, the colors can be changed by the user. The horizontal axis has ticks of “x dB”, which indicates the power of the signal. Drag the color or the bright patches and the colors can be adjusted or offset.

So here comes the answer, after some more readings on different topics written by different people. The Spectral Pan Display and Spectral Phase Display, firstly introduced in version 2.0, make a useful analysis for STEREO signal. The Pan Disp shows how the sound is biased in the terms of “left or right”. The vertical axis shows the degree of bias, -180 as totally left and +180 right. Different colors indicates different power (dW/dt, or an Inverse Fourier Transform of the Spectrum F-1{S(w)} in fact) of the signal. An overlay of signals will generate beautiful ribbons since the colors will be added according to superpositioning of the light, as you will see when handling with common music tracks. The Phase Disp provides a straight insight of how the sound is spread among the “stereo” space. The vertical axis shows in what position you can feel and locate the sound (using both of your ears, and of course a bit of imagination! ). The color definition is the same as Pan Disp.

In the end, we shall come back to Spectral Frequency View. These three plots are pretty interesting to me, as each of them depicts one unique character of a stereo sound, all of which can be precisely defined by mathematics, and computed out using numerical analyses. Although the colors look “different”, strange, or to me a little uncomfortable in Pan and Phase Display, they have sheerly the SAME meanings as the Freq Disp. So, if the “default” colors Adobe has determined when releasing its new software consent with each other in the three displays, maybe it would be easier for us to understand how they work.

– END –

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