The lissajous pattern on oscilloscope display

The digital oscilloscope is a very ideal device for displaying lissajous patterns. The lissajous pattern it created by feeding a repetitive sine wave into the vertical axis of the digital oscilloscope display and another into the horizontal display axis. The result is a pattern that can show the relative frequency or phase of one compared with the other. For example, if a 1Khz signal were fed into one axis and a 2Khz signal were fed into the other axis the lissajous would display a pattern with one side being twice the figure of the other. Here is what it would look like, below.

If two sine wave signals were at a ratio of 3/2 in frequency it would look lik this:

In the early days of stereophonic recording it was important to know the degree of “separation” between the two channels there was. The oscilloscope was an important tool for the recording and mixing engineers to learn the degree of separation between for the overall “stereo effect”. The same requirements were needed for FM broadcasting and transmission channels.
The degree of “stereoness” can easily be seen on an oscilloscope display pattern. Below is what a typical pattern may look like:

(image: picoscope)

The fullness of the pattern indicates the stereo is more broadly separated. If the two channels were monophonic the pattern would merely becom a single horizontal line.
In my early days as an equipment designer we wanted to keep the separation of the stereophonic signals as far apart as possible throughout the low and middle frequencies because that is where the human ear will hear the greatest stereo effect. But, at higher frequencies it was essential to bring the channels closer together in order to minimze the noise that is generated by most FM demodulators; in other words we used the oscilloscope to help us design circuits that would present to the ear a legitimately sounding stereo playback but with reduction of noise from the high frequency reproduction system.
You can imagine how useful this would be in the recording, mixing and broadcast studios. The digital oscilloscope makes a very viable tool for them to use.

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