Stereo Separation Metering

Goniometer

The Goniometer presents the stereo separation signal measurement in the audio metering world. A Goniometer plots the signal on a two-dimensional area so that the correlation between the two audio channels becomes visually apparent. The goniometer proves useful because it provides very dense information in an analog and surprisingly intuitive form. From the display, one can get a good feel for the audio levels for each channel, the amount of stereo, and its compatibility as a mono signal. Even to some degree, the frequencies that the signal contains. Experts may even be able to determine the probable arrangement of microphones after the recorded signal.

Audio Vector Oscilloscope

Another name for the Goniometer is “Audio Vector Oscilloscope.” It is an instrument that can give a detailed picture of the relationships in a stereo signal. The idea behind the instrument was first developed by Holger Lauridsen, the Chief Engineer at Danmarks Radio, in the 1940s and early 1950s. It involves an oscilloscope with one input for the X-deflection and the other input for the Y-deflection. In comparison with a normal oscilloscope, the image is rotated by 45 degrees. This means that when the same signal is applied in the left and right channels (X and Y) it leads to a vertical deflection (a vertical line). If the signal is otherwise identical but in the opposite phase, then a deflection will occur in the horizontal plane. When the signals are different, the display changes from straight lines to spatial figures. What is ingenious about the instrument is that it can show many different parameters in the stereo signal simultaneously. As long as a major portion of the display lies within +/- 45 degrees related to the vertical axis, then there is a higher degree of mono compatibility. When the curves begin to bend at the top and the bottom, it can indicate nonlinearity in the relationship between the two channels. This can arise if overloading or limiting is occurring in only one channel. It can stem from time delays between the channels that may arise in the older digital systems or tape units.

The Process

An audio technician would typically begin a session by adjusting the equipment (usually with a 1 kHz monotone). The output should produce a vertical plot line. If you phase-invert one channel, the plot line is horizontal instead of vertical. This is a sure sign of problems. As for mono signals, a half-inverted signal would be reduced to (near) silence. The persistence of a CRT display is a desired effect on goniometers because the signal display is very dynamic, and the overall shape or envelope of the signal is the object of interest. In fact, good digital and software goniometers provide artificial and even user-adjustable persistence. The goniometer proves useful because it provides very dense information in an analog and surprisingly intuitive form. From the display, one can get a good feel for the audio levels for each channel, the amount of stereo and its compatibility as a mono signal, and even, to some degree, what frequencies the signal contains. Experts may even be able to determine the probable arrangement of microphones.

Additional resources & source texts

Goniometer