Vectorscope
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Vectorscope.jpg
Vectorscope_graticule.png
A vectorscope is a special type of oscilloscope used in both audio and video applications. Whereas an oscilloscope or waveform monitor normally displays a plot of signal vs. time, a vectorscope displays an X-Y plot of two signals, revealing valuable details about the relationship between these two signals.
Video Applications
In video applications, a vectorscope supplements a waveform monitor for the purpose of measuring and testing television signals, regardless of format (NTSC or PAL). While a waveform monitor allows a broadcast technician to measure the overall characteristics of a video signal, a vectorscope is used to visualize chrominance, which is encoded into the video signal as a subcarrier of specific frequency. The vectorscope locks exclusively to the chrominance subcarrier in the video signal (at 3.58 MHz for NTSC, or at 4.43 MHz for PAL) to drive its display.
A vectorscope uses a circular display, or graticule, for visualizing chrominance signals, which is the best method of referring to the QAM scheme used to encode color into a video signal. Chrominance is measured using two methods—color saturation, encoded as the amplitude, or gain, of the color subcarrier signal, and hue, encoded as the subcarrier's phase. The vectorscope's graticule represents saturation as distance from the center of the circle, and hue as the angle, in standard position, around it. The graticule is also embellished with several elements corresponding to the various components of the standard colorbar video test signal, including boxes around the circles for the colors in the main bars, and perpendicular lines corresponding to the U and V components of the chrominance signal (and additionally on an NTSC vectorscope, the I and Q components). NTSC vectorscopes have one set of boxes for the color bars, while their PAL counterparts have two sets of boxes, due to the fact that the R-Y chrominance component in PAL reverses in phase on alternating lines. Another element in the graticule is a fine grid at the nine-o'clock, or -U position, used for measuring differential gain and phase.
The reference signal used for the vectorscope's display is the color burst that is transmitted before each line of video, which for NTSC is defined to have a phase of 180°, corresponding to the nine-o'clock position on the graticule. The actual color burst signal shows up on the vectorscope as a straight line pointing to the left from the center of the graticule. In the case of PAL, the color burst phase alternates between 135° and 225°, resulting in two vectors pointing in the half-past-ten and half-past-seven positions on the graticule, respectively.
Audio Applications
In audio applications, a vectorscope is used to measure the difference between channels of stereo audio signals. One stereo channel drives the horizontal deflection of the display, and the other drives the vertical deflection. A monaural signal, consisting of identical left and right signals, results in a straight line with a slope of positive one. Any stereo separation is visible as a deviation from this line, creating a Lissajous figure. If the straight line (or deviation from it) appears with a slope of negative one, this indicates that the left and right channels are 180° out of phase.