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The vertical resolution (number of horizontal lines) on a TV or computer screen. The term typically is used with TVs; for example, HDTVs have 720 or 1,080 lines, while analog TVs used 525 or 625 lines, several of which were invisible between frames.

Most spectra display only a single dot for the highest amplitude in each frequency bar, so the graph appears as a jagged line reflecting those peaks. The highest frequency on the horizontal axis is called the fmax, and the number of bars in the graph is known as the “number of lines of resolution.”

Setting the number of lines of resolution and the fmax are essential first steps for the vibration analyst. For most common analysis tasks, 400 lines of resolution is adequate, while more specialized tasks may require higher resolution. Setting the proper fmax is a little more complex.

The frequency resolution of each spectral line is equal to the Sampling Rate divided by the FFT size. For instance, if the FFT size is 1024 and the Sampling Rate is 8192, the resolution of each spectral line will be: 8192 / 1024 = 8 Hz. Larger FFT sizes provide higher spectral resolution but take longer to compute.

The line frequency (50/60Hz) is the frequency at which AC power is supplied to the motor; this causes changes in magnetic attraction between the rotor and stator at twice the line frequency. The varying magnetic forces cause small dimensional changes in the iron material, leading to vibration.

For "500 lines of horizontal resolution", this means if you inscribed a circle in the TV set, and displayed a pattern consisting of alternating white & black vertical lines, you could count 500 distinct lines across the diameter of the circle without the lines blurring together into a grey blob.

The frequency resolution of the vibration spectrum is equal to the Fmax divided by the number of lines (and multiplied by the window factor - which we will ignore here for now). So, if we collect a spectrum with an Fmax of 1,600 Hz with 800 lines of resolution, the resolution on our plot will be 1,600 / 800 or 2 Hz.