I understand that this has something to do with difference between refresh rate of monitor (or line frequency used by the ballast in case of Fluoroscent lamp) and frame capture rate of video recorder.

When learning about DSP in the past, I vaguely remember that a certain difference in frame capture rates of video recorder against the refresh rate of screen will cause the movement of bands upwards or downwards (assuming screen drawn left to right, top to bottom) ?

Please help me understand the phenomenon (the name, if there is one for this) or direct me to appropriate text on this.

Thank you.

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    $\begingroup$ Aliasing. Similar phenomena include the stroboscopic effect and the "Wagon-wheel" effect. $\endgroup$ – pichenettes Mar 28 '14 at 16:21

This effect is most obvious in light generators that brighten and dim cyclicly over time, as in an AC current powered photon generator (bulb), or the phosphors on an old CRT periodically hit by a refresh beam.

In some ways this is similar to audio beating between two different but close audio frequency signals or pitched notes. If the frequencies are slightly different (including the fundamental of one to a strong harmonic of another), then the phase will drift in and out over time. In audio, this creates changing constructive or destructive interference. In video recording this might capture the brightest portion of the beam or lamp when in phase, or not capture it when out of phase, or a capture over time while the phase is continually changing in between, resulting in rolling of the brightest band.

Note that the NTSC frame refresh rate is 29.97 Hz, which will beat against half an AC line frequency of 30 Hz.

| improve this answer | |
  • $\begingroup$ Is there a jargon for this phenomenon of generating beats ? $\endgroup$ – Bleamer Apr 1 '14 at 8:58

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