Analog video signals are written onto tape as a frequency-amplitude modulated (FAM) radio-frequency signal. The frequency modulated (FM) part represents the luminance (brightness) information, and the amplitude modulated (AM) part represents the chrominance (color) information. Early systems used a relatively low-frequency range for the FM, but as the sizes of the magnetic particles in the tape became smaller with improvements in the technology, higher frequency recordings became possible, with consequent improvements in the available video bandwidth.
This led VTR manufacturers to release high-band versions of their recorders (e.g. S-VHS and Hi-8 ). The high-band machines can record and play the earlier tapes in the low-band format, but the old low-band machines cannot play the high-band tapes, and cannot record or erase them properly because higher magnetic field-strengths are required. A high-band machine can however, make low-band recordings on a high-band tape for playback in a low-band machine.