This term describes a low level off-tape signal. This can be due to a poorly made recording on broken or mis-calibrated machine/record heads, or the use of poor quality video tape stock. Dirty playback heads caused by oxide buildup will output low RF, as well. Some preservationists maintain that age alone isn’t a factor in RF deterioration or magnetic media lifespan. Some VTRs have a meter which displays RF levels during playback. Others have warning lamps that activate when a low level is detected.
Can it be fixed?
If oxide build-up on the tape heads is causing this issue, then yes, cleaning the heads should help. If the source media itself has been exposed to a strong magnetic field (unshielded speakers, motors, high-voltage transformers, etc.) that has caused RF to drop, then there is no remedy for this problem.