Historically, the original Macrovision technology was considered a nuisance to some specialist users because it could interfere with other electronic equipment. For example, if one were to run their video signal through a VCR before the television, some VCRs will output a ruined signal regardless of whether it is recording. This ... occurs in some TV-VCR combo sets. Apart from this, many DVD recorders mistake the mechanical instability of worn videotapes for Macrovision signals, and so refuse to make what would be perfectly legal DVD dubs of people's home old movies and the like. This widespread problem provides a legitimate basis for the sale of devices that defeat Macrovision. The signal has also been known to confuse home theater line doublers (devices for improving the quality of video for large projection TVs) and some high-end television comb filters.