the record manufacturers had also realised that by reducing the amplitude of the lower frequencies recorded in the groove, it was possible to decrease the spacing between the grooves and further increase the playing time. these low frequencies were then restored to their original level on playback. furthermore, if the amplitude of the high frequencies was artificially boosted on recording the disc and then subsequently reduced to their original level on playback, the noise introduced by the disc would be reduced by a similar amount. this gave rise to an equalization frequency response applied during record coupled with an inverse of the response applied on playback. each disc manufacturer applied their own version of an equalization curve (mostly because each manufacturer's equalization curve was protected by interlocking patents). low-end reproduction equipment applied a compromise playback equalization that reproduced most discs reasonably well. however, amplifiers for audiophile equipment were equipped with an equalization selector with a position for most, if not all, disc manufacturers. the net effect of equalization is to allow longer playing time and lower background noise while maintaining full fidelity of music or other content.