the first lp records introduced used fixed pitch grooves just like their 78 predecessors. the use of magnetic tape for the production of the master recordings allowed the introduction of variable pitch grooves. the magnetic tape reproducer used to transfer the recording to the master disc was equipped with an auxiliary playback head positioned ahead of the main head by a distance equal to one revolution of the disc. the sole purpose of this head was to monitor the amplitude of the recording. if the sound level from both the auxiliary and main magnetic heads was loud, the cutting head on the disc recording lathe was driven at its normal speed. however, if the sound level from both magnetic heads was quieter, then the disc cutting head could be driven at a lower speed reducing the groove pitch with no danger of the adjacent grooves colliding with each other. the playing time of the disc was therefore increased by an amount dependent on the duration of quieter passages.