Discreet magnetic particles no matter how fine being pulled by the heads will result in a certain level of tape hiss.... It's a simple fact of life. This "hiss" is uniform, and although at a low level, it is still objectionable. The energy distribution of the "hiss" is normally right in the middle of the higher frequencies of the recorded material, resulting in a lower signal to noise ratio in the low level "brighter" sections of the material. Since as we stated that tape hiss is a fixed given, the only "out" is to increase the high end frequencies of the recorded material so as to improve the signal to noise ratio (which is just a fancy way of saying that we intended to "drown out" the tape hiss by recording the material louder. It's more than just turning up the recording level and recording everything "hotter" however. To preserve the dynamic range, one must do it somewhat selectively by boosting only the frequencies or sections in a way that will not introduce clipping.... The 3 main techniques for this are Pre-Emphasis/De-Emphasis, DBX & Dolby.