Pre-emphasis is the first part of a noise reduction technique in which a signal's weaker, higher frequencies are boosted before they are transmitted or recorded onto a storage medium. Upon playback, a de-emphasis filter is applied to reverse the process. Pre- and de-emphasis can be collectively referred to as just emphasis. Emphasis was sometimes used in digital storage media in the late s through early s, including on a small percentage of audio CDs. Emphasis is akin to Dolby noise reduction for tapes, or the RIAA equalization curve for vinyl records. Some also used noisy "brick wall" filters to remove frequencies higher than the Nyquist frequency Hz. The resulting noise introduced by these converters and filters could be made relatively quiet by using pre-emphasis: boosting the signal especially the higher frequencies in the recording before it was put onto CD, and embedding flags in the disc's subcode to tell a CD player to apply de-emphasis on playback.