Noise in the playback of phonographs takes different forms and has different origins. "Hiss," a steady background noise generally low in level relative to the program material, results both from noise originally recorded along with the program material during the making of the record (such as noise inherent to the recording equipment), and from dust and wear on the record itself. "Clicks," "pops" and other types of high-level, irregularly occurring noise stem from scratches in the the record or large particles of dust or dirt settled on the surface of the record. These two types of noise are generally present in combination during phonograph playback.
For each type of noise, there is a correlating type of noise reduction developed to combat it. There are both "de-hissing", and "de-clicking" hardware and software solutions available. "De-hissing" systems analyze the frequency and dynamic content of hiss-type noise, and use equalization and dynamic control to reduce the level of hiss. "De-clicking" systems identify the strong transient fingerprint of clicks, and selectively reduce the volume of the clicks. You must carefully adjust de-clicking to avoid removing transients from musical elements in the recording such as drums and percussion.
There are both hardware and software systems available for vinyl phonograph restoration and noise removal. Some of these operate in real time, meaning that you can put them into the audio signal path after the phonograph and use the noise reduction as while listening, without the need to record the phonograph to a digital file. Hardware units from CEDAR operate in real time, and are therefore also useful for broadcast playback situations. Software noise reduction is generally a plug-in for existing audio software. Using software solutions, audio engineers record the phonograph into an audio workstation and process the resulting file within the program. Plug-ins from Waves and Sound Forge are popular choices within audio engineering circles. The free program "Audacity" also offers a noise reduction function, making it a good choice for phonograph enthusiasts with a computer, but who do not have a dedicated audio recording setup.
Whether you are performing noise reduction in real time or processed to an audio file recorded from a phonograph, the signal chain remains basically the same. First, connect the output of the phonograph to a phonograph preamp for playback. If you are recording the phonograph to digital, connect the output of the preamp to the analog-to-digital converter (input) of a digital recorder. Next, place the noise reduction units, whether hardware or software, in line with the signal. Place click removal before hiss removal, but listen carefully when adjusting the units; it is always best to adjust audio processing by ear! Finally, connect the output of the noise reduction units the playback system for final amplification over speakers or for broadcast. In the case of a digital recording, record the noise reduction software/plug-in processing to a new file, and you can preserve the noise-reduced recording for later playback.