5.4.11 Noise Reduction The signal recorded onto a tape may have been encoded in such a way as to mask the inherent noise of the carrier. This is known as noise reduction. If the tape has been encoded while recording, it must be decoded using the same type of decoder appropriately aligned. The most common noise reduction systems include Dolby A, and Dolby SR (professional), Dolby B and Dolby C (domestic), dbx types I (professional) and II (domestic)although rarely used and TelCom. The alignment of the record and replay characteristics of the tape machine are critical to the adequate operation of noise reduction systems and characteristic line up tones are often included on professionally recorded tapes. The output level, as well as the frequency response can alter the response of the decoding system and it is also important to note that noise reduction may be applied to either IEC or NAB equalisation and must be replayed correctly. Dolby B and Dolby C have routinely been included in most professional cassette decks of recent years and generally do not have line up tones and have a less obvious effect on the signal than the professional systems. Though it is possible to transfer the audio from an encoded tape for decoding at a later time, the multiple variables in alignment can compound the errors and make it difficult to decode accurately once the tape has been transferred. Decoding is better undertaken at the time of transfer. Unless documented, it is difficult to assess whether compact cassettes have encoded with a noise reduction system. As with equalisation, the lack of documentation may require the operator to make such decisions aurally. The right replay is generally characterised by an even level of background hiss, while the fluctuation of this level indicates a wrong playback setting. A spectrum analysis tool can be helpful. If it cannot be determined, copies of cassettes should be made flat.