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A mastering engineer is one skilled in the practice of taking audio (typically musical content) that has been previously mixed in either the analog or digital domain as mono, stereo, or multichannel formats and preparing it for use in distribution, whether by physical media such as a CD, vinyl record, or as some method of streaming audio.
Education and Experience
The mastering engineer is responsible for a final edit of a product and preparation for manufacturing copies. An audio mastering engineer (especially in music) should have complete knowledge of audio engineering, and may hold an audio or acoustic engineering degree. Most audio engineers master music or speech audio material. The best mastering engineers might possess arrangement and record production skills, allowing them to 'trouble-shoot' mix issues and improve the final sound. Generally, good mastering skills are based on experience which can be the result of many years of practice.
Many mastering engineers use special audio signal chain equipment to process audio. The equipment used in mastering is almost entirely dedicated to this goal and is usually very expensive. Some advocates for digital software claim that plug-ins are capable of handling mastering processes without signal degradation. The quality of the results varies according to the algorithm these programs use to operate.
Some mastering engineers rely not only on what they hear, but on what they see. Through the guidance of frequency spectrum analyzers, phase oscilloscopes and sensitive peak meters, the pre-mastering process can be taken to an unprecedented level of precision and maximum sonic delivery.
Aspects of their work
Most mastering engineer accolades are given for their ability to make a mix consistent with respect to subjective factors based on the perception of listeners, regardless of their playback systems and the environment. This is a difficult task due to the varieties of systems now available and the effect it has on the apparent qualitative attributes of the recording. For instance, a recording that sounds great on one speaker/amplifier combination playing CD audio, may sound drastically different on a computer-based system playing back a low-bitrate MP3. Some engineers maintain that the main mastering engineer's task is to improve upon playback systems translations while the position of others is to make a sonic impact.
Prolonged periods of listening to improperly mastered recordings usually leads to hearing fatigue that ultimately takes the pleasure out of the listening experience. A professional mastering engineer renders mixes that have a good harmonic balance. Harmonic balancing can be accomplished by correcting and removing the tonal imbalances that result from problem mixes, thus producing a more naturally pleasing and enjoyable sound to the listening ears, and is another aspect to a mastering engineer's job. This is why mastering is considered an art as well as an "audio engineering" discipline.