Mastering is about two things. Firstly it’s about careful listening by an experienced engineer, on a reference-quality reproduction system in a familiar, acoustically controlled environment. In that way, decisions on sound quality, and any changes felt necessary, may be made with utmost confidence.

Secondly, mastering is about preparing the pre-master to be used for replication or duplication. As well as the audio, this pre-master carries information such as the track start/finish information and barcode. This pre-master needs to be prepared in the way expected by the manufacturing plant, and should be free of faults which would cause problems, which at that stage can be very costly.

After listening to a recording in controlled conditions, it may be decided that it would benefit from some EQ (‘equalisation’) to alter the tonal balance, some extra reverberation, or some other process. In addition, it is often possible to remedy some minor blemishes using Cedar Retouch or iZotope RX, resulting in a more polished sounding recording. Lastly there is also the issue of levels. It’s important that the master is made at an appropriate level, which for digital audio (like CD) means that it should peak just below the maximum possible (0dBFS). More obvious to the listener is the average level, which depends on the dynamic range of the music. In many cases it’s best to leave the listener to enjoy the full dynamic range in a natural way. There are times, however, when an excessively low average level needs to be raised, which means that the dynamic range must be controlled. Whether this is achieved manually (‘over-controlling’) or automatically (compression) will depend on the dynamic nature of the recording, and the subjective effects of these processes. When done subtly and sympathetically, very few need be aware of an ‘invisible hand’ exerting some control over levels.

The position of track markers is contained in the ‘PQ’ subcode, and this information is added at the mastering stage, along with a barcode and the International Standard Recording Code (ISRC) for each track, where appropriate. See here for more details on ISRCs:


With most releases now being made available online as well as in physical form, an extra stage has now been introduced to ensure that when the master files are converted to their compressed format (AAC or MP3) for delivery by iTunes or other digital download / streaming platforms, levels are maintained within peak limits.  This process is known as MFiT or Mastered for iTunes and Classical Media is an approved MFiT supplier to Apple.

Recordings of a higher resolution than the release format (which for CD is 44.1kHz/16-bit) are processed at the mastering stage, with noise-shaped dither applied following re-quantisation for optimum performance.

We can prepare pre-masters in several ways, the most common of which is a DDP fileset which can be delivered electronically and securely direct to the manufacturing plant.

Case Study: Oboe Classics – Janet Craxton
Specialist label Oboe Classics wanted to release a disc featuring the late British oboist Janet Craxton in modern chamber repertoire. The recordings were originally made by the BBC in various locations at different times. Some were live concerts, others sessions produced for later broadcast. Auditioning them in our edit suite, it was obvious that the recordings varied greatly in level, tonal balance and recorded perspective. One had a huge left-right imbalance, while another was extremely wide. In order to present a more enjoyable listening experience, EQ was applied to reduce the tonal differences between the different recordings, that left-right imbalance was evened up, and the wide recording narrowed to be closer to the others in perspective.
It was possible (and felt desirable) to enhance ensemble in a few places, and correct wrong/fluffed notes in one or two others. Gaps between movements were tidied, and many noises off and blemishes (clicks, bumps, dropouts) were remedied using Retouch. In addition, two of the pieces benefited from the ‘bloom’ of a little artificial reverberation. After establishing gaps between pieces, the track markers were placed (PQ encoding), and the barcode and ISRCs entered. Finally a PMCD was burnt onto a branded CDR at single speed.