In the 1990s, electro-mechanical processes were largely superseded by digital technology, with digital recordings transferred to digital masters by an optical etching process that employs laser technology. The digital audio workstation (DAW) became common in many mastering facilities, allowing the off-line manipulation of recorded audio via a graphical user interface (GUI). Although many digital processing tools are common during mastering, it is also very common to use analog media and processing equipment for the mastering stage.
Just as in other areas of audio, the benefits and drawbacks of digital technology compared to analog technology for some, is still a matter of debate. However, in the field of audio mastering, the debate is usually over the use of digital versus analog signal processing rather than the use of digital technology for storage of audio.
Although in reality there isn't such a thing as an "optimum mix level for mastering", the example on this picture to the right only suggests what mix levels are ideal for the studio engineer to render and for the mastering engineer to process. It's very important to allow enough headroom for the mastering engineer's work. Many mastering engineers working with digital equipment would agree that a minimum of 3 to 6 dB of available headroom is critical to perform good mastering. Ideal peak levels should not exceed -3dBFSD and the average sum of the left and right channels should be at around -10 to -18 dBFSD.
The audio mastering process is much different than the recording/mixing process. The manipulation of the final mix requires flat response speakers within a flat response environment. In today's market mastering exists within 3 budget catagories. The "small range" $100.00 per track independent artist. The "mid range" $250.00 per track independent label and the "high end range" $500.00 per track major label recording artist. The variables being the studio and experience of the mastering engineer. It should be noted that many mastered CD's costing $20,000 still sounded like crap and sold ziltch... while many $1,500 CD's ended up sounding fantastic and sold millions. The point being, it's not always the money. It's the engineer. The time devoted. Great care should be taken in this process. Without any doubt, there are mastering engineers and mastering studios well worth the $20K to $40K price tag, but there are probably less than 20 of them in the entire world... and the waiting list is long. Sometimes many months. Yet, this final process may be the most important of all !
The source material is processed using equalization, compression, limiting, noise reduction and other processes. Subsequently, it is rendered to a medium such as CD or DVD. This mastered source material is also put in the proper order at this stage. This is commonly called the assembly or track sequencing. More tasks such as editing, pre-gapping, leveling, fading in and out, noise reduction and other signal restoration and enhancement processes can be applied as part of the mastering stage.
The specific medium varies, depending on the intended release format of the final product. For digital audio releases, there is more than one possible master medium, chosen based on replication factory requirements or record label security concerns.
A mastering engineer may be required
to take other steps, such as the creation of a PMCD (Pre-Mastered Compact Disc),
where this cohesive material needs to be transferred to a master disc for mass
replication. A good architecture of the PMCD is crucial for a successful
transfer to a glass master that will generate stampers for reproduction.
The process of audio mastering varies depending on the specific needs of the audio to be processed. Steps of the process typically include but are not limited to the following:
> transferring the recorded audio tracks into the Digital Audio Workstation (DAW) (optional)
> sequence the separate songs or tracks (the spaces in between) as they will appear on the final product (for example, an audio CD)
> process or "sweeten" audio to maximize the sound quality for its particular medium
> transfer the audio to the final master format (i.e., Red Book-compatible audio CD or a CD-ROM data, half-inch reel tape, PCM 1630 U-matic tape, etc.)
Examples of possible actions taken during mastering:
> edit minor flaws
> apply noise reduction to eliminate hum and hiss
> adjust stereo width
> add ambience
> equalize audio between tracks
> adjust volumes
> dynamic expansion
> dynamic compression
> peak limit the tracks
The guidelines above are mainly descriptive of the mastering process and not considered specific instructions applicable in a given situation. Mastering engineers need to examine the types of input media, the expectations of the source producer or recipient, the limitations of the end medium and process the subject accordingly. General rules of thumb can rarely be applied.