Remaster (and its derivations, frequently found in the phrases digitally remastered or digital remastering) refers to quality enhancement of sound and/or picture to a previously existing recording.
To understand the concept of remastering, it is beneficial to understand that often a pyramid of copies would be made from a single original "master" recording, which might itself be based on previous recordings. For example, sound effects (a door opening, punching sounds, falling down the stairs, a bell ringing, etc.) might have been added from copies of sound effect tapes similar to modern sampling to make a radio play for broadcast.
A master is the recording which experts state will be the definitive copy that is duplicated for the end user usually into other formats i.e. LP records, CDs, DVDs etc.
Problematically, several different levels of masters often exist for any one audio release. As an example, examine the way a typical music album from the 1960s was created. Musicians and vocalists were recorded on multi-track tape. This tape was mixed to create a stereo or mono master. A further master tape would likely be created from this original master recording consisting of equalization and other adjustments and improvements to the audio to make it sound better on record players for example.
More master recordings would be duplicated from the equalized master for regional copying purposes (for example to send to several pressing plants). Pressing masters for vinyl recordings would be created. Often these interim recordings were referred to as Mother Tapes. All vinyl records would derive from one of the master recordings.
Thus, mastering refers to the process of creating a master. This might be as simple as copying a tape for further duplication purposes, or might include the actual equalization and processing steps used to fine-tune material for release. The latter example usually requires the work of mastering engineers.
With the advent of digital recording in the late 1970s, many mastering ideas changed. Previously, creating new masters meant incurring an analogue generational loss; in other words, copying a tape to a tape meant reducing the signal-to-noise ratio. This means how much of the original intended "good" information is recorded against faults added to the recording as a result of the technical limitations of the equipment used (noise, e.g. tape hiss, static, etc.). Although noise reduction techniques exist, they also increase other audio distortions such as azimuth shift, wow and flutter, print-through and stereo image shift.
With digital recording, masters could be created and duplicated without incurring the usual generational loss. As CDs were a digital format, digital masters created from original analog recordings became a necessity.
Remastering is the process of making a new master for an album, movie, or any other creation. It tends to refer to the process of porting a recording from an analogue medium to a digital one, but this is not always the case.
For example, a vinyl LP originally pressed from a worn-out pressing master many tape generations removed from the "original" master recording could be remastered and re-pressed from a better-condition tape. All CDs created from analogue sources are technically digitally remastered.
The process of creating a digital transfer of an analogue tape remasters the material in the digital domain, even if no equalization, compression, or other processing is done to the material. Ideally, because of their high resolution, a CD or DVD (or other) release should come from the best source possible, with the most care taken during its transfer.
Additionally, the earliest days of the CD era found digital technology in its infancy, which sometimes resulted in poor-sounding digital transfers. The earliest days of the DVD era were not much different, with early DVD copies of movies frequently being produced from worn prints, with low bitrates and muffled audio. When the first CD remasters turned out to be bestsellers, companies soon realized that new editions of back-catalogue items could compete with new releases as a source of revenue. Back-catalogue values skyrocketed, and today it is not unusual to see expanded and remastered editions of fairly modern albums.
Master tapes, or something close to them, can be used to make CD releases. Better processing choices can be used. Better prints can be utilized, with sound elements remixed to 5.1 surround sound and obvious print flaws digitally corrected. The modern era gives publishers almost unlimited ways to touch up, doctor, and "improve" their media, and as each release promises improved sound, video, extras and others, producers hope these upgrades will entice people into making a purchase.
Remastering music for CD or even digital distribution first starts from locating the original analog version. The next step involves digitising the track or tracks so it can be edited using a computer. Then the track order is chosen. This is something engineers often worry about because if the track order is not right, it may seem sonically unbalanced.
When the remastering starts, engineers use software tools such as a limiter, equaliser and a compressor. The compressor and limiters are ways of controlling the loudness of a track. However, this is not to be confused with the volume of a track, which is controlled by the listener during playback.
The dynamic range of an audio track is measured by calculating the variation between the loudest and the quietest part of a track. In recording studios the loudness is measured with negative decibels, zero designates the loudest recordable sound. A limiter works by having a certain cap on the loudest parts and if that cap is exceeded, it is automatically lowered by a ratio preset by the engineer.
Remastered audio has been the subject of criticism. Many remastered CDs from the late 1990s onwards have become casualties of the loudness war, where the average volume of the recording is increased at the expense of clarity and dynamic range, making the remastered version sound louder at regular listening volume than an uncompressed version. Some have also criticized the overuse of noise reduction in the remastering process, as it affects not only the noise, but the signal too, and can leave audible artifacts. Equalisation can change the character of a recording noticeably. As EQ decisions are a matter of taste to some degree, they are often the subject of criticism. Mastering engineers such as Steve Hoffman have noted that using flat EQ on a mastering allows listeners to adjust the EQ on their equipment to their own preference, but mastering a release with a certain EQ means that it may not be possible to get a recording to sound right on high end equipment. 
To remaster a movie digitally for DVD and Blu-ray, digital restoration operators must scan in the film frame by frame at a resolution of at least 2,048 pixels across (referred to as 2K resolution). Some films are scanned at 4K, 6K, or even 8K resolution to future proof for higher resolution delivery formats. Scanning a film at 4K - a resolution of 4096x3092 for a full frame of film - generates at least 12 Terabytes of data before any editing is done.
Digital restoration operators then use specialist software such as MTI's Digital Restoration System (DRS) to remove scratches and dust from damaged film. Restoring the film to its original color is also included in this process.
As well as remastering the video aspect, the audio is also remastered using such software as Pro Tools to remove background noise and boost dialogue volumes so when actors are speaking they are easier to understand and hear. Audio effects are also added or enhanced, as well as surround sound, which allows the soundtrack elements to be spread among multiple speakers for a more immersive experience.
An example of a restored film is the 1939 film The Wizard of Oz. The color portions of Oz were shot in the three-strip Technicolor process, which in the 1930s yielded three separate black and white negatives created from red, green, and blue light filters which were used to print the cyan, magenta, and yellow portions of the final printed color film answer print. These three negatives were scanned individually into a computer system, where the digital images were tinted and combined using proprietary software.
The cyan, magenta, and yellow records had suffered from shrinkage over the decades, and the software used in the restoration morphed all three records into the correct alignment. The software was also used to remove dust and scratches from the film by copying data, for example, from the cyan and yellow records to fix a blemish in the magenta record. Restoring the movie made it possible to see what could not be seen before: when the Scarecrow says "I have a brain", burlap is noticeable on his cheeks. It was also not possible to see a rivet between the Tin Man's eyes prior to the restoration.
Remastered movies have been the subject of criticism. When the Arnold Schwarzenegger film Predator was remastered it was felt that the process was overdone, resulting in Schwarzenegger's skin looking waxy. As well as complaints about the way the picture looks there have been other complaints about digital fixing. One notable complaint is from the 2002 remastered version of E.T. the Extra-Terrestrial (1982), where director Steven Spielberg replaced guns in the hands of police and federal agents with walkie talkies. A later 30th anniversary edition released in 2012 saw the return of the original scene.
Remastering a video game is more difficult than remastering a film or music because the video game's graphics show their age. This can be due to a number of factors. For example, modern televisions tend to have higher display resolutions than the televisions available when the video game was released. Because of this, classic games that are remastered typically have their graphics redesigned or their original graphics re-rendered at the higher resolutions used by high-definition televisions.
An example of a game that has had its original graphics re-rendered at higher resolutions is Hitman HD Trilogy, which contains two games with high resolution graphics: Hitman 2: Silent Assassin and Hitman: Contracts. Both were originally released on PC, PlayStation 2, and Xbox. The original resolution was 480p on Xbox. With the remaster, the games are displayed at 720p on XBOX 360.
There is some debate regarding whether new graphics of an older game at higher resolutions make a video game look better or worse than the original artwork, with comparisons made to colorizing black-and-white-movies.
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