A history of mastering

Mastering is the last step before music reaches listeners: making the finished mix work on the format it will be released on, and making the songs on a release sound like they belong together. It started as a technical job, getting sound into a groove without the needle jumping out, and became a creative one. Its story is also the story of the loudness war, and of how streaming ended it.

Before mastering: the master was a metal disc

Until the late 1940s there was nothing to master. The performance was cut straight into a disc, and that disc was used to make the metal parts that pressed the records. The word master comes from that metal part, not from a stage of production.

Tape creates the transfer engineer (1948 onwards)

Once studios recorded on tape, someone had to transfer the finished tape onto a lacquer disc on a cutting lathe, and that turned out to be a craft of its own. These transfer engineers, or cutting engineers, had to balance things that fight each other on vinyl:

• Level against playing time. A louder cut makes wider grooves, so fewer minutes fit on a side. Too quiet, and the surface noise shows.
• Bass. Big, uneven bass makes the groove swing so far that the needle can jump out. With stereo records from the late 1950s, bass that differs between left and right also cuts the groove up and down, so cutting engineers kept the low end in the centre. That's still good advice today; see The low end.
• Harsh highs and loud "s" sounds can distort on playback, so they were tamed on the way in.

In 1954 the record industry agreed on one standard EQ for cutting and playing records, the RIAA curve, so every record sounded right on every player. As engineers began to use EQ and compression to make records sound better, not just fit, the transfer engineer became the mastering engineer, and independent mastering studios appeared. Loudness was already part of the job: jazz engineer Rudy Van Gelder designed a more powerful cutting amplifier in the 1950s partly so his clients' records would hold their own on jukeboxes and the radio.

The CD and the loudness war

The CD (1982) removed vinyl's physical limits. There was no groove to jump out of and no trade between level and playing time, only a hard digital ceiling: 0 dBFS, the largest number the format can store. In the 1980s, CD masters mostly left their peaks below that ceiling. Then louder started to win: a louder song sounds better in a quick comparison, on the radio and in the shop, so every release had to match or beat the last.

In 1994 Waves released the L1, the first mass-produced digital brick-wall limiter with lookahead. Limiters like it could shave the peaks off a mix and turn the rest up, so that a CD's average level kept rising while its peaks stayed at 0 dBFS. Through the late 1990s and 2000s masters got denser and denser: the loudest moments stopped standing out, drums lost their snap, and some records clipped and distorted outright. The low point is often given as Metallica's Death Magnetic (2008): fans found the CD audibly clipped, the version made for the video game Guitar Hero sounded more open, and thousands signed a petition asking for a new master.

Measuring loudness the way we hear it

The problem was partly that nobody measured loudness the way ears do: meters showed peaks, and peaks say little about how loud something sounds. In 2006 the International Telecommunication Union published ITU-R BS.1770, a way to measure loudness that matches hearing, and in 2010 the European Broadcasting Union's R128 recommendation built on it, introduced the unit LUFS, and set TV and radio to −23 LUFS. Loud adverts stopped being louder than the programmes. More in Measuring loudness.

Streaming turns the volume knob for you

Streaming services adopted the same measurement for music. Spotify, for example, adjusts tracks to −14 LUFS by default, measured with ITU 1770; YouTube, Tidal, Amazon and Apple Music do the same kind of thing at their own levels. A master that is squeezed louder simply gets turned down to match everything else. What remains is the sound of the squeezing, so a heavily limited master now often sounds smaller next to an open one, not bigger. That is why MasterScope talks about density (PLR) and about peaks after encoding rather than chasing a loudness number.

Mastering today

Mastering engineers still cut vinyl, still prepare CDs, and now deliver for streaming, spatial audio and video too. Their work is still the same three things: make it sound its best, make the release consistent, and make sure nothing goes wrong on the way to the listener. What has changed is that the tools are within everyone's reach, which is why it helps to understand what they do. Liminal and MasterScope come out of that history: a limiter that measures peaks the way streaming encoders create them, and an analyser that judges a master by how it will actually be played.