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Mastering 101
Old 12th May 2008
Let me take a moment to type up a PQ primer...

PQ codes refer to 2 of the 8 subcode channels P, Q, R, S, T, U, V, W, that run alongside the audio data on a CD. The P and Q bits contain basic info like track start, pause (end), index, and the basic TOC (Table Of Contents) data. Some other things included are ISRC codes, UPC code, emphasis flag, and copy inhibit flag.

ISRC is short for International Standard Recording Code and is a unique identifier for each track that lists the country of origin, registrant (releasing entity, usually the label), year, and designation code (unique identifier created by the registrant). This code stays with the audio recording for the life of it. Even if it later appears on a compilation, the same ISRC will accompany it.

It is not for the compposition, however, simply the recording. If a new recording of a song is made, it will receive a new ISRC code. In the US the codes are administered by the RIAA. They can help with anti-piracy and royalty collection, though most US radio isn't very good about using the codes. There is better support for them in Europe.

The UPC code is the Universal Product Code, which is essentially the number represented by the barcode on the back of the packaging. These are administered by the UCC, or Uniform Code Council. While an ISRC refers to a single track, the UPC code is for the entire album. Each unique physical product that may be put on a store shelf has a unique code. In addition to the barcode on the back, you can actually encode this into the PQ information.

The code is compatible with the Europenan EAN code, which is why it is often referred to as a UPC/EAN code. The UPC has 12 digits, while the EAN has 13, so if you are encoding a UPC onto the disc, you simply add a leading zero and the rest of the numbers are identical.

Copy Prohibit status and emphasis are less likely to be used much these days. However, early digital recordings did indeed use emphasis, so if you are doing a reissue, compilation, or career retrospective, you may indeed need to know how to deal with pre-emphasis.

CD-Text information (not supported by most players, and not to be confused with CDDB which supplies titles to computer players) is included in one of the other bits (R through W), as is karaoke info, graphics, and other extended features not standard to the original red book CD spec.

Some other "books" besides red include yellow book, which is a data CD (or CD-ROM, often used for DDPi masters), orange book, which is recordable CD, and the extended, +G, and enhanced versions like blue, green, and purple. SACD (Super Audio CD) with its DSD recording is Scarlet book. SACD and DVD are not put on CD media, but rather UDF (Universal Disc Format) higher capacity discs (flavors of DVD).
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