大多數時候，我們最喜歡的音樂CD播放軟體提供從線上資料庫下載相關資訊，但這一步真的有必要嗎？音樂CD上是否已經有了所需的全部資訊？今天的超級使用者問答帖子回答了一位好奇的讀者的問題。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

照片由約翰·沃德（Flickr）提供。

問題

超級使用者讀者cipricus想知道大多數音樂CD是否包含曲目所需的元資料：

I see that many audio players (multimedia software like Winamp or Foobar2000, for example) have the ability to retrieve music (song) information from online databases like CDDB. This information should already be available on the music CDs though, right? Is it actually there?

Some audio players display a CD’s contents while others do not. Is that information taken from the CDs or retrieved from the Internet?

大多數音樂CD是否包含曲目所需的元資料？

答案

超級使用者貢獻者RedGrittyBrick為我們提供了答案：

This information should already be available on the music CDs though, right?

I think most of us, as c***umers, would say yes.

Is it actually there?

Almost never in my experience. The software that I have used to rip CDs to MP3s never seems able to obtain this information from the CDs themselves, though I have read about a few excepti*** (notably Sony since 1997).

There are probably several reas*** for this, including:

• The music industry’s business model
• Inertia
• The rise of digital distribution

Music Industry Business Model

The music industry traditionally made money from the sales of vinyl records, cassette tapes, and audio CDs. Protection of their copyright was seen by the industry as essential for their survival. In order to combat illegal copying of tapes, they persuaded legislators to impose a levy on blank tape sales.

The music industry felt that facilitating playback on personal computers was facilitating the infringement of their copyright, thus facilitating their own destruction. So decisi*** concerning audio CD content and formats were heavily skewed against making anything easier for personal computer users.

Inertia

The audio CD has been established for a long time now and there is no point in making new CDs incompatible with existing CD players. This means that care has to be taken if adding digital content to audio CDs. Digital data and audio data on CDs use completely different and incompatible underlying formats. This makes it tricky to mix both (though it can be done).

Given a large population of older CD players, the industry has evidently not seen any benefit in improving the audio CD format.

Their perceived usage case is: you buy a CD, you put it into a dedicated audio CD player attached to an audio amplifier and loudspeakers, you sit down and read the track information printed on the CD’s cover.

Digital Distribution

These days the trend is shifting to downloadable content. At least purchased MP3 files generally contain metadata listing the artist, album name, year, genre, etc.

It therefore seems unlikely that the music industry has any interest whatsoever in doing anything new with their CD pressing process. It is a dying business after all. From a 2011 blog post:

• One of the greatest, coolest, but sadly least known and least often used tech things about CDs is CD-Text. … This has been out for 14 years and I can count on one hand the number of times I have actually seen a CD in my car have text associated with it.

Make that nearly 20 years now and no sign of general adoption by the music industry.

Why Did CDs not Include Metadata Originally?

It is worth remembering that the audio CD was merely a more durable and convenient sized replacement for the pressed 12″ vinyl album disc.

The latter was a purely ****ogue form with no digital information on it, just the ****ogue audio waveform in the form of vertical and horizontal undulati*** in a continuous spiral groove, with no distinction between tracks other than a section of silence (no undulati***) and wider spacing of the spiral (visible to humans but not detectable by a record player). Any information about track names, etc. was present on the printed paper sleeve notes or on the printed cardboard sleeves themselves.

So when audio CDs were invented, they took the same approach. They expected CDs to be played in dedicated CD music players, not in computers. Therefore, the music was not stored on CDs with the type of file system that a computer would normally use for data files. Details of tracks were printed on the paper insert in the plastic CD-case and not placed with the CD contents in any way.

Similarly, the audio data on an audio CD was encoded on a single continuous spiral track. This is very different from the low-level formatting of computer data disks (hard, floppy, CD data, etc.) which typically have a large number of circular tracks arranged concentrically and divided into sectors.

There was no provision for data, probably because this had not been needed for vinyl records and because it would have complicated the manufacturing of audio CD players, making them more expensive at a time when the industry presumably wanted to encourage sales of CDs as a premium (more profitable) product.

Note that, to identify a CD, programs on computers have to extract some of the audio data (i.e. the list of song offsets in the lead-in section of the track or the waveform of part of the first song) and use that as a key for searching in a database, typically a remote database elsewhere on the Internet. This is how software retrieves artist names, album names, track names, etc.

Some programs do look for CD-Text, sometimes only if they are offline and cannot contact a remote database. So the presence and use of CD-Text is a relative rarity. There is no computer readable metadata in most audio CDs, not even an identifying product number.

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