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Metadata on Audio Discs

November 19, 2011

One of the more frequent questions that come in is about audio discs. I am specifically referring to the sort of disc that plays in a ordinary CD player, not a disc containing MP3 files. These discs can have computer applications on them as an extension and such discs are referred to as “enhanced music CDs”. The key is that all but perhaps one track is an audio track.

Audio tracks exist only on CDs – there is no provision for such a think on a DVD. There were at one time music DVDs but they have pretty much gone by the wayside as far as the consumer is concerned.

So what is an audio track and how is it different? To start with understand that CDs came about as an audio playback medium – in 1982 you could buy a music CD and a player and that was all there was. The idea of putting data onto a CD came quite a bit later. So the foundation of all CD structure is in fact the audio track.

An audio track consists of a collection of frames which are sometimes referred to as subcode blocks. It isn’t really accurate to call them sectors, although most drives will treat an audio track as having sectors today. Each frame holds 588 stereo samples with two 16-bit binary numbers in each one. This, when played at 44,100 samples per second results in the original audio being played.

Now going back to 1982 you have to realize that this was the dawning of microprocessors. They were starting to come out in consumer devices, like microwave ovens and the like but were still rather primitive. In many ways the design of the structure of an audio CD was geared to what was possible and cheap to do in the early 1980s. Therefore, unlike many standards you find today with huge amounts of metadata the CD audio standard doesn’t provide for any.

That’s right. None. Nothing. No information whatsoever about the music that is being played. No dates, no titles or other textual information. There are two times that refer to the content of the disc Atime and Ptime which translate to Absolute Time (the position on the entire disc) and Program Time which is really the duration into the track itself.

From the Ptime and other information on the disc it is possible to calculate the remaining time in a track which some players show. But the original players with primitive microprocessors would display only the duration in the current track.

The table of contents (or TOC) is simply a set of pointers into the disc indicating the starting point for each of the tracks and where the disc ends. While being played there is also information in the frames that can be used to identify the end of a track.

This is about the time that people say “But …” because they have put a music disc into a computer and seen a list of track names with dates and times. In the class “CD and DVD Forensics” we spend about 30 minutes with a music disc going over this specifically in depth.

For now, let me just say that what you are seeing is a construct from Windows and all of the timestamp information you are seeing is constructed by Windows. There are no dates on the disc. Check it out with a couple of different discs from as widely spread out time period as you can find. An old Who CD from the 1980s would be something interesting to check this out with.

The real problem for the forensic examiner comes when their prosecutor puts a music disc into a computer and sees the date and time that Windows made up. They then come back and demand an explanation of this and want the examiner to spend lots of time either explaining this or researching it in depth.

If such an unlucky examiner has a copy of CD/DVD Inspector, they are always free to call up and get an education on the subject, at least enough so they can go back to the prosecutor with a good explanation of why they don’t need to waste any more time on this.

Yes, CD/DVD Inspector does a really good job of digging out all of the information present on music CDs. It will also display the RID information that can be present on such discs, IF and only IF, the disc was made on a stand-alone audio recorder. Discs made on a computer do not generally have any sort of identifying information.

The key item to take away from this post is that there isn’t any real metadata on a music disc that you can use. Stop looking for it. If someone tries to convince you that there is something important there that can say when the disc was made or what computer it came from you are going to need some more information – maybe from InfinaDyne – to refute this and get back to real work.

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