• The start of a track; this is how a digital recorder knows where the tracks start even when you're copying a CD with continuous tracks
• The source of the audio (CD, DAT, DCC, MD, analog source)
• The fact if the audio is an original or a copy
• The fact whether the audio is copy-protected or not.

Basically, you are allowed to copy a CD or a prerecorded tape (i.e. an original) to another tape, but you're not allowed to copy a tape that already is a copy of such a copy-protected source (by default, all CD's and prerecorded media are copy-protected, even the ones that were made before SCMS was invented).

However, you _can_ copy a tape indefinitely if it's your own recording, including analog copies of copy-protected media (the issue was about digital copies of prerecorded work, right?) or if it was recorded from a digital source that has the "copying allowed" bit enabled in subcode. Of course it is also possible to make as many copies of an original as you want (even simultaneously if you have some kind of digital signal splitter) but not from a copy.

Interesting note: I have a CD (CD-maxisingle of "Children" by Robert Miles, pressed by EOD in Tilburg, the Netherlands) that has the copy-allowed bit set to ON (probably because of a mistake with the mastering) but CANNOT be recorded with either of my DCC recorders; it appears the "copy allowed" bit (which is encoded slightly differently on a CD from what I hear) is misinterpreted by the DCC as a second-generation CD.

By the way, it is relatively easy to make a digital copy of an audio CD if you have a CD-recorder: all CD-recorders (and many CD-ROM drives) can read audio CD's digitally to harddisk. See the CD-R FAQ for more details on Digital Audio Extraction (DAE).

• The first version had to be built into the equipment (and customized for that purpose too, not for the faint-hearted). There is no version for DCC recorders I think.
• The Dutch January 1996 issue published a stand-alone version that can also be used for conversion from optical to electrical format and vice versa. I bought one of these as a kit for about $100 and it works brilliantly.
• The Dutch October 1996 issue had a sample-rate converter (again with both optical and electrical digital inputs and outputs) that happens to defeat SCMS as well, even if the input sample rate is the same as the output rate. This works because the digital signal is decoded, resampled and completely rebuilt from scratch. It should cost about $230 as a kit.
• The Dutch December 1997 issue features a version of the defeater that works on the basis of an EPROM instead of a $50+ EPLD (as the January 1996 version does). This makes it much cheaper and easier to build if you only have the schematics and no way to buy a kit somewhere.

There are other more elaborate ways around SCMS. For example, if you record audio to harddisk using the DCC175 + PC-link you can copy it back to another tape no problem. Also there are ways to copy from a digital source to a computer and back using a digital audio card such as the CardD (See the CD-R FAQ).