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Player recorder differences
To create copy-proof audio or video discs you have to be a little cleverer than just creating discs with errors. You have to exploit differences between the hardware/software that plays a disc and the hardware/software that records a disc.
For example, a recorder uses a preformatted disc with a spiral track that can record a fixed maximum amount of data. CDs are supposed to last 74mins and hold 650MBytes but by using space at the edge of the disc this can be increased to 76mins/670MBytes and even to 80mins/700MBytes.
This is “Overburn” and it is still used as a copy protection mechanism. Some CD Writers and their supplied software simply cannot cope with a CD that is bigger than 650Mbytes even if you can find a blank disc that can but all players will follow the data spiral to the very end.
Of course with the right CD writer overburn is easy to defeat and so it isn’t a good copy protection method but again it illustrates the way that differences between reading and writing mechanisms can be used.
Of course using such differences are a risk because it is always possible that some players will be designed with the same features as a copy device with the result that a legitimate user cannot use the disc. For example, back in the early days of CD writers they tended to use a particular decoder chip which crashed if fed an unusual data pattern which was supposed never to occur. Most CD players didn’t use this chip and so for a short while discs could be made uncopiable by including a small burst of special data which overloaded the decoder in a copying drive but not in a reading drive. Of course this was a short-lived success because it generated complaints from users with players that also couldn’t read the discs and the CD-RW drives soon moved on to new decoder chips that didn’t have the fault.
Subchannels and RAW copy
There is another way that a disc copy can be different from the original.
Much of the data on a disc is control data such as padding between frames or a byte to mark the start of a block of data. Most CD players read this data and use it internally but they don’t pass it on to software reading the disc. They simply pass on the “real” data corresponding to the audio, video or software recorded on the disc. When this is re-written to a blank disc the writing hardware automatically generates the necessary control data to produce a legal format. However some control data is meaningful and is use to create “sub-channels”.
The control byte, which is included at the start of every frame, is actually used to record some extra data the sub-channel data. A total of 98 frames are read to assemble a single sub-channel block of 98 bytes. There are eight types of sub-channel – called P to W – and these do various jobs including encoding a table of contents etc. Most CD readers don’t pass the sub-channel data on to software and so this makes it a good place to use special coding tricks to induce errors etc. Of course, after copy protection methods based on using data in sub-channels were invented, CD RW drives and CD drives in general started to offer special modes that would read and pass on the sub-channel data making it possible to create exact copies.
The key idea is that as copy protection develops copying hardware has to become better at creating exact copies – complete with errors, error coding, illegal data patterns and all control data including sub-channels. This all depends on the reading device passing on all of the raw data – the so-called RAW reading mode. Drives can either do RAW reading or they can’t. Ideally what you need is RAW+96, which in principle is capable of supplying the information a writer needs to make a very good copy or all of the data and all of the sub-channels.
At this point it sounds as if you should always use RAW copying to defeat copy protection but what if the disc has a real error on it caused by a scratch? Presumably then you want the software to make a permanent correction and not an exact copy.
The latest batch of copy protected CDs make use of a “higher level” difference between CD players and CD drives.
A CD can have multiple sessions recorded on it – each one is like a complete CD recording. The session data is recorded on the CD and software in a multi-session player works out how to handle each session. However, many CD players are single session players and will simply ignore any additional sessions recorded on an audio disc. CD drives on the other hand are more flexible and they nearly all try to read all of the data on a multi-session disc.
So how can this be used for copy protection – simply create a second session that contains illegal data. The average CD player will simply ignore everything but the first session, which will play perfectly. The average CD drive will try to make sense of all the sessions, discover the error, post an error message and then give up. This not only makes the disc uncopiable in a PC, it makes it unplayable as well!
This method of copy protection is has being used with variation by companies such as Sony on audio discs.