Steve, I don't pretend to have a full understanding of the secure digital technology, but have relied on the non-technical descriptions in PR material provided by the industry group that is pushing the technology.
Without going back to those reports, I can only recall that the description is not quite what you are assuming. The MP3 file is not modified when it is downloaded. The host computer merely looks for an identification from the user so that it knows where the fee, if any, for downloading can be charged. When the the file has been transferred to the SD card, if the copy is to be transferred to another device, such as a CD or hard disk, the SD card also looks for identification, and if the ID is the same, then the copying is done routinely. That's my understanding, but I don't have the information on how this works from an engineering point of view.
Of course, it would be easy to make copies from an MP3 file by merely routing the audio output signal that normally goes to the speakers to a casette or other recording device, with only a small amount of degradation from the original. That is, the protection would apply only to unauthorized copies of the MP3 file, not the music contained in the file. Somebody please help me out if this is incorrect! |
