I worked on a QC device in the late 80's at Philips when the CD-A standard was nicely established, and the up and coming thing was CD-ROM, in various format flavours.

I had to adapt a combined CD-V/LaserDisc player for use as a test instrument so that I could read out the error flag information being generated by the readback circuitry as it pulled the encoded symbols off the disc.

There were three classifications of error flag, E1 E2 and E3, and for each class, three levels. This means a range of 9 error criticalities, of which (generally) the Video playback circuitry used correction strategies at class 2, and the audio at class three.

Class E2.2 & 3 audio symbol errors usually resulted in a "mute" being applied at the audio output by the better players, or a "click" if no suppression was applied. Class E3.3 usually meant an irrecoverable tracking error, either in the motion servo tracking or in the focus servo tracking. This either meant the player would "jump" or it would "stick" as it tried to recover.

The strategy of muting, re-tries, or whatever was usually determined by the software engineering team who wrote the firmware for the player; this was usually controlled by the marketing team who (in one case where I actually have knowledge) decided that a player "sticking" rather than bailing out from playing the track and either moving on to the next track or stopping playing altogether was the best strategy - a user would be more likely annoyed by the player stopping playing, and would return the player to the shop. If it "stuck", then based on decades of experience with sticking LPs, they would blame the medium rather than the player and take the disc back to the shop, thereby handily switching the bias of the blame away from the player. Crafty bit of psychology, eh?

Anyway, the point of all this is that audio symbol errors on a CD-A are usually of very little consequence, since the player can to a limited extent compensate before giving up and muting (E3.3 errors are actually quite rare due to the quality control procedures used in manufacturing). However, tracking servo information is pretty critical, so this (to me) is where this protection method would work, by artificially introducing symbols into the servo control info at a level where the more rigidly enforced tracking standards required by any CD-ROM drive capable of running at above 2X speed would cause the drive to get upset.

Don't forget that a CD-ROM drive has no recovery strategy since it is not primarily intended for audio playback. Software CD players using CD-ROM drives have to look at reported errors and compensate accordingly; RIPpers like Audio Catalyst have to monitor tracking errors during data capture from the CD-A to prevent the introduction of errors in the audio stream. The only thing in common between a modern CD-ROM drive and a CD-A player is their common beginnings - like us and the apes.

I think this method will be used, without publicity, and the industry will go through an initial phase where they get massive returns of discs from customers who believe that they are defective. They will tolerate this, and with experience, they will eventually get the balance between playability and returns down to what they regard as an acceptable level against what they view as a revenue-robbing problem. They will also be doing this in the knowledge that the older CD-A players will gradually be discarded and replaced by newer players which will be prime candidates for technical mods to include latent protection methods that will be developed as the industry fights to put the lid back on the bottle. If they do decide to publicise it, it will be on the basis that the discs are being changed to include "more music" or "better sound quality" or some other almost unquantifiable, mushy consumer "benefit". People will (as is human nature) moan about it, then walk down to the nearest Comet or Dixons and buy the latest, newest, flashiest player they can get their hands on, and their old gear will be history.

I say this having followed the same b****** story with DAT recorders (available for nearly 4 years in the lab where I worked before they were released while management and marketing agonised over "protection" of the CD-A market) and the many abortive protection schemes and failed protected audio products that have appeared (and sunk) over the years. The CD-A market is too big for all concerned for it to lose profits by freely-exchangeable, legitimate music.

It's too late - but that won't stop 'em from trying....