SafeAudio There are two different approaches to copy protection: main-channel and control-channel. "Main-channel systems alter the way the audio data are recorded on the disc," explains Peter Newman, VP of engineering for Macrovision Europe. "Control-channel systems alter the data in the subcode channels, which are used for steering and error recovery. SafeAudio is a main-channel system."
SafeAudio, which Macrovision codeveloped with TTR Technologies, introduces deliberate errors into both the audio data and the error-correction codes in the main channel. The protected CD can still play on a computer or CD player, which interpolate data to bridge the gaps using electronic guesswork, but the errors spoil the sound on any copy.
Macrovision contends that main-channel protection poses less of a risk of incompatibility than control-channel protection, which prevents a disc from playing at all in some existing players. But critics say that the necessary interpolations could have an audible effect because the player is constantly guessing at the corrupted data.
"Not so," insists Newman. "The uncorrectable errors are so carefully sited, with such similar information on either side of the gap, that there is an accurate way to bridge the gap. The bridging is so tiny, so narrow, and so perfect that the human ear can't detect it."
Macrovision claims that SafeAudio discs will play on virtually all CD audio equipment and has opened its lab to demonstrate its ambitious compatibility-testing program. No other copy-protection developer has done so or even revealed how it tests for compatibility.
Cactus Data Shield (CDS) Midbar Tech has patents for control-channel protection, which it might or might not be using as part of various technologies employed on CDS-protected discs. The company won't explain its technologies, but every Cactus disc released so far prevents computers from playing the standard audio tracks on a CD by denying access to the disc's Table of Contents.
In compensation, CDS-protected discs automatically install a software player from EverAd on the computer that lets users play a version of the album as a compressed music file. When you hold a Cactus-encoded disc's play surface to the light at the correct angle, you can actually see two data areas, separated by a clear band where no data are written. The inner band is standard audio, supposedly playable on all standalone CD equipment; the outer band is the compressed music file.
The CDS system uses either a 32- or 44.1-kHz sampling rate at a 80- or 128-kilobits-per-second data transfer rate for its compressed files, compared with the CD standard's 44.1-kHz sampling and 1.4-megabits-per-second data transfer. Midbar recently claimed that all known playback problems with standalone players had been resolved in the latest version of CDS, Cactus Data Shield 200.
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