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Technology, Theory and Design

SACD: Up Close and Personal

Andrew Demery, Director of Field Services for Sony’s SACD hardware division, kicked off a series of SACD talks with an introduction to the SACD format, and a report on the current state of the SACD hardware market.

There are about 1 million SACD-capable players in the US, which can play some 1,000 titles SACD titles worldwide, with 600 directly available in the US. More than half of the 1,000 titles are multi-channel, and this number is expected to grow in the future as more SACD releases will have multi-channel mixes along with the mandatory 2-channel mix.

Part of SACD’s appeal to music publishers is its content-protection, which Sony claims is practically guaranteed by several proprietary features:

1. Lead-in scrambling, or Super Audio CD mark. This prevents drives that are not licensed for SACD to read the lead-in data area of an SACD, which contains data to enable the drive to read the rest of the disc. This protects against PC-based copying where hackers may try to read the raw data off the disc.

2. Content encryption. DSD data is encyrpted with an 80-bit key on the disc, and the keys are cleverly hidden to prevent access. Physical watermarks called Pit Signal Processing Physical Disk Mark, or PSP-PDM, which stores part of the decryption key in the physical shape of the pits prevents a bit-for-bit copy from working if DSD data is copied to a re-writable format. Unlike audio watermarks, such as the infamous Verance watermarks used on some DVD-Audio releases, physical watermarks have no effect on the sound of a disc. There are also no plans to release a writable SACD format, analogous to CD-R.

The key is never transmitted on any bus, or anywhere outside the proprietary SACD chipset, and thus cannot be snooped with logic analyzers, therefore the only known attack is an exhaustive search of all 80-bit keys – trying every 80-bit value as a key until one successfully decodes the data. The number of possible 80-bit values is approximately 1 million million million million, or a 1 followed by 24 zeroes. Put another way, if each key took 1 second to try, it would require approximately 30,000 million million (a 3 followed by 16 zeroes) years to try every key.

This effort of an exhaustive key search must be repeated for every SACD title, because the key is unique for each release. So if one SACD title is ever cracked, all other SACD discs are still safe.

SACD discs are also careful about leaving open cleartext information that could be used in the analysis of the key. Failure to do this on DVD allowed DVD’s encryption to be broken, and spread as the DeCSS program.

3. Proprietary knowledge. SACD’s encryption and decryption are available only in carefully licensed and controlled hardware, and never in software. This makes reverse engineering of the algorithms very difficult. There are no plans to make these algorithms available as software.

Disc production equipment, necessary to write the lead-in scrambling and PSP-PDM will also be carefully licensed and controlled. These encoders are rented to licensed manufacturers, and will not be available on the used market, thereby tightly controlling access to the equipment.

SACDs can be manufactured as hybrid discs, with a Red Book-compatible (otherwise known as Compact Disc) optical layer that allows playback in CD players, and this is especially appealing to stores because they can stock only one disc that has both SACD and CD layers. While hybrid SACD discs will play back in virtually all CD players, they do not play back in all DVD players. SACD’s physical layer is essentially a DVD, with proprietary content as described above. Some DVD players will read the DVD layer first instead of the Red Book layer, and some small fraction of those players will be confused by SACD’s non-standard content, and either reject the disc or misbehave in some way (reports from users have included DVD players refusing to eject the disc and crashing players). In theory, this should be fixable in software because the player could be taught to try the Red Book layer first.

Another appeal of SACD is its simplicity and similarity to CD in terms of mastering. Unlike DVD, which can be overwhelming in its possibilities of disc layout and content, SACD contains a mandatory two-channel audio track up to 256 minutes in length. A six-channel multi-channel audio track is optional. A level-matched 44.1kHz 16-bit downsampling of the DSD signal for the Red Book-compatible layer on hybrid SACDs, or for a separate CD release, is easily created with Sony’s SBM Direct software.

The only non-audio frill SACD offers is text for each track with up to 8 languages. This feature is commonly used to store music titles. SACD has some extra space and bandwidth reserved for future data, such as video, but there are no current plans to use this space.

As a result of SACD’s simplicity, an SACD can be authored in a few hours, with the actual authoring time set mainly by the quality check of the disc, wherein the audio tracks must be listened to by a human in real time. []


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SMR's Surround 2002 coverage is co-sponsored by High Fidelity Review
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Show report last updated: 2nd February 2003.

 

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