What is Super Bit Mapping (SBM)?
Super Bit Mapping is Sony technology associated with reducing higher-precision digital audio to a 16-bit output while shaping quantisation noise so that less of it falls in the most audible frequency regions.
The short answer
SBM became closely associated with 1990s 20-bit mastering. It did not create a 20-bit CD. It was part of the process used to make a carefully quantised 16-bit signal from higher-precision material.
An SBM CD is still an ordinary 16-bit / 44.1 kHz audio CD. No special SBM decoder is required for playback.
Why is reducing bit depth a problem?
If low-order bits are simply discarded, the quantisation error can become correlated with the music and behave like low-level distortion. Dither randomises that error, while noise shaping can redistribute much of the added noise toward frequencies where hearing is less sensitive.
What does SBM do?
Sony describes Super Bit Mapping as using information from higher-precision data when creating a 16-bit result, with noise reduction concentrated in the audible band. In 20-bit-to-16-bit workflows, the goal is to retain more useful low-level information than straightforward truncation would preserve.
Conceptually, SBM belongs to the same family of mastering ideas as dither and perceptually shaped quantisation. The implementation is Sony-specific.
What is actually stored on the CD?
Standard 16-bit PCM. The mastering process has already created the final sample values before the disc is authored.
This differs from HDCD, where control information can be carried inside a compatible 16-bit PCM stream and a decoder can change playback behaviour. SBM is fundamentally a mastering-side process.
What about SBM Direct?
SBM Direct appeared on later Sony mastering/reissue material, particularly in workflows connected with DSD. It should still be understood as a mastering/bit-reduction process rather than as a new consumer audio format.
Packaging terminology varied between releases, so the mastering credits are more informative than assuming every SBM or SBM Direct logo represents an identical source chain.
Does SBM guarantee a better-sounding CD?
No. A careful bit-reduction stage is useful, but source quality, transfer, EQ, compression, limiting and other mastering decisions can have much larger audible effects. SBM describes one part of the production process.
Sources and further reading
Related pages
20-bit remaster · Dither and noise shaping · XRCD and K2 · Mastering markings