Firstly, do not confuse Super Bit Mapping (SBM) with Super
Bit Mapping-Direct (SBM-Direct). SBM is a noise-shaping/dithering scheme based
on the audibility of the average human ear. Human hearing is more sensitive in
the 1 kHz to 3 kHz range than it is at low and high frequencies. SBM (or
noise-shaping in general) takes advantage of this to move noise to frequencies
where the ear is less sensitive. The result is an effective 20-bit performance
in the sensitive range, and worsening performance with increasing frequency for
a 16-bit system like CD. Weiss, Prism and Meridian also made noise-shapers that
utilized the U-shaped ear sensitivity curve while Apogee's UV-22 had a better
than 16-bit performance from DC to about 13 kHz before the SNR went almost vertical.
SBM-Direct is the confusing name that Sony in Japan gave to
the process for decimating DSD data to 16-bit/44.1 kHz PCM to make the CD layer
of hybrid discs. It is possible that SBM noise-shaping is part of this process,
but that has never been made clear (as far as I know).
