Keith O. Johnson stands as one of the original co-founders of Reference Recordings. Over the course of four decades, he has steadfastly served as the Technical Director and mastering engineer for the label. Collaborating initially with producer and RR founder Tam Henderson, and more recently with other accomplished producers, Johnson has contributed to the recording and release of an impressive catalog comprising over 140 compact discs, LPs, and surround-sound SACD titles. This eclectic array of releases spans across various musical genres, including classical, jazz, world, and blues.

The distinctive 'RR Sound' owes its unique character to Johnson's distinctive methodologies and equipment, much of which he has personally handcrafted or extensively modified. His microphone techniques exhibit a broad spectrum, ranging from purist approaches to intricate setups, tailored to the specific musical forces and acoustical environments involved.

Referred to as 'Prof.' Keith Johnson, he has dedicated over 50 years to building a reputation as an innovator, technical virtuoso, and accomplished musician. He occupies a rarefied position within the audio industry, akin to only a select few visionaries. Johnson's contributions extend to pioneering designs and patented inventions in both the professional and consumer audio domains, including the groundbreaking HDCD encoding process.

In 2011, Keith Johnson was honored with a GRAMMY® award for Best Surround Sound Album, with an impressive tally of eight additional nominations in categories such as Best Engineered Album (Classical). His illustrious career has also earned him a slew of other industry awards and nominations, including the prestigious Audio Engineering Society Silver Medal Award in 2008. This award, given in recognition of outstanding contributions to the field of audio engineering, has previously been bestowed upon luminaries such as Ray Dolby, Paul Klipsch, Robert Moog, and Willi Studer.

HDCD technology, developed by Keith Johnson and Michael Pflaumer of Pacific Microsonics Inc. between 1986 and 1991, became publicly available in 1995. In 2000, Microsoft acquired the company along with all its intellectual properties. Starting from Windows Media Player 9 and onwards, an HDCD decoder has been included.

This process involves the challenge of fitting the enhanced audio quality of 24-bit recordings into the confines of a standard CD's 16-bit format.

Peak Extension is a unique feature in HDCD technology. It's a restorable soft peak limiter that comes into play when HDCD decoding is used. It enables peak levels to reach up to 6 dB above the standard full-scale peak level (+6 dBfs) on 16-bit HDCD-encoded recordings without causing clipping or distortion ("overs"). This limiter is designed with a carefully crafted "easy over" curve, which aims to replicate the warm saturation characteristic of analog tape. It operates within an input signal level range of -3 dBfs to +6 dBfs, effectively compressing the top 9 dB of the input signal's range into the top 3 dB of the 16-bit recording.

When HDCD-decoded playback occurs, the peak limiting effect is reversed by the HDCD decoder chip, using a precisely mapped inverse of the limiting curve controlled by a hidden code. This process restores the dynamics of the original material, allowing peak levels to extend up to +6 dBfs. This extension effectively broadens the dynamic range by up to 6 dB.

Unfortunately, not all CDs fully utilize this technology. Some lack the peak extension feature, and the heavy limiting compression applied cannot be reversed during decoding, thus undermining the original intent of preserving the dynamic range and audio quality of the master.